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23 Sep 2024
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Health Physics, Nuclear Medicine, Radiation Safety Versant Physics 0 comment

Dr. Thomas Morgan: An Inspirational Impact on Health Physics

Dr. Thomas Morgan, a Senior Health Physicist with Versant Physics, was recently made a Fellow by the Health Physics Society. As a long-standing scientist within the world of radiation science and health physics, this is an achievement he more than deserves. A 1983 graduate of the University of California, Irvine, Dr. Morgan has built a distinguished career spanning several decades. His academic and professional achievements reflect a deep commitment to advancing the field and training future generations of scientists.

Academic Foundations and Early Training

Dr. Morgan’s academic path laid a strong foundation for his future contributions to radiation science. At UC Irvine, he earned bachelor’s degrees in both biology and chemistry. These disciplines provided him with a robust understanding of the scientific principles underlying medical physics and radiological sciences. Dr. Morgan’s educational journey didn’t stop there. He continued at UC Irvine to obtain both a master’s degree and a Ph.D. in radiological sciences, specializing in medical physics.

During his time as a graduate student, Dr. Morgan underwent rigorous training in operating nuclear reactors. He became licensed by the Nuclear Regulatory Commission as a Senior Operator of the TRIGA Mk 1 nuclear reactor on campus. This hands-on experience with nuclear technology and safety would become a cornerstone of his career.

Contributions to Research and Publications

Dr. Morgan’s research contributions are significant and varied. His work spans several critical areas, including radiation biology and physics, cancer biology, clinical cancer research, and radiation safety. Over the years, he has directed and conducted valuable research. This work has advanced our understanding of how radiation affects biological systems. The research also produced options for how it can be used safely and effectively in medical treatments.

The scholarly output of Dr. Thomas Morgan is impressive, with more than 35 peer-reviewed publications to his name. Additionally, he has co-authored three books and six book chapters. This further solidifies his reputation as a leading expert in his field. In the Versant Physics newsroom, we covered the publication of one of his more recent publications with HPS. Publications like these not only reflect his deep knowledge but his dedication to sharing that knowledge with the broader scientific community.

Commitment to Education and Training

Dr. Morgan’s commitment to education is evident from his extensive teaching experience. He was hired by the Southern California Permanente Medical Group in Los Angeles, California, to teach radiation biology resident physicians in the Radiation Therapy Department. He was appointed as an Adjunct Professor of Health Sciences at California State University of Long Beach. This was where he taught radiation biology to radiation therapy radiologic technologists. Dr. Morgan took every role in educating these future professionals with determination to better the future. This underscores his belief in the importance of training and mentoring the next generation of radiation practitioners.

Dr. Thomas Morgan also taught health physics to medical physics graduate students at Columbia University in New York City. There he served as an Adjunct Professor of Applied Physics and Mathematics. Through these roles, Dr. Morgan has not only imparted his knowledge but has also helped shape the careers of the next generation of medical physicists.

Professional Roles Through a Dedicated Career

Dr. Morgan’s professional career includes several prestigious positions in radiation safety and environmental health. He served as the Radiation Safety Officer (RSO) at the Southern California Permanente Medical Group’s Los Angeles campus. Dr. Morgan was also the RSO at the University of Rochester and Strong Memorial Hospital in Rochester, New York. His expertise in managing radiation safety protocols in these settings was crucial for maintaining safe and compliant operations.

Before his retirement in 2018, Dr. Morgan held a prominent role as the Executive Director of Environmental Health and Safety and Chief Radiation Safety Officer at Columbia University and the Columbia University Irving Medical Center. In this capacity, he was responsible for overseeing all aspects of environmental health and safety. This included radiation safety, ensuring that the institution adhered to the highest standards.

Leadership in Professional Organizations

Dr. Morgan’s contributions extend beyond his research and teaching. He has always been an active member of the Health Physics Society. Dr. Morgan even served as past president of the Western New York and Florida chapters. His leadership roles also include chairing several committees and serving as a Director of the Society. In recognition of his service and contributions to the field, he was named a Fellow of the Health Physics Society in 2023.

His editorial roles further illustrate his commitment to advancing the field. Dr. Morgan is currently an Associate Editor of the Health Physics Journal. He also previously served as an Associate Editor of Applied Physics Research. These positions highlight his ongoing engagement with the latest research and developments in health physics.

Certification and Licensure

Dr. Morgan’s professional qualifications are extensive. He is certified by the American Board of Health Physics in the practice of comprehensive health physics and is licensed to practice medical health physics in New York, Florida, and Texas. These credentials underscore his expertise and commitment to maintaining the highest standards in his practice.

Personal Life and Volunteer Work

Outside of his professional life, Dr. Morgan enjoys residing in Sarasota, Florida, with his wife, Diane. He remains actively involved in his community through volunteer work. He serves as a docent at the Mote Marine Laboratory, where he helps educate the public about marine science. Additionally, his involvement with the Manasota Medical Reserve Corps reflects his dedication to supporting public health and safety.

A Continuing Devotion to Radiation Science and Health Physics

Headshot of Dr. Tom Morgan.

The career of Dr. Thomas Morgan is a testament to his passion for radiation science and health physics. From his early academic achievements to his significant research contributions, teaching roles, and leadership positions, his impact on the field is profound. His dedication to education, professional service, and community involvement underscores a lifetime of commitment to advancing science and improving safety. As a newly named Fellow of the Health Physics Society, Dr. Morgan’s legacy continues to inspire and shape the future of radiation science.

29 Aug 2024
Cancer Treatment, Nuclear Medicine Versant Physics 0 comment

Celebrating the Achievements of Dr. Darrell R. Fisher:  A Journey of Scientific Excellence

Earlier this year, the Columbia Chapter of Health Physics Society and the Georgian Health Physics Association nominated Dr. Darrell R. Fisher for the 2024 Distinguished Scientific Achievement Award of the Health Physics Society (HPS). Fisher is a nuclear medicine physicist with Versant Medical Physics and Radiation Safety. He received the Distinguished Scientific Achievement Award in July 2024, during the 16th International Congress of the International Radiation Protection Association and 69th HPS Annual Meeting in Orlando, Florida. The award recognized Dr. Fisher’s scientific contributions over a career spanning more than five decades in applied radiation safety sciences, including internal dosimetry, radiobiology, radiochemistry, and design and testing of radiopharmaceuticals and medical devices. His innovative research has led to numerous significant advancements in the applied radiation health sciences. 

Discoveries in Chelation Therapy and Radiotoxicity

One of Dr. Fisher’s early discoveries was identifying the toxicity and congenital birth defect teratogenicity of the heavy-metal chelating agent CaNa3DTPA compared to the non-toxic ZnNa3DTPA. This research showed the importance of maintaining cellular zinc and manganese availability during critical growth periods to ensure uninterrupted cell division.  His work was instrumental in enhancing the safety and effectiveness of emergency decorporation therapy for nuclear workers accidentally exposed to plutonium, neptunium, and americium. By establishing the safety of ZnNa3DTPA, Dr. Fisher’s work paved the way for safe and effective treatments after actinide contamination events, significantly impacting emergency response protocols and worker safety in nuclear industry.

Advances in Applied Microdosimetry

Dr. Fisher performed the first microdosimetry calculations for plutonium-238 and plutonium-239 in beagle dog lung tissue specimens. His research disproved the “hot particle hypothesis” claimed by Thomas Cochran and Arthur Tamplin; Fisher showed that their hypothesis could not be supported by dosimetry or radiobiology.  Fisher described the fundamental science underlying relative biological effectiveness (RBE) relationships between alpha-particle dose and two critical biological endpoints: cellular mutation and primary DNA damage. This work demonstrated that varying the specific activity and spatial geometry of plutonium microsphere sources could result in vastly different biological outcomes at constant absorbed dose. These findings were pivotal to better understanding radiation effects at the microscopic level, influencing safety standards, and advancing what would later become new cancer treatment approaches with alpha-emitting radionuclides.

Innovations in Cancer Treatment

Dr. Fisher was an early proponent of short-lived alpha emitters such as radium-223 and actinium-225 for cancer treatment. He performed the first cellular-level microdosimetry calculations for alpha-emitter-radiolabeled monoclonal antibodies used in cell-targeted radioimmunotherapy. His pioneering work in developing treatment-planning dosimetry for clinical radioimmunotherapy supported the high-dose treatment of relapsed lymphoma, leukemia, and multiple myeloma patients.  In the laboratory, he helped to develop and test several innovative alpha-emitter complexing agents, including macrocyclic cages and nanoparticle delivery constructs for next-generation therapies.

Contributions to Uranium Toxicology

Dr. Fisher’s assessments of dosimetry and health effects from uranium intakes led to a modified biokinetic model for inhaled uranium hexafluoride compounds. His research provided crucial insights following the Sequoyah Fuels accident in Oklahoma, where workers were exposed to uranium hexafluoride. Dr. Fisher’s model improved the understanding of uranium’s behavior in the body, leading to better understanding of uranium toxicity. 

Pioneering Work in Radionuclide-Polymer Composites

Dr. Fisher conceived, designed, patented, and tested novel radionuclide-polymer composites employing insoluble crystalline yttrium-90-phosphate microspheres administered in a thermo-reversible hydrogel delivery matrix. This innovation has enabled direct interstitial therapy of nonresectable solid tumors, providing new avenues for cancer treatment in both humans and veterinary animals.  Precision radionuclide therapy by direct intra-tumoral injection maximizes therapeutic ratios, a key measure of treatment safety and efficacy.  His work has enabled highly efficient delivery of radiation to tumors, minimizing damage to surrounding healthy organs and tissues and associated side-effects of radiation therapy.

Enhancing Patient Safety in Nuclear Medicine

Dr. Fisher helped develop and published practical methods for patient-specific dosimetry to characterize the severity of inadvertent radiopharmaceutical extravasations in diagnostic and therapeutic nuclear medicine.  Fisher has specialized in medical internal radiation dosimetry, an essential element of managing patient safety and improving therapy outcomes. 

Lifelong Commitment to Radiation Safety

Darrell Fisher’s journey in health physics began in 1973 as an undergraduate research assistant at the University of Utah’s Radiobiology Laboratory. His early research on extracting americium-241 from skeletal surfaces in live mice laid the foundation for his future contributions to radiation safety and radiobiology.  Over the years, his work has led to significant advancements in understanding and mitigating the effects of radiation exposures in medical and occupational settings. His dedication to the field is evident in his continuous efforts to improve safety standards, develop innovative treatment methods, and educate future generations of health physicists.

The 2024 Distinguished Scientific Achievement Award recognizes Dr. Fisher’s commitment to scientific excellence and discovery for work that not only advances scientific knowledge but also improves patient care and radiation safety.  His diverse contributions to the field of applied radiation safety sciences, in collaboration with highly respected research associates and colleagues worldwide, have been impactful. His innovative research, spanning over 50 years, has led to significant advancements in worker safety, dosimetry and microdosimetry, radionuclide therapy treatment planning, direct interstitial treatment of inoperable tumors, and patient safety in nuclear medicine.

Dr. Fisher Headshot, 2024.
27 Mar 2024
Airplane flight crew character design. Pilot and stewardess flat vector illustration
Background Radiation, Health Effects, Ionizing Radiation, Radiation Safety Versant Physics 0 comment

Flight Crews and Radiation Exposure

Flight crews are among the occupational groups most exposed to ionizing radiation, with an average annual effective dose surpassing that of other radiation-exposed workers in the United States, excluding astronauts.1 This elevated exposure is primarily due to the high levels of cosmic radiation encountered at flight altitudes, which can pose significant health risks to pilots and cabin crew members.2 In this blog post, we’ll explore the nature of cosmic radiation, its potential health effects, current exposure levels for aircrews, as well as the guidelines and regulations in place to ensure their safety.

What Is Cosmic Ionizing Radiation?

As we’ve touched on in a previous blog, cosmic ionizing radiation–or simply cosmic radiation–originates from beyond Earth’s atmosphere. Additionally, it consists of two main components: galactic cosmic radiation (GCR) and solar particle events (SPEs).3,4

Galactic Cosmic Radiation

GCR is a constant background radiation that permeates interstellar space, originating from distant stars and galaxies. It is composed primarily of high-energy protons (85%) and alpha particles (14%). There is also a small fraction of heavier nuclei (1%) ranging from lithium to iron and beyond. These particles span a wide energy range, and as a result some reach extremely high energies capable of penetrating deep into the Earth’s atmosphere and passing through aircraft shielding.5

Solar Particle Events (Solar Flares)

Solar particle events, on the other hand, are sporadic bursts of intense radiation associated with solar flares and coronal mass ejections. During an SPE, the Sun ejects a large number of high-energy protons and other particles that can reach Earth within hours to days. While less frequent than GCR, SPEs can dramatically increase radiation exposure for flight crews, particularly those on polar routes where the Earth’s magnetic field provides less protection.6,7

At higher altitudes, such as those typically encountered during air travel, the Earth’s atmosphere provides less shielding against cosmic radiation, resulting in increased exposure for flight crews and passengers.

Several studies that have investigated the difference in cosmic ray levels at various altitudes versus ground level found that the dose rate of cosmic radiation at a cruising altitude of 30,000 feet was approximately 10 times higher than at sea level.

The specific increase in cosmic ray exposure at higher altitudes is influenced by several factors, including the solar cycle (solar maximum vs. solar minimum), geomagnetic field strength, and also the path of the flight (polar routes are exposed to higher levels of cosmic rays). For example, during periods of high solar activity (solar maximum), the increased solar wind can actually shield the Earth from some cosmic rays, slightly reducing the exposure at high altitudes. Conversely, during a solar minimum, the cosmic ray intensity can be higher.

Estimates of the number of hours that have to be flown in order to receive an effective dose of 1 mSv at 30,000 feet are 510 hours at a latitude of 30o South and 1,330 hours at the equator.8

Health Effects and Uncertainties

The health risks associated with radiation exposure are generally well-documented. Prolonged exposure to high levels of radiation can increase the risk of cancer, cataracts, as well as other adverse health effects.  However, quantifying the specific risks associated with the chronic low-dose radiation experienced by flight crews remains a challenge.

The World Health Organization’s International Agency for Research on Cancer (IARC) acknowledges that ionizing radiation causes cancer in humans and is also associated with reproductive problems. However, when it comes to cosmic ionizing radiation, several uncertainties remain:

  1. Cancer Risk: Most radiation health studies have focused on groups exposed to much higher doses from different types of radiation (such as atomic bomb survivors or patients receiving radiation therapy).9 Due to this, the specific link between cosmic ionizing radiation and cancer risk is not yet fully understood.
  2. Reproductive Health: Miscarriages and birth defects related to cosmic radiation exposure are still not definitively established.10

Despite the limitations of current research, several studies have suggested that flight crews may have a higher incidence of certain cancers compared to the general population. These include breast cancer, melanoma, as well as non-melanoma skin cancers.11,12 However, the causal link between cosmic radiation exposure and these increased risks has not been definitively established. Other factors, such as lifestyle and genetic predisposition, may also play a role.13

Exposure Levels for Flight Crews

Recent dose and risk assessments by a wide variety of investigators have demonstrated the need to dedicate further attempts to quantify potential radiation exposure.14 The National Council on Radiation Protection and Measurements (NCRP) reports an average annual effective dose of 3.07 mSv for flight crews; most of this exposure comes from natural radiation:

  • Estimates of annual aircrew cosmic radiation exposure range from 0.2 to 5 millisieverts (mSv) per year depending on factors such as flight routes, altitude, and solar activity.
  • Solar particle events occur less frequently, but during events, exposure levels can increase substantially and potentially lead to higher doses over short periods.

Guidelines and Regulations

While there are no official dose limits specifically for aircrew in the United States, national and international guidelines provide context:

  • International Commission on Radiological Protection (ICRP): Recognizes aircrew as radiation-exposed workers. They also recommend an effective dose limit of 20 mSv per year averaged over 5 years (totaling 100 mSv in 5 years) for radiation workers. However, for the general public, the recommended limit is 1 mSv per year.15
  • Pregnant Aircrew: The ICRP recommends a dose limit of 1 mSv throughout pregnancy.16

Current regulations aim to limit radiation exposure for flight crews, but there is room for improvement. The International Commission on Radiological Protection (ICRP) sets guidelines for radiation protection and also includes dose limits for occupational exposure. However, these guidelines may not adequately address the unique challenges faced by flight crews. To improve current radiation safety regulations for aircrews, a multi-faceted approach is necessary. This should include:

Improved Monitoring and Data Collection

Implementing advanced radiation monitoring systems on aircraft in addition to encouraging the use of personal dosimeters by flight crews can provide more accurate and comprehensive data on exposure levels17. This information can help refine risk assessments as well as guide the development of more effective protection strategies.

Aircraft Shielding and Design

Continued research into advanced shielding materials in addition to aircraft design modifications can help reduce the radiation dose received by flight crews and passengers18. This may also involve the use of novel composite materials or the incorporation of additional shielding in critical areas of the aircraft.

Route Optimization and Flight Planning

By carefully planning flight routes and altitudes, airlines can minimize exposure to cosmic radiation, particularly during solar particle events19. This may also involve rerouting flights to lower latitudes or reducing flight time at higher altitudes when necessary.

Education and Awareness Programs

Providing flight crews with comprehensive information about the risks of cosmic radiation exposure in addition to the importance of proper protection measures can empower them to make informed decisions about their health and safety20. This should include training on the use of personal protective equipment, such as dosimeters, as well as guidelines for managing exposure during pregnancy.

Regulatory Harmonization and Enforcement

Strengthening international collaboration to harmonize radiation protection standards for flight crews in addition to ensuring consistent implementation and enforcement of these standards across the aviation industry can help create a safer working environment for all aircrews2121.

Conclusion

Although no regulations officially set dose limits, radiation exposure is still a concern to be evaluated for airplane flight crews due to their occupational exposure to cosmic radiation. While the specific health risks associated with this chronic low-dose exposure remain uncertain, continued efforts are essential ensure a safe working environment. By implementing measures such as personal dosimetry devices, increased monitoring, staff training, and encouraging airplane manufacturers to consider shielding and design modifications, airlines can better protect their flight crews. Ensuring a safer career for every radiation worker will require time, dedication, and collaboration. However, the benefits for the health and safety of all industries, including aircrews, make it a worthwhile endeavor.

Versant Physics is a full-service medical physics and radiation safety consulting company based in Kalamazoo, MI. Contact us for all of your regulatory, radiation safety, and personnel dosimetry needs.

Sources

  1. Friedberg, W., & Copeland, K. (2003). What aircrews should know about their occupational exposure to ionizing radiation. Oklahoma City, OK: Civil Aerospace Medical Institute, Federal Aviation Administration. ↩︎
  2. United Nations Scientific Committee on the Effects of Atomic Radiation. (2008). Sources and effects of ionizing radiation: UNSCEAR 2008 report to the General Assembly, with scientific annexes. New York: United Nations. ↩︎
  3. Validation of modelling the radiation exposure due to solar particle events at aircraft altitudes. Radiation Protection Dosimetry, Volume 131, Issue 1, August 2008, Pages 51–58. https://doi.org/10.1093/rpd/ncn238 ↩︎
  4. Wilson, J. W., Townsend, L. W., Schimmerling, W., Khandelwal, G. S., Khan, F., Nealy, J. E.,  & Norbury, J. W. (1991). Transport methods and interactions for space radiations. NASA Reference Publication, 1257 ↩︎
  5. O’Sullivan, D. Exposure to galactic cosmic radiation and solar energetic particles. Radiat Prot Dosimetry. 2007;125(1-4):407-11. https://pubmed.ncbi.nlm.nih.gov/17846031/ ↩︎
  6. Turner, R. E. (2007). Solar particle events from a risk management perspective. Radiation Protection Dosimetry, 127(1-4), 534-538. ↩︎
  7. Lantos, P., & Fuller, N. (2003). History of the solar particle event radiation doses on-board aeroplanes using a semi-empirical model and Concorde measurements. Radiation Protection Dosimetry, 104(3), 199-210. ↩︎
  8. Cosmic Radiation Exposure for Casual Flyers and Aircrew, https://www.arpansa.gov.au/understanding-radiation/radiation-sources/more-radiation-sources/flying-and-health ↩︎
  9. National Research Council. (2006). Health risks from exposure to low levels of ionizing radiation: BEIR VII phase 2 (Vol. 7). National Academies Press. ↩︎
  10. CDC – Aircrew Safety and Health – Cosmic Ionizing Radiation – NIOSH Workplace Safety & Health Topics. Centers for Disease Control and Prevention. Published 2019. https://www.cdc.gov/niosh/topics/aircrew/cosmicionizingradiation.html ↩︎
  11. Pukkala, E., Aspholm, R., Auvinen, A., Eliasch, H., Gundestrup, M., Haldorsen, T., & Tveten, U. (2003). Cancer incidence among 10,211 airline pilots: a Nordic study. Aviation, Space, and Environmental Medicine, 74(7), 699-706. ↩︎
  12. Rafnsson, V., Hrafnkelsson, J., & Tulinius, H. (2000). Incidence of cancer among commercial airline pilots. Occupational and Environmental Medicine, 57(3), 175-179. ↩︎
  13. Hammer, G. P., Blettner, M., & Zeeb, H. (2009). Epidemiological studies of cancer in aircrew. Radiation Protection Dosimetry, 136(4), 232-239. ↩︎
  14. Olumuyiwa A. Occupational Radiation Exposures in Aviation: Air Traffic Safety Systems Considerations. International Journal of Aviation, Aeronautics, and Aerospace. Published online 2020. doi:https://doi.org/10.15394/ijaaa.2020.1476 ↩︎
  15. International Commission on Radiological Protection. (2007). The 2007 recommendations of the International Commission on Radiological Protection. Annals of the ICRP, 37(2-4), 1-332. ↩︎
  16. International Commission on Radiological Protection. (2000). Pregnancy and medical radiation. Annals of the ICRP, 30(1), iii-viii, 1-43. ↩︎
  17. Bartlett, D. T. (2004). Radiation protection aspects of the cosmic radiation exposure of aircraft crew. Radiation Protection Dosimetry, 109(4), 349-355. ↩︎
  18. Wilson, J. W., Miller, J., Konradi, A., & Cucinotta, F. A. (1997). Shielding strategies for human space exploration. NASA Conference Publication, 3360. ↩︎
  19. Copeland, K. (2014). Cosmic radiation and commercial air travel. Radiation Protection Dosimetry, 162(3), 351-357. ↩︎
  20. International Civil Aviation Organization. (2012). Manual of Civil Aviation Medicine. https://www.icao.int/publications/Documents/8984_cons_en.pdf ↩︎
  21. International Atomic Energy Agency: Cosmic radiation exposure of aircrew and space crew. https://www.iaea.org/sites/default/files/20/11/rasa-cosmic.pdf ↩︎
18 Dec 2023
Large size motorized 3D water phantom system for dose distribution measurement of radiation therapy beams in real daily routine practice used as a part of quality control of radiation therapy.
Cancer Treatment, Dosimetry, Nuclear Medicine, Radiation Therapy, Uncategorized Versant Physics 0 comment

What’s Inside Matters Most: Internal Dosimetry

Medical physics has been an integral part of medicine and healthcare over the greater part of the last century. Applying physics theory, concepts, and methods, scientists have created patient imaging, measurement, and treatment techniques that revolutionized the medical world. One product of medical physics has been the evolving specialty called radiopharmaceutical dosimetry, the calculation of absorbed dose and optimization of radiation dose delivery in cancer treatment. Today, we address internal dosimetry, the subset of medical physics that aims to optimize treatment and protect the patient from any undesirable side effects.

What is internal dosimetry?

Dosimetry is the measurement of radiation energy imparted to body organs and tissues. Radionuclides emit beneficial ionizing radiation that is useful for both diagnostic imaging of various diseases as well as for cancer treatment. Thus, medical internal dosimetry is the assessment of internal radiation dose from incorporated radionuclides associated with such life-saving radiopharmaceuticals.1 Radiation dose is the amount of energy imparted by radiations emitted during disintegration of radioactive atoms that constitute part of the radiopharmaceutical chemistry. Dose to organs of the body is quantified per unit mass (or weight) irradiated tissue. Dosimetry provides the fundamental quantities needed for several important purposes, including record-keeping, radiation protection decision-making, risk assessment, and cancer-treatment planning.2 The purpose and objective is to optimize medical benefit while minimizing potential radiation damage to body cells, tissues, and organs.

Dosimetry is a complex physical and biological science. Internal dosimetry provides critical information needed to better understand the biological mechanisms governing radionuclide uptake, translocation, and excretion from the body. The radiation dose imparted depends on the type, amount, and distribution of radionuclides, as well as specific nuclear properties, such as energy emitted.

Internal dosimetry differs from “external” dosimetry, which deals with the radiation dose from sources outside the body. Devices such as dosimeters measure external dosimetry directly, while internal dosimetry relies on indirect methods of radioactivity inside the patient using bioassay and imaging measurements.3 Bioassay is the measurement of the activity or concentration of radionuclides in biological samples. Samples can include urine specimens, feces, blood, or breath. Imaging techniques, such as whole-body counters or gamma cameras, can detect the radiation emitted by the radionuclides inside the body. This helps to provide information on their location and quantity.1

When is internal dosimetry used in healthcare?

Internal dosimetry mainly benefits patients who receive radionuclide therapy, a treatment that involves administering radioactively labeled proteins, such as monoclonal antibodies, to target specific types of cancer.3 It also helps to evaluate and account for unique patient variations in biodistribution—the way that different subjects respond to treatment. Internal dose assessments analyze radionuclide behavior in both normal (healthy) organs, as well as tumors. For example, imaging measurements provide physicists with important information to determine tumor uptake, retention, and clearance. In doing so, administered activity can be tailored according to patient health status, age, size, sex, and basal metabolic rates.

How does internal dosimetry produce useful data?

The main challenge of internal dosimetry is to track and follow the uptake, redistribution, metabolism, and clearance of the administered radiopharmaceutical inside the body over extended time periods after administration.4 Tracking sometimes involves mathematical modeling to describe the absorption, distribution, metabolization, and excretion of radionuclides by the body. Biokinetic models may be developed from the study of population groups, knowledge of radionuclide behavior in different organs, and the unique chemistry of each radiopharmaceutical. Biokinetic models incorporate mathematical compartments representing a particular organ or a tissue, and descriptions of the transfer rates that reflect the movement of radionuclides from one body compartment to another.1

Summary

Internal dosimetry is an important tool for radiation protection, especially in the fields of nuclear medicine, occupational health, and environmental monitoring. Dosimetry helps to customize or personalize nuclear medicine in cancer patients. In a broader sense, internal dosimetry is also applied to occupational and environmental health to prevent or reduce the exposure to radionuclides, by providing information on the sources, pathways, and levels of intake, and by suggesting appropriate measures, such as respiratory protection, contamination control, or dose limits. Internal dosimetry can also help to verify the adequacy of workplace controls, to demonstrate regulatory compliance, and to provide medical and legal evidence in case of accidental or intentional exposure.

Special software tools have been developed for the clinical nuclear medicine setting to facilitate medical imaging and calculate internal doses.

QDOSE® Multi-purpose Voxel Dosimetry (Personalized Dosimetry in Molecular Radiotherapy) is a complete, one-stop solution software for all internal dosimetry needs with multiple parallel workflows. With USFDA 510(k) clearance granted in August 2023, QDOSE® has proven its quality and compliance. To learn more, visit our QDOSE® webpage or schedule a meeting with our team.

Sources

  1. Sudprasert W, Belyakov OV, Tashiro S. Biological and internal dosimetry for radiation medicine: current status and future perspectives. J Radiat Res. 2022;63(2):247-254. doi:10.1093/jrr/rrab119
  2. Bartlett R, Bolch W, Brill AB, et al. MIRD Primer 2022: A Complete Guide to Radiopharmaceutical Dosimetry. Society of Nuclear Medicine & Molecular Imaging; 2022.
  3. Chapter 7 External and Internal Dosimetry. Accessed November 15, 2023. https://www.nrc.gov/docs/ML1121/ML11210B523.pdf
  4. What is Internal Dosimetry – Definition. Radiation Dosimetry. Published December 14, 2019. Accessed November 16, 2023. https://www.radiation-dosimetry.org/what-is-internal-dosimetry-definition/

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13 Sep 2023
Supervisor use the survey meter to checks the level of radiation
Ionizing Radiation, Medical Physics, Radiation Safety, Radioactive Material Versant Physics 0 comment

The Units to Measure Radiation: Explained

The history of radiation units ties closely to the development of our understanding about radiation and its effects. The discovery of x-rays and radioactivity in the late 19th century by scientists like Wilhelm Roentgen, Henri Becquerel, and Pierre Curie paved the way for the exploration of radiation measurement.

As our knowledge of radiation’s effects on living organisms grew, the need for standardized units became evident. The roentgen was one of the earliest units to measure ionization, followed by the introduction of the curie to measure radioactivity. Over time, advancements in our understanding of radiation’s biological effects led to the development of units like the rem and the sievert.

Creating Radiation Units

The development of the SI system (International System of Units) established a standardized set of units to provide a coherent and universal way to measure radiation. The gray and the sievert were introduced as the primary units for absorbed dose and equivalent dose, respectively, within the SI system.

Four distinct yet interconnected units quantify radioactivity, exposure, absorbed dose, and dose equivalent. The mnemonic R-E-A-D creates a simple way to recall these units, which consist of a combination of commonly used (British, e.g., Ci) and internationally recognized (metric, e.g., Bq) units.1 Below, we will detail the mnemonic and discuss the history of the radiation units along with their relevant scientists:

Radioactivity defines the release of ionizing radiation from a substance. Whether it emits alpha or beta particles, gamma rays, x-rays, or neutrons, the radioactivity of a material is a measure of how many atoms within it decay over a specific period of time. The curie (Ci) and becquerel (Bq) units quantify radioactivity.1


Antoine Henri Becquerel was a French physicist, engineer, and Nobel laureate who discovered evidence of radioactivity. Becquerel’s earliest works centered on the subject of his doctoral thesis: the plane polarization of light, with the phenomenon of phosphorescence and absorption of light by crystals. Early in his career, Becquerel also studied the Earth’s magnetic fields. In 1896, Becquerel discovered evidence of radioactivity while investigating phosphorescent materials such as some uranium salts. For his work in this field, he shared the 1903 Nobel Prize in Physics with Marie Curie and Pierre Curie. The SI unit for radioactivity, becquerel (Bq), is named after him.2

The curie (Ci) unit was created in 1910 by the International Congress of Radiology to measure radioactivity. Pierre Curie, another French physicist, and his wife Marie Curie, who also sat on the committee that named the unit, were the inspirations for the name through their radioactive studies. The original definition of the curie was “the quantity or mass of radium emanation in equilibrium with one gram of radium (element)”.3 In 1975, the becquerel replaced the curie as the official radiation unit in the International System of Units (SI) where 1 Bq = 1 nuclear decay/second4. The relationship between the two units is 1 Ci = 37 GBq (giga becquerels).

Exposure quantifies the extent of radiation going through the atmosphere that reaches a person’s body or a material. Numerous radiation monitors gauge exposure, utilizing the units of roentgen (R) or coulomb/kilogram (C/kg).1

The roentgen is a legacy unit of measurement for the exposure of X-rays and gamma rays. This unit is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air and has the value 2.58 x 10-4 C/(kg air).5 It was named after Wilhelm Roentgen, a German physicist who discovered X-rays and was awarded the first Nobel Prize in Physics for the discovery.

In 1928, the roentgen became the first international measurement quantity for ionizing radiation defined for radiation protection. This is because it was, at the time, the most easily replicated method of measuring air ionization by using ion chambers.6 However, although this was a major step forward in standardizing radiation measurement, the roentgen had a disadvantage: it was only a measure of air ionization rather than a direct measure of radiation absorption in other materials, such as different forms of human tissue. As a result, it did not take into account the type of radiation or the biological effects of the different types of radiation on biological tissue. Consequently, new radiometric units for radiation protection came to be which took these concerns into account.7

The SI unit for measuring exposure to ionizing radiation is coulomb per kilogram (C/kg). Interestingly, unlike other SI radiation units, this unit does not have a specific name. It officially replaced the previous unit, the roentgen, in 1975, with a transition period of at least ten years.8 The SI unit of electric charge, the coulomb, was named in honor of Charles-Augustin de Coulomb in 1880. Charles-Augustin de Coulomb was a French physicist whose best-known work is his formulation of Coulomb’s law. This law states that the force between two electrical charges is proportional to the product of the charges and inversely proportional to the square of the distance between them. He also made important contributions to the fields of electricity, magnetism, applied mechanics, friction studies, and torsion.9

Absorbed dose refers to the quantity of energy that is absorbed by an object or person where the energy is deposited by ionizing radiation as it passes through materials or the body. The units, radiation absorbed dose (rad) and gray (Gy), measure absorbed dose.1

In 1953, the International Commission on Radiation Units and Measurements (ICRU) adopted the unit rad at the Seventh International Congress of Radiology. This was the unit that replaced the rep, roentgen equivalent physical (detailed later in this blog). Although many believe that the rad is an abbreviation of “radiation absorbed dose”, the ICRU never identified it as such. This suggests that the term “rad” was as a standalone word to be a unit for absorbed dose. There was no documented discussion regarding the use of the rad prior to the Seventh International Congress of Radiology. The closest discussion was during the meeting in 1951 when they determined the need for this type of unit. In 1975, the gray (Gy) replaced the rad as the SI unit of absorbed dose where 1 Gy = 100 rad.10

Louis Harold Gray was a 20th century English physicist who worked mainly on the effects of radiation on biological systems. He was one of the earliest contributors to the field of radiobiology. He worked as a hospital physicist at Mount Vernon Hospital in London and developed the Bragg–Gray equation in collaboration with the father and son team of William Henry Bragg and William Lawrence Bragg. Bragg-Gray theory is the basis for the cavity ionization method of measuring energy absorption by materials exposed to ionizing radiation. Gray’s contributions to radiobiology were numerous. Amongst many other achievements, he developed the concept of RBE (Relative Biological Effectiveness) of doses of neutrons and initiated research into cells in hypoxic tumors and hyperbaric oxygen.11 Gray defined a unit of radiation dosage (absorbed dose) which was later named after him as an SI unit, the gray.

Dose equivalent, also known as effective dose, is a measurement that combines the amount of radiation absorbed and the impact it has on the human body. When it comes to beta and gamma radiation, the dose equivalent is equal to the absorbed dose. However, for alpha and neutron radiation, the dose equivalent surpasses the absorbed dose because these types of radiation have a greater biological impact resulting from their increased ability to damage tissue. To quantify dose equivalent, we use the units of roentgen equivalent man (rem) and sievert (Sv).1

Roentgen equivalent man, or “rem”, was first proposed for use in 1945, but under a different abbreviation. The roentgen was the only unit capable of expressing a radiation exposure at the time. However, it fell short being specifically measurable for photons. Workers came into contact with many other forms of radiation such as alpha particles, beta particles, and neutrons, so Herbert Parker, British-American physicist, created units that would be able to gauge exposures to many types of radiation. These were the roentgen equivalent physical (rep) and the roentgen equivalent biological (reb). Due to similarity in pronunciation between rep and reb, reb was eventually renamed to roentgen equivalent man or mammal (rem) to avoid confusion.

The first appearances of the rem unit in scientific literature were not until 1950.10 The rem related to the rad by multiplying the latter by a quality factor (QF) used to account for the varying biological effects of the different types of radiation. The rad in turn may be obtained from the roentgen by multiplying a dose conversion factor. In air, the dose conversion factor relationship between rad and roentgen is 1 R = 0.88 rad. The absorbed dose to a material is in turn found by multiplying 0.88 by the ratio of the mass energy absorption of the material to that of air.12

Rolf Maximilian Sievert was a Swedish medical physicist. He is best known for his work on the biological effects of ionizing radiation and his pioneering role in the measurement of doses of radiation, especially in its use in the diagnosis and treatment of cancer.13 Sievert contributed significantly to medical physics, earning him the title of “Father of Radiation Protection”. The sievert (Sv), the SI unit representing the stochastic health risk of ionizing radiation, is named after him. The sievert officially replaced the rem as the international SI unit in 1979 with 1 Sv = 100 rem.14

Conclusion

In summary, the history of radiation units is a journey that reflects the progress in our understanding of radiation’s properties and its impact on living organisms. The development of these units has enabled safer and more accurate measurement and assessment of radiation exposure and its effects on human health. By understanding the intricate relationship between radiation and our bodies, we can now take proactive measures to mitigate its harmful effects and promote a safer environment for all.

Sources

  1. NRC: Measuring Radiation. Nrc.gov. Published 2017. https://www.nrc.gov/about-nrc/radiation/health-effects/measuring-radiation.html
  2. The Nobel Prize. The Nobel Prize in Physics 1903. NobelPrize.org. Published 2019. https://www.nobelprize.org/prizes/physics/1903/becquerel/biographical/
  3. Curie – Unit of Radioactivity | nuclear-power.com. Nuclear Power. https://www.nuclear-power.com/nuclear-engineering/radiation-protection/units-of-radioactivity/curie-unit-of-radioactivity/
  4. ‌ Bell DJ. Becquerel (SI unit) | Radiology Reference Article | Radiopaedia.org. Radiopaedia. https://radiopaedia.org/articles/becquerel-si-unit
  5. ‌ Bashir U. Roentgen (unit) | Radiology Reference Article | Radiopaedia.org. Radiopaedia. Accessed August 29, 2023. https://radiopaedia.org/articles/roentgen-unit?lang=us
  6. ‌ Roentgen – Unit of Exposure | nuclear-power.com. Nuclear Power. Accessed August 29, 2023. https://www.nuclear-power.com/nuclear-engineering/radiation-protection/radiation-exposure/roentgen-unit-of-exposure/
  7. ‌ Roentgen (unit) explained. everything.explained.today. Accessed August 29, 2023. http://everything.explained.today/Roentgen_(unit)/
  8. ‌ Bell DJ. Coulomb per kilogram | Radiology Reference Article | Radiopaedia.org. Radiopaedia. Accessed August 29, 2023. https://radiopaedia.org/articles/coulomb-per-kilogram
  9. ‌ Laboratory NHMF. Charles-Augustin de Coulomb – Magnet Academy. nationalmaglab.org. https://nationalmaglab.org/magnet-academy/history-of-electricity-magnetism/pioneers/charles-augustin-de-coulomb/
  10. ‌ Why Did They Call It That? The Origin of Selected Radiological and Nuclear Terms. Museum of Radiation and Radioactivity. Accessed August 29, 2023. https://orau.org/health-physics-museum/articles/selected-radiological-nuclear-terms.html#rad
  11. ‌ LH Gray Memorial Trust: About L.H. Gray. www.lhgraytrust.org. Accessed August 29, 2023. http://www.lhgraytrust.org/lhgraybiography.html
  12. ‌Dosimetric Quantities and Units. U.S. NRC. Published October 25, 2010. Accessed September 11, 2023. https://www.nrc.gov/docs/ML1122/ML11229A688.pdf
  13. Aip.org. Published 2023. Accessed August 29, 2023. https://pubs.aip.org/physicstoday/Online/8433/Rolf-Sievert
  14. ‌ Bell DJ. Sievert (SI unit) | Radiology Reference Article | Radiopaedia.org. Radiopaedia. https://radiopaedia.org/articles/sievert-si-unit?lang=us
30 Jun 2023
Diagnostic and Radiation Therapy Examples
Brachytherapy, Cancer Treatment, Diagnostic Physics, Medical Physics, Molecular Imaging, Nuclear Medicine, Radiation Therapy Versant Physics 0 comment

Other Forms of Radiation Therapy and Diagnostic Machines

In our last blog post, we explored the timeline of diagnostic imaging and radiotherapy developed using x-rays. It was the original discovery of x-rays by Röntgen that led to the creation of these medical practices. However, other researchers of the 20th century also invented diagnostic tools and radiation therapy without x-ray involvement at all. We will explore a few of these imaging and radiotherapy machines or procedures in this blog.

Diagnostic Imaging

MRI

One of the most well-known imaging procedures is MRI, which stands for Magnetic Resonance Imaging. Research for MRI began early in the 1970s, but investigation of the magnetic resonance principles started as early as 1945. By happenstance, Felix Bloch, a Swiss physicist working at Stanford, and American physicist, Edward Mills Purcell at Harvard, conducted—nearly simultaneously—an experiment for a new mode of nuclear induction. This led to published papers from both physicists on nuclear magnetic resonance. After publication, both Purcell and Bloch won the 1952 Nobel Prize for this research.1

The process to produce 2D images in an MRI using nuclear magnetic resonance (NMR) was discovered by Paul Lauterbur and Peter Mansfield. Lauterbur published the first nuclear magnetic image in 1973, eventually producing 3D images as well. However, he still had long to go before these images would be practical for application with actual patients. Mansfield continued to expand on Lauterbur’s work, developing the echo-planar imaging technique to improve image quality and scan time in the late 1970s.  By 1977, physician Raymond Damadian created the first full body MRI machine. Through continuous improvements and further research, today’s MRI provides a safe, effective way of capturing images in the brain, portions of the body, the cardiovascular system, and central nervous system. MRIs provide information on size, location, classification, and grade of lesions so as to aid diagnosing stroke, MS, blood supply issues, and any brain damage.2

Ultrasound

Today, ultrasound is most widely known in the OB/GYN field of medicine. Through its invention, however, this diagnostic tool proved its range of utility before becoming a staple in wellness checks during pregnancies. The origin of ultrasonography is often credited to Lazzaro Spallananzi, a physiologist who discovered echolocation throughout 1794 from various experiments involving bats. The principles that serve as a basis for echolocation are also those functioning for medical ultrasound technology today. In 1942, neurologist Karl Dussik first used ultrasonic waves in a diagnostic procedure while attempting to find brain tumors. He later published a report of musculoskeletal ultrasonography in 1958, laying the groundwork for diagnostic musculoskeletal ultrasound.3

Professor Ian Donald from the University of Glasgow was responsible for the development of medical ultrasound in clinical practice. During his research from the late 1950s into the 1960s, there was clinical skepticism about the use of ultrasound. Many doctors felt that manual abdominal and pelvic examinations had proven adequate for medical diagnoses. With his co-workers, Donald performed several studies to show the likelihood of misdiagnoses of a cyst versus a malignant mass. By publishing the findings in 1958, this became a critical point in time for encouraging use of the medical ultrasound. Donald and his colleagues eventually developed an automatic scanner that utilized a full bladder for early pregnancy detection in 1963. This set the path to the now common practice of ultrasounds in the OB/GYN field today.4

PET Scans

Experimental physicist, C.D. Anderson, observed particles from cosmic rays in 1932 that were the same mass as an electron, but which moved in a strong magnetic field along a path opposite to that of an electron indicating a positive charge. He named these particles as “positrons” or positive electrons.

Emission of these positrons was the proof that Irène Joliot-Curie and Frédéric Joliot-Curie used to prove the artificial creation of radioactive elements to win the 1935 Nobel Prize in Chemistry. The creation of these artificial elements was the main focus over the following years due to their significance to military research from the late 1930s until the 1950s. Imaging studies began taking strides as a result of the desire by Louis Sokoloff, an American neuroscientist who hoped to relate mental function to brain function. This encouraged research through the 1950s until the first tomography unit attempt in 1968. This was the turning point in which PET scan development accelerated into the machines we are more familiar with today.5

PET scans can provide imaging through the detection of the gamma ray pairs that are created when a positron annihilates with an electron inside the tissue where the positron emitting isotope has collected. Whereas other diagnostic scans capture the form of an object, PET scans correlate with the metabolic functions of the tissues that uptake the radioactive compound. The compound with the radioactive tracer is typically a form of glucose called FDG (fluorodeoxyglucose), which is used as an energy source by cells and is given to the patient by means of an IV injection. The more rapid the metabolism of the cells in a tissue, as you have with cancer cells, the more uptake of the FDG. This differential uptake is then seen as an enhanced region on a PET scan. 5

Radiation Therapy

Gamma Rays – Gamma Knife (and Cobalt Therapy)

Gamma radiation from radioactive cobalt sources has been harnessed to create an alternative method for producing therapeutic radiation beams. Similar in design to the previously discussed LINAC machines, a Cobalt unit uses a single Co-60 source to produce the radiation beam for treatment. A machine of a fundamentally different design, the Gamma Knife was developed for the treatment of brain tumors. The Gamma Knife is a stereotactic radiosurgery technique that allows for functional neurological surgery. This machine uses anywhere from 192 to 201 C0-60 arrayed in a semi-spherical shield with each source collimated to point their radiation to the same point in space. This type of precision allows for the potential to treat brain tumors and, in turn, pain, movement disorders, and even some behavioral disorders for patients who had not been responsive to other treatment.6

The Gamma Knife was created through the inspirations of Swedish professors Borje Larsson and Larks Leksell in the 1950s. After an investigation using proton beams with stereotactic devices capable of pinpointing targets in the brain, the researchers eventually gave up on that approach due to cost and complexity. Motivated to find an alternative, Larsson and Leksell went on to create a prototype Gamma Knife system in 1967. The prototype unit was a success, used in Sweden for twelve years  and led to a second Gamma Knife in 1975, before other units began appearing globally in the 1980s.6

Proton Therapy

Protons were discovered in the early 1900s. New Zealand physicist Ernest Rutherford’s research during this time led to his discovery of a nuclear reaction that led to the “splitting” of an atom, where he found protons. As a charged particle, the protons have a finite range in matter. The proton’s interaction with matter produces ionization as the proton slows down along its path, losing energy. A peak of deposited dose then occurs at a depth that is proportional to the proton’s original energy.7

The cyclotron, invented by the American physicist Ernest O. Lawrence along with his associates in 1929, proved an efficient way to produce beams of protons. The cyclotron was capable of accelerating protons to high enough energy that, towards the middle of the 20th century, it was suitable for application in cancer treatments.

Dr. Robert Wilson, another American physicist, was the one to spark the idea of proton therapy for battling cancer. He wrote a seminal paper on the functionality of particle beams (comprised of either protons or other heavy-charged particles) and their ability to disperse their energy in the body—the initial release would be small, then exponentially grow when the beam reaches the end of its path for maximum effect against the tumor. Dr. Wilson published the paper in 1946 and received credit to being the inspiration of further research on proton therapy and the continued development of cyclotrons.7

Brachytherapy and Interstitial Therapy

Brachytherapy is an internal radiation therapy technique involving direct placement of radiation inside a patient’s body cavity. This procedure involves small, radioactive source implants that are generally positioned in a body cavity to be near a cancerous tumor. The radioactive material, typically encapsulated within suitable housing material such as titanium, constantly exposes the tumor to a stream of radiation until removal of the radioactivity. These implants will be temporary, and the patient is either hospitalized or kept in a special suite for the duration of the treatment.

Another form of therapy, interstitial therapy (or interstitial brachytherapy), uses sources that are placed directly into the tissue. These may either be temporary or permanent implants of small, encapsulated sources on the order of the size of a rice grain. For treatments with permanent implants, it is safe enough for the patient to go home with a few simple safety guidelines to follow.8

Brachytherapy was first used in 1901 during an attempt to treat lupus. Alexandre Danlos and Paul Bloch completed this treatment with a radioactive sample from Marie Curie. Shortly after, in 1903, Margareth Cleaves used brachytherapy to treat cervical cancer. It became a popular radiotherapy technique to treat breast, cervical, and prostate cancer. As technology advanced along with the use of brachytherapy through the 1900s, imaging-guidance became a valuable asset. More precise dosimetry became possible and allowed for better brachytherapy planning when doctors could use diagnostic modalities such as CTs, MRIs, or ultrasounds. Today, brachytherapy is a more popular treatment for conditions ranging through gynecological, genitourinary, ocular, and head & neck cancers.9

Summary and Conclusion

The 20th century saw tremendous growth and development in diagnostic machinery and radiotherapy. From soundwave technology that gave way to ultrasound, magnetic resonance inspiring MRIs, and the discovery of positrons, medical and non-medical fields can run diagnostic imaging that suits their needs exactly for the most effective information. Radiation therapy research continuously improves the technology and practices used for the wellbeing of patients. Precision instruments such as the Gamma Knife, Cyberknife, and modern LINACs permit for types of treatments that would otherwise be impossible whilst at the same time reducing the side effects associated with external beam therapy. Proton therapy calculations allow for precise travel through the body for maximum effect against cancerous tumors. Brachytherapy provides a treatment option that directly targets tumors without having radiation travel through the body to the target tissue. Modern image guidance ensures source placement exactly where treatment is necessary and that externally directed therapy beams hit their target. As these modalities evolve, we witness the growing reality of safer, more effective diagnoses and cancer treatments within our lifetimes.

The Versant Physics team has experience that covers a range of equipment. This includes dental units, mobile c-arms and Cone-beam CTs, as well as high energy LINACs and even Proton Therapy units and Cyclotrons. To learn more about how our services can help you, contact us to set up a meeting.

Sources

1. NMR Basics | Nuclear Magnetic Resonance Spectroscopy Facility | University of Colorado Boulder. www.colorado.edu. https://www.colorado.edu/lab/nmr/nmr-basics

2. The History of the MRI – DirectMed Parts & Service. https://directmedparts.com/history-of-the-mri/

3. D. Kane and others, A brief history of musculoskeletal ultrasound: ‘From bats and ships to babies and hips’, Rheumatology, Volume 43, Issue 7, July 2004, Pages 931–933, https://doi.org/10.1093/rheumatology/keh004

4. History of Ultrasound – Overview of Sonography History and Discovery. Ultrasoundschoolsinfo.com. Published December 27, 2021. https://www.ultrasoundschoolsinfo.com/history/

5. Henry N. Wagner, A brief history of positron emission tomography (PET), Seminars in Nuclear Medicine, Volume 28, Issue 3, 1998, Pages 213-220, ISSN 0001-2998, https://doi.org/10.1016/S0001-2998(98)80027-

6. History and Technical Overview | Neurosurgery. Neurosurgery. Published 2018. https://med.virginia.edu/neurosurgery/services/gamma-knife/for-physicians/history-and-technical-overview/

7. History of Proton Therapy – NAPT. NAPT. Published 2018. https://www.proton-therapy.org/about/history-of-proton-therapy/

8. Radiation Oncology. Radiation Answers. Accessed May 31, 2023. https://www.radiationanswers.org/radiation-sources-uses/medical-uses/radiation-oncology.html

9. Mayer C, Kumar A. Brachytherapy. PubMed. Published 2021. https://www.ncbi.nlm.nih.gov/books/NBK562190/

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