PURPOSE:The accuracy of dose delivery to all patients treated with medical linacs depends on the accuracy of beam calibration. Dose delivery cannot be any more accurate than this. Given the importance of this, it seems worthwhile taking another look at the expected uncertainty in TG-51 photon dose calibration and a first look at electron calibration. This work builds on the 2014 addendum to TG-51 for photons and adds to it by also considering electrons. In that publication, estimates were made of the uncertainty in the dose calibration. In this paper, we take a deeper look at this important issue. METHODS:The methodology used here is more rigorous than previous determinations as it is based on Monte Carlo simulation of uncertainties. It is assumed that mechanical QA has been performed following TG-142 prior to beam calibration and that there are no uncertainties that exceed the tolerances specified by TG-142. RESULTS/CONCLUSIONS:Despite the different methodology and assumptions, the estimated uncertainty in photon beam calibration is close to that in the addendum. The careful user should be able to easily reach a 95% confidence interval (CI) of ± 2.3% for photon beam calibration with standard instrumentation. For electron beams calibrated with a Farmer chamber, the estimated uncertainties are slightly larger, and the 95% CI is ±2.6% for 6 MeV and slightly smaller than this for 18 MeV. There is no clear energy dependence in these results. It is unlikely that the user will be able to improve on these uncertainties as the dominant factor in the uncertainty resides in the ion chamber dose calibration factor N D , w 60 Co $N_{D,w}^{{}^{60}{\mathrm{Co}}}$ . For both photons and electrons, reduction in the ion chamber depth uncertainty below about 0.5 mm and SSD uncertainty below 1 mm have almost no effect on the total dose uncertainty, as uncertainties beyond the user's control totally dominate under these circumstances.

Electron beams are often used to treat superficial lesions of the lip, cheek, nose, and ear. Lead is frequently used to block distal structures. It is customary to place an internal bolus of low atomic number in between the tissue and the lead to reduce electron backscatter from the lead. Space for the lead and the internal bolus is quite limited. A previous method for estimating the thickness of the lead plus internal bolus is not self-consistent and leads to a larger than necessary thickness. A new method is described here to provide a quick, accurate, and self-consistent estimate of the minimum necessary thickness of the internal bolus and the lead for incident electron beam energies of 4, 6, 8, 9, and 10 MeV as a function of the thickness of the overlying tissue. This method limits the dose enhancement at the tissue/bolus interface due to the underlying lead to 10%. Measurements made with gafchromic film validate this methodology.

Recent publications have called into question the accuracy of reference tenth-value layer (TVL) data cited in official reports for linac primary concrete barriers. Doubts have arisen based on both experimental and theoretical evidence. Most of the standard reference TVL values trace back to a publication that appeared in 1984 that used beam spectra that are not representative of modern linacs. This study reports a new set of TVL data for concrete based on modern linac beam spectra and a definition of the barrier transmission that is consistent with its use in shielding calculations. TVL values have been computed for concrete using Monte Carlo simulation for beam energies of 4, 6, 10, 15, and 18 MV. The barrier transmission depends on the field size at the barrier and the distance from the distal surface of the barrier to the point of observation. The TVL values reported here lead to barrier transmission values that are up to a factor of 4 larger than those in official reports. The air kerma rate beyond the barrier does not obey an inverse square law as the barrier now acts like a new (non-point) source of radiation. For distance greater than 0.3 m from the distal side of the barrier, inverse square predictions of the air kerma rate are low by up to a factor of 2. The average energy of the transmitted photons declines rapidly for all beam energies with increasing barrier thickness up to a thickness of about 50 cm and then slowly increases with increasing thickness.

There is widespread consensus in the literature that flattening filter free (FFF) beams have a lower primary barrier transmission than flattened beams. Measurements presented here, however, show that for energy compensated FFF beams, the barrier transmission can be as much as 70% higher than for flattened beams. The ratio of the FFF barrier transmission to the flattened beam barrier transmission increases with increasing barrier thickness. The use of published FFF TVL data for energy compensated FFF beams could lead to an order of magnitude underestimate of the air kerma rate. There are little data in the literature on the field size dependence of the barrier transmission for flattened beams. Barrier transmission depends on the field size at the barrier, not at isocenter Measurements are presented showing the relative dependence of barrier transmission on the field size, measured at the barrier, for 6 MV and 10 MV beams. An analytical fitting formula is provided for the field size dependence. For field sizes greater than about 150 cm in side length, the field size dependence is minimal. For field sizes less than about 100 cm, the transmission declines rapidly as the field size decreases.

Radiation Oncology is a highly multidisciplinary medical specialty, drawing significantly from three scientific disciplines—medicine, physics, and biology. As a result, discussion of controversies or changes in practice within radiation oncology involves input from all three disciplines. As a result, we have adopted this “team-science” approach to the traditional debates featured in this journal. This article is part of a series of special debates entitled “Three Discipline Collaborative Radiation Therapy (3DCRT)” in which each debate team has included three multidisciplinary team members, with the hope that this format would be both engaging for the readership and foster further collaboration in the science and clinical practice of radiation oncology. Previous 3DCRT debates have included a radiation oncologist, medical physicist, and radiobiologist on each team. For this debate, we break that trend and include a seasoned radiation oncologist, an early career radiation oncologist, and a medical physicist on each team. We hope these perspectives add valuable insight to this particular debate. Physics is one of the fundamental scientific pillars of radiation oncology. Its principles form the foundation for everything from the creation of the radiation we use, to how it interacts in the patient, to how we create and deliver our treatments. As such, it represents one of the core didactic elements of radiation oncology residency training. However, radiation oncology has undergone a staggering increase in technological complexity over the past few decades and our medical residents must be trained to understand and apply these new advances. So how do we add new content to our training curriculum without eliminating or condensing existing content? Medical physics didactic training could be expanded, but this would encroach on other important aspects of training, and these are also expanding. As expressed by Vapiwala in a recent editorial, “keeping up with our field, and oncology in general, is frankly overwhelming”.1 If we were instead to eliminate or condense some component of our medical physics didactic curriculum to make room for these new additions, what component would it be? Is it time to retire the most basic and timeworn content? Here we will arbitrarily define “basic fundamental physics” to include the explicit constituents of the “Basic Physics” section of the radiation oncology physics examination blueprint from the American Board of Radiology—“fundamental physics, atomic and nuclear structure, production of kV x-ray beams, production of MV x-ray beams, and radiation interactions”. Some would support eliminating this content, arguing that basic fundamental physics is the least “clinically relevant” component and minimizing it would create space for training on current clinical applications of medical physics. After all, when was the last time the factors that determine nuclear stability came up when discussing a patient's treatment in chart rounds? Others would argue that basic fundamental physics is the most important element since it underlies not only all current applications but all new applications that will be developed after the resident has completed training. In other words, “teach your residents about current technology and they will practice effectively for the day, but teach them fundamental physics and they will practice effectively with technological advances for life”. As we continue to expand our training curriculum to produce the most effective future practitioners of radiation oncology, it seems that something may have to give. Should it be basic fundamental physics? This is the subject of this edition of the 3DCRT debate. Arguing for the proposition will be Drs. Sushil Beriwal, Jeffrey Ryckman, and Dandan Zheng. Dr. Sushil Beriwal worked as a radiation oncologist at UPMC from 2004 to 2021 in various roles, including Residency Program Director, Director of Brachytherapy, and Deputy Director for the UPMC Hillman Cancer Center Network. He has published more than 350 peer-reviewed articles and book chapters and has received the ABS president award, ARRO teacher of the year, and fellowship from ABS and ASTRO. He is currently Professor of Radiation Oncology at Drexel University, Academic Chief for the Allegheny Health Network, and VP of Medical Affairs for Varian Medical Systems. Dr. Jeff Ryckman is an Assistant Professor at West Virginia University, where he practices as a generalist. He is the chief editor of Rad Onc Review, a free, comprehensive textbook including all aspects of oncology, and the chief content creator for Rad Onc Calc, a free mobile and web application about treating safely. He is also interested in global standardization as evidenced by his involvement with TG-263U1 working group. Dr. Dandan Zheng is the Director of Medical Physics and Professor in the Department of Radiation Oncology at the University of Rochester. Aside from her involvement in Medical Physics trainee education, she has also been teaching the physics course throughout her career to Radiation Oncology residents at three different universities. Arguing against the proposition will be Drs. Kelsey L. Corrigan, Patrick N. McDermott, and May N. Tsao. Dr. Kelsey L. Corrigan is a PGY-5 in radiation oncology at UT MD Anderson Cancer Center who recently passed the American Board of Radiology Physics Board Examination for radiation oncology residents. Dr. Patrick N. McDermott is the Director of Physics Education at Corewell Health (formerly Beaumont Health). He is the coauthor of the textbook “The Physics and Technology of Radiation Therapy,” which was written explicitly for radiation oncology residents. He has been teaching medical residents for 25 years. Dr. May N. Tsao is the Vice Chair of Education and Associate Professor in the Department of Radiation Oncology at the University of Toronto. She, along with her physics colleague Steve Babic, coordinate the yearly applied physics course for the University of Toronto Radiation Oncology and Radiation Physics residents. If the medical physics course is like a beautiful and intricate garden with carefully arranged and well-balanced flowers to create a harmonious landscape, adding new flowers without removing any old ones would eventually overwhelm and destroy the garden with the overcrowded flowers competing for space, sunlight, and nutrients. Let's take a look at the new flowers in our field. Over the past few decades, technologies such as IMRT,2 SRS,3 SGRT,4 and particle therapy5 have revolutionized our clinical practice landscape. More recently, artificial intelligence (AI) clinical tools are being developed and implemented at an explosive speed,6 and new large language model AIs are poised to multiply these efforts even further.7 At the same time, our treatment paradigm is rapidly shifting from “what you see on paper is what the patient receives” to evaluating the actual delivery and exploring online adaptive RT8 and real-time motion adaptation.9 Innovations like Cherenkov imaging10 and PET-Linac11 promise real-time in-vivo dosimetry and biologically guided RT. Research directions such as FLASH12 and PULSAR13 challenge conventional biological and physics models and open new doors. It would be a disservice to the future of our field not to expose radiation oncology residents to these developments or new directions or to prepare them to further these and other endeavors. To create space for the new additions to flourish and prevent our garden from collapsing under its own weight, we must carefully select and remove less relevant flowers. In this debate, we maintain that basic fundamental physics should be removed. Radiation oncology is a team sport. Medical physics is the domain of the physicist, and radiation oncologists are never without a physicist! Dr. Ryckman (who is on our team) is an MD with a master's degree in Medical Physics, but he confesses that he has never once had to correct his physicist on fundamental physics; instead, he relies on them for those types of questions. If every department has a medical physicist, is it necessary for the physician to “know it all” or just the part that directly influences our practice? If our multidisciplinary practice of radiation oncology is like a Venn diagram, do we have to know the details of every circle or only our own and the parts where our specialties overlap? This is particularly of concern, as the circles keep getting bigger and bigger! Despite the human nature of wanting to be omniscient, the “know it all” expert is unattainable, especially in modern times, as the body of knowledge has evolved so far and become so complex. Today's radiation oncology practice and innovation require a high level of technical expertise and knowledge of complex treatment planning software, advanced imaging modalities, and computer and data science, resulting in a shift towards technology-driven approaches in radiation oncology. We believe that the field has become so technologically complex that it is no longer necessary for radiation oncology residents to focus their studies on fundamental physics. Instead, residents need a high-level understanding of many topics and familiarity with advanced tools and applications for radiation oncology's sustained and expanded future. It is true that physics principles are essential and that a lack of understanding may lead to errors. But the burden of applying fundamental physics for an effective and safe radiation oncology clinical operation rests upon medical physicists instead of radiation oncologists. As fundamental physics rarely comes up in radiation oncologists' clinical discussions with patients, among peers, or with multidisciplinary clinicians, a requirement for its mastery seems very cost-ineffective in their training. If a fundamental physics question arises, it can be effectively referred to the physician's physicist colleague. For residents, it is more important to understand the limitations of technology than basic physics details. However, important practical training is lacking because currently there is too much emphasis on basic physics in the physics training and testing. Another critical mission for radiation oncologists is to innovate our field. These innovations will likely come from effective collaborations with medical physicists, biologists, and other experts from diverse disciplines, instead of from radiation oncologists that “know it all”. For residents, the time spent on learning fundamental physics concepts could be better spent on hands-on training with sophisticated treatment planning and imaging systems, learning new technology and critical topics, clinical research, quality improvement initiatives, and interprofessional collaboration. This way, the residents catch up with the rapidly evolving field and are prepared to propel the next wave of innovation. Even NIH grants are moving increasingly towards multiple-PI grants than single-PI grants because, in this day and age, innovations come more from collaborations between experts from different domains than from a single lab or discipline. In conclusion, subtracting contents is not a sign of weakness or failure but a strategic decision to maintain the integrity and sustainability of the training. Removing basic fundamental physics from the modern curriculum for radiation oncology residents ensures that the remaining important concepts have room to blossom and provide a clear and meaningful path for the residents' learning journey. By omitting the mastery requirement for radiation oncology residents on basic fundamental physics, we can curate a garden of physics knowledge that continues to inspire and engage while avoiding the risk of overwhelming the learners and compromising the overall effectiveness of the training. Aristotle defined a first principle as “the first basis from which a thing is known”.14 These basic fundamentals drive technological advances and are critical to the mastery of any field. As such, having a solid knowledge of basic fundamental physics is critical to the complete understanding of therapeutic radiation. There is a prima facia case against the proposition in this debate. The question should not be whether basic fundamental physics should be taught but what basic physics should be taught. The “staggering increase in technological complexity” argues for an increased emphasis on basic radiation physics. The technology of today will be gone tomorrow. What will remain, however, is basic radiation physics. Although basic physics is not always obviously applicable in the clinic, it helps the physician understand what we are doing and how we are accomplishing it. These basic fundamental principles will not only guide present practice but will also help with problem solving. Additionally, there are still many clinical scenarios which are based on simple beam planning (e.g., direct orthovoltage, superficial x-rays, or direct electrons for certain skin cancers). These clinical scenarios require a fundamental knowledge of the differences between these simple beam modalities (e.g., shielding differences, use of bolus or not, how the beam is produced, superficial dose in bone etc.). If we eliminate the requirement to understand basic physics, physicians may lose this fundamental knowledge and could become little more than technicians. The fundamental building-blocks of any new technology are basic science and mathematics, and understanding basic physics will help in adopting and understanding more complex radiation technologies. For example, it is only through insight of single photon beam fundamental physics that allows for understanding of multi-beam arrangements, IMRT, VMAT, SBRT, SRS and how these are produced by specialized radiation machines. The understanding of charged particle physics provides crucial insight into the various proton and heavy ion particle therapy (e.g., helium or carbon) as compared to photons. It is very difficult to say for certain where technology will take us in the future, but basic fundamental physics is the most important element since it underlies not only all current applications but all future applications. As long as radiation remains a therapeutic modality, practitioners will need to know about radiation. The study of the interaction of radiation with matter is a basic pre-requisite for radiobiology, which is the basis for the profession of radiation oncology. How can you treat patients with radiation if you don't know what radiation is? Radiation oncologists are writing prescriptions for absorbed radiation dose and are liable for these prescriptions. They should understand what this means. Furthermore, this is applicable to radiation oncologists who practice both in academic medical centers and in the community. In academic medical centers, many research projects in radiation oncology require a sophisticated knowledge of physics. In community treatment centers, radiation oncologists play a vital role for radiation protection and often serve on hospital radiation safety committees. All of these responsibilities require an understanding of radiation and, consequently, the basic physics behind radiation therapy. “I don't know what's the matter with people: they don't learn by understanding; they learn by some other way—by rote or something. Their knowledge is so fragile!” In the opening statement, our opponents quoted Feynman to support their point. It is worth noting that the “Feynman Technique” prioritizes learning through simplicity to build depth of understanding. One might argue that radiation oncology has some of the highest usages of complexity and jargon of any specialty, and our specialty would greatly benefit from teaching physics through the Feynman Technique. Also, given the misinformation regarding radiotherapy in the media, it would help us to train residents who can convey these complex concepts to the general public. Indeed, the fundamental medical physics knowledge needed for residents to safely and effectively practice is straightforward. Still, high-level radiation physics dominates the first half of dedicated oncology training for busy radiation oncology residents, and mastery is expected. Fundamental physics displaces other vital topics early in training. The historical era was largely dominated by conventional fractionation and substantially less clinical trial data, but in the modern era, the portion of conventional fractionation keeps getting smaller. We must ask ourselves if we are maintaining all aspects of basic fundamental physics in the curriculum, that it is not as a “rite of passage,” but to prioritize deep understanding of the concepts that drive clinical care on a day-to-day basis, especially in the context of all of the other vital information radiation oncology residents are expected to know in the modern era. For instance, one of the most important goals of residency should be to have a deep understanding of plan quality and evaluation so that residents can drive the modernization of clinical practice once they become attending physicians. Still, plan technique/evaluation is often not prioritized until PGY-4, when the radiation physics board examination has been passed. Radiation oncology education should focus on mastering advanced tools and principles of radiotherapy planning and advanced image utilization rather than too much emphasis on rote memorization of equations and performing hand calculations, which radiation oncologists hardly use in clinical life. A recent Twitter poll of over 150 radiation oncology participants acknowledged nearly 90% of participants had not done hand calculations in the previous year.15 However, rote memorization and hand calculations represent a substantial portion of the resident physics course and board exam. Instead of rote memorization, the focus should be on ensuring residents know where to find the equations and how to perform the calculations in the clinical setting appropriately. The current training introduces fragility into our knowledge base by expecting mastery of basic fundamental physics instead of encouraging understanding of concepts important in day-to-day practice. Our opponents argue that “it is a false dichotomy to insist that we must choose to teach either basic physics or new technology—we should continue to teach both”. In an ideological world, yes, but “know-it-all” experts do not exist, or at least not on a mass professional training level. Instead, we have a multidisciplinary team that works together, where we always have medical physicists who we can rely on for solid knowledge of basic physics. In our opponents’ example of “radiation oncologists play a vital role for radiation protection and often serve on hospital radiation safety committees”, their role on these committees does not require solid knowledge of basic physics-that is the role of the radiation safety officer and the medical physicist on the committee. And that is precisely why we appoint a committee rather than a single person! In the realistic world where time constraints limit the luxury of comprehensive and deep understanding in every subject, radiation oncologists’ role is best played with mastery in their domain, familiarity with modern radiation technology, knowledge of new and future directions, and critical thinking to know where/from whom and what information to locate and also how to interpret and apply that to practice in a multidisciplinary manner. Fundamentals will always remain relevant. Technical trends will come and go. The simplistic garden analogy fails to recognize that the most beautiful garden relies on older plant species with established roots. The point is to control weeds and sow new seeds, but not to uproot established majestic trees in the process. We are surprised that our esteemed colleagues state that “basic fundamental physics should be removed”. Radiation oncology is an interdisciplinary specialty, and it is important for each member of the team to have some knowledge of all facets. To become a competent and thoughtful radiation oncologist requires an expert level of understanding of the tools of our trade, namely fundamental physics, such as atomic and nuclear structure, bremsstrahlung x-ray production, and radiation interactions. Despite the emergence of new technologies, what has not changed is the basic physics knowledge that is required to understand these technologies. Moreover, these technological changes may be even more confusing for patients. It is the job of the radiation oncologist to not only safely treat patients with these new technologies, but also to explain to patients what is about to happen to their bodies. This is exemplified by these patient questions that are commonly asked during appointments: “How does radiation work?” “What kind of radiation am I receiving?” “What is the difference between external and internal radiation that you are planning for me for my cervical cancer? Why do I need both?” “Why is my skin cancer being treated with this machine, whereas my cousin's skin cancer was treated with a different machine down the hall?” “What is the blue light that I see while I am getting treated?” What happens to the confidence and reassurance regarding treatment safety that patients impart on their radiation oncologists if their treating radiation oncologist replies: “Sorry I don't know, the field has become so technologically complex, I was not taught nor did I learn that. Let me get my physics colleague to answer all your questions!” How can you participate in the delivery of “new technology” particle therapy if you do not understand the basic physics of the interactions of protons or heavy ions with matter, and cannot effectively explain this to patients? Our opponents claim that there has been “a shift towards technology-driven approaches in radiation oncology”. We disagree, technology has always been there from the very beginning, but it has certainly changed! What has not changed is the basic physics knowledge that is required to understand these changes. To solve the dilemma of adding more information to the physics curriculum, we could remove information regarding old technologies, such as Co-60 teletherapy units, silver halide radiographic film, conventional simulators, cast blocks, or tissue compensators, which are not used anymore. In other words, remove the “weeds” in the garden analogy. We could also consider removing the requirement to memorize content that could be simply looked up, such as equations, conversion factors, and particle masses. Instead, this information could be provided for residents during their board examination, which would leave space open for new content covering new technologies. This testing method is already commonly done in graduate physics and engineering classes to test students on how they use knowledge instead of unnecessary rote memorization. These examples are logical and feasible strategies for creating space in curricula to introduce new concepts. In conclusion, we must keep the coverage of relevant basic physics in radiation oncology resident curriculums so that physicians will understand new technology when it comes along, as it surely will. In our garden analogy: new weeds will need to be removed continually, but the foundations on which these new plants are developed will remain the same. “Teach your residents about technological advances and they will practice effectively for the day, but teach them basic physics and they will practice effectively for life”. Can't argue with that! All authors were responsible for preparation of arguments, and writing and reviewing the manuscript. The authors have nothing to report. The authors declare no conflicts of interest.

AbstractIt has been shown that a widely quoted formula for estimating medical linac photon skyshine equivalent doses is erroneous. Monte Carlo calculations have been performed to develop an easy method for quickly and accurately estimating skyshine radiation levels and to gain improved physical insight into the skyshine phenomenon. Calculations of linac photon skyshine have been performed for 4, 6, 10, 15, and 18 MV beams for 10 × 10 cm2 and 40 × 40 cm2 fields and for a range of room dimensions and roof thicknesses. The effect of flattening filter free beams has been considered. Air kerma rates (AKRs) can be accurately fitted to a simple algebraic formula that is a function of the horizontal distance from the isocenter with a single energy dependent fitting parameter. The AKR, at a height of 1.3 m above level ground, reaches a local maximum at a distance dmax = 1.5dw + 1.1h, where dw is the horizontal distance from the isocenter to the outside of the side wall, and h is the vertical distance from the isocenter to the top of the roof. For thin roofs, low energy beams lead to significantly more skyshine than high energy beams because low energy photons are more easily scattered through large angles. In the absence of a roof, the maximum skyshine dose rate is on the order of 8 × 10−7 times the dose rate at isocenter. The average energy of the skyshine photons is about 0.15 MeV, and it is remarkably independent of almost all parameters. A simple methodology is outlined for the evaluation of photon skyshine.

Primary barrier design for linac shielding depends very sensitively on tenth value layer (TVL) data. Inaccuracies can lead to large discrepancies between measured and calculated values of the barrier transmission. Values of the TVL for concrete quoted in several widely used standard references are substantially different than those calculated more recently. The older standard TVL data predict significantly lower radiation levels outside primary barriers than the more recently calculated values under some circumstances. The difference increases with increasing barrier thickness and energy, and it can be as large as a factor of 4 for 18 MV and concrete thickness of 200 cm. This may be due to significant differences in the beam spectra between the earlier and the more recent calculations. Measured instantaneous air kerma rates sometimes show large variations for the same energy and thickness. This may be due to confounding factors such as extra material on, or inside the barrier, variable field size at the barrier, density of concrete, and distal distance from the barrier surface. In some cases, the older TVL data significantly underestimate measured instantaneous air kerma rates, by up to a factor of 3, even when confounding factors are taken into account. This could lead to the necessity for expensive remediation. The more recent TVL values tend to overestimate the measured instantaneous dose rates. Reference TVL data should be computed in a manner that is mathematically consistent with their use in the calculation of air kerma rate outside barriers directly from the linac "dose" rate in MU/min.

A widely used formula for the prediction of photon skyshine has been shown to be very inaccurate by comparison with numerous measurements. Discrepancies of up to an order of magnitude have been observed. In addition to this, the formula does not predict the observed dependence on field size, nor the fact that skyshine dose rates exhibit a local maximum. A scaling formula is derived here, with a single fitting parameter, which properly accounts for these properties, provides physical insight into the skyshine phenomenon, and is more accurate. The location of the maximum dose rate depends on the ratio of the roof height above isocenter to the distance from the isocenter to the outer surface of the sidewall. For nominal linac room dimensions, the maximum dose occurs at a distance from the outer wall of approximately two times the height of the roof above the isocenter. The skyshine dose rate is proportional to the field area and not Ω1.3 , as predicted by the standard formula, where Ω is the solid angle subtended by the beam. For lightly shielded roofs (concrete thickness less than about 0.5 m), the photon skyshine for 6 MV exceeds that for 18 MV. Evidence is presented that at intermediate distances the skyshine declines as one over the distance and not one over the distance squared. Predictions of skyshine dose rates depend critically on accurate knowledge of the roof transmission factor. If a roof is shielded so as to avoid designation as a "high radiation area," photon skyshine will be negligible.

The biologically relevant depth for acute skin reactions in radiotherapy is 70 µm. The dose at this depth is difficult to measure or calculate and can be quite different than the dose at a depth of as little as 1 mm. For breast radiotherapy with medial and lateral tangential beams, the skin dose depends on both the contribution from the entrance beam and the exit beam. The skin dose has been estimated in a breast model hemi-ellipse accounting for field size, beam energy, obliquity, lack of backscatter, fractionation, size and shape of the hemi-ellipse. The dose has been held constant along the axis of symmetry of the hemi-ellipse by introducing modulation as in clinical IMRT practice. Dose distributions have been computed as a function of the polar angle from the center of the hemi-ellipse. The exit dose always dominates the entrance dose for all realistic parameters. As a result, the surface dose is higher for 18 MV than 6 MV over the entire surface for all reasonable sizes and shapes of the hemi-ellipse. The results of these calculations suggest that substituting an 18 MV beam for a 6 MV beam to achieve greater skin sparing may have just the opposite effect. The ratio of the surface dose to the mid-depth dose ranges from about 35% at polar angle 0o to up to 70% at polar angle 80o. The dose rises sharply at angles above 30o. The surface dose rises moderately at all angles as the size of the hemi-ellipse increases. The effect of shape is somewhat complex: as the breast becomes flatter, doses at intermediate angles increase, but doses at small and large angles decrease. The biologically effective dose for erythema and moist desquamation is about 2 to 3 Gy higher at all polar angles for conventional fractionation (2.00 Gy × 25 fractions) than for hypofractionation (2.66 Gy × 16).

It's no wonder more and more colleges are adopting The Physics & Technology of Radiation Therapy for their radiology and medical physics programs. Radiation therapy is a difficult subject to understand and teach, so it helps to have a book written by two renowned experts who have explained the field's tough concepts to students for many years with the perfect mix of depth, insight, and humor. The critics agree. From Thomas Lowinger in IOMP Journal... " This is an excellent book; the presentation of the book diagrams, figures, pictures (many in color), and selection of problems are clear and logical and make this book a classic. It is evident that the authors have taught this subject for a long time and were able to distill and explain concepts in a clear and interesting way. The authors have written a spellbinding textbook that belongs on the bookshelf of every medical physicist, both as a reference and as guide. …it surely belongs in the pantheon of great medical physics textbooks. I wish I had this book when I was a student." From Joseph P. Driewer in JACMP... " The 2nd edition of The Physics & Technology of Radiation Therapy by McDermott and Orton is an accessible textbook for radiation therapists, medical dosimetrists, and radiation oncology residents. It could be a great primary text for a first course in radiation therapy physics as well as a great study resource for board exams in these areas." The 2nd edition has been guided by the 2018 ASTRO core curriculum for radiation oncology residents. Novice physicists will find the book useful when studying for board exams, with helpful chapter summaries, appendices, and extra end-of-chapter problems and questions. It features new material on digital x-ray imaging, neutron survey meters, flattening-filter free and x-band linacs, biological dose indices, electronic brachytherapy, OSLD, Cerenkov radiation, FMEA, total body irradiation, proton therapy, and more. Also included: Updated graphics in full color for increased understanding. Appendices on board certifications in radiation therapy for ABR, AART, and Medical Dosimetrist Certification Board. Dosimetry Data A full index

Accelerating Waveguides; Bending Magnets; Sources of Microwave Power; The Treatment Head; The Invention of the Cavity Magnetron

Purpose: The American Society for Radiation Oncology (ASTRO) Physics Core Curriculum Subcommittee (PCCSC) has updated the recommended physics curriculum for radiation oncology resident education to improve consistency in teaching, intensity, and subject matter.Methods and Materials: The ASTRO PCCSC is composed of physicists and physicians involved in radiation oncology residency education. The PCCSC updated existing sections within the curriculum, created new sections, and attempted to provide additional clinical context to the curricular material through creation of practical clinical experiences. Finally, we reviewed the American Board of Radiology (ABR) blueprint of examination topics for correlation with this curriculum.Results: The new curriculum represents 56 hours of resident physics didactic education, including a 4-hour initial orientation. The committee recommends completion of this curriculum at least twice to assure both timely presentation of material and re-emphasis after clinical experience. In addition, practical clinical physics and treatment planning modules were created as a supplement to the didactic training. Major changes to the curriculum include addition of Fundamental Physics, Stereotactic Radiosurgery/Stereotactic Body Radiation Therapy, and Safety and Incidents sections, and elimination of the Radiopharmaceutical Physics and Dosimetry and Hyperthermia sections. Simulation and Treatment Verification and optional Research and Development in Radiation Oncology sections were also added. A feedback loop was established with the ABR to help assure that the physics component of the ABR radiation oncology initial certification examination remains consistent with this curriculum.Conclusions: The ASTRO physics core curriculum for radiation oncology residents has been updated in an effort to identify the most important physics topics for preparing residents for careers in radiation oncology, to reflect changes in technology and practice since the publication of previous recommended curricula, and to provide practical training modules in clinical radiation oncology physics and treatment planning. The PCCSC is committed to keeping the curriculum current and consistent with the ABR examination blueprint. (C) 2016 The Authors. Published by Elsevier Inc. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

To the Editor: We thank Dan Odero for his review of our book The Physics and Technology of Radiation Therapy in the June 2011 issue ( 1 Odero D. Book review: The physics and technology of radiation therapy. Int J Radiat Oncol Biol Phys. 2011; 80: 637 Abstract Full Text Full Text PDF Google Scholar ). When reviewing an 850-page book, it is usually not practical to read the entire volume. We believe that this has led to some misunderstandings. We have been criticized for leaving some things out of the book that are, in fact, in the book. Regarding the reference to the “Mevalac” linear accelerator (linac), we are criticized because “there exists no such linac or manufacturer.” The book clearly and repeatedly states (on Pages 12.5, 15.2, 15.5, and 15.23, as well as in the index [Page I.10]) that this is a fictitious linac. We give detailed dosimetry data for this fictitious machine in Appendix C. The title page to the appendix again states that this is a fictitious linac. It is stated that radiochromic dosimetry is not mentioned, yet this topic is discussed on Page 8.33. Regarding the use of thermoluminescent dosimeters for “external agency machine output verification,” if this is referring to the RPC mailed dosimetry program, the RPC no longer uses thermoluminescent dosimeters. This is discussed in Chapter 11 (Page 11.13). Dr. Odero states that the physician reader needs to understand the importance of positron decay and electron capture, particularly for 125I and 103Pd. The decay of 125I is discussed on Pages 3.33, 16.5, and 16.41. The decay of 103Pd is discussed on Pages 16.7 and 16.41. The Physics and Technology of Radiation TherapyInternational Journal of Radiation Oncology, Biology, PhysicsVol. 80Issue 2PreviewThis introduction to the physics and technology of radiation therapy developed from years of classroom lecture notes taken by radiation oncology residents is designed to take the reader from elementary mathematic concepts through the basic physics of radiation to applications in clinical medical physics. It presents a wide range of information in a simple yet concise manner, omitting detailed explanations for clarity but using clinical examples for emphasis. Full-Text PDF Letter to Editor on “Comment on ‘The Physics and Technology of Radiation Therapy’ Book Review”International Journal of Radiation Oncology, Biology, PhysicsVol. 82Issue 2PreviewTo the Editor: I thank Drs. McDermott and Orton for their comment to the editor regarding my review of their book. I concede that the text alerted the reader that the “Mevalac” is a fictitious machine, although had the authors named the fictitious machine “LINAC,” it would have been clearer. The authors have correctly pointed out that the decay processes are discussed in Chapter 3, the inclusion of which, in my review, I commended for the physician reader. Full-Text PDF

Purpose: In 2004, the American Society for Radiation Oncology (ASTRO) published its first physics education curriculum for residents, which was updated in 2007. A committee composed of physicists and physicians from various residency program teaching institutions was reconvened again to update the curriculum in 2009.Methods and Materials: Members of this committee have associations with ASTRO, the American Association of Physicists in Medicine, the Association of Residents in Radiation Oncology, the American Board of Radiology (ABR), and the American College of Radiology. Members reviewed and updated assigned subjects from the last curriculum. The updated curriculum was carefully reviewed by a representative from the ABR and other physics and clinical experts.Results: The new curriculum resulted in a recommended 56-h course, excluding initial orientation. Learning objectives are provided for each subject area, and a detailed outline of material to be covered is given for each lecture hour. Some recent changes in the curriculum include the addition of Radiation Incidents and Bioterrorism Response Training as a subject and updates that reflect new treatment techniques and modalities in a number of core subjects. The new curriculum was approved by the ASTRO board in April 2010. We anticipate that physicists will use this curriculum for structuring their teaching programs, and subsequently the ABR will adopt this educational program for its written examination. Currently, the American College of Radiology uses the ASTRO curriculum for their training examination topics. In addition to the curriculum, the committee updated suggested references and the glossary.Conclusions: The ASTRO physics education curriculum for radiation oncology residents has been updated. To ensure continued commitment to a current and relevant curriculum, the subject matter will be updated again in 2 years. (C) 2011 Elsevier Inc.

Breast radiotherapy is associated with an increased risk of contralateral breast cancer (CBC) in women under age 45 at the time of treatment. This risk increases with increasing absorbed dose to the contralateral breast. The use of intensity modulated radiotherapy (IMRT) is expected to substantially reduce the dose to the contralateral breast by eliminating scattered radiation from physical beam modifiers. The absorbed dose to the contralateral breast was measured for 5 common radiotherapy techniques, including paired 15 degrees wedges, lateral 30 degrees wedge only, custom-designed physical compensators, aperture based (field-within-field) IMRT with segments chosen by the planner, and inverse planned IMRT with segments chosen by a leaf sequencing algorithm after dose volume histogram (DVH)-based fluence map optimization. Further reduction in contralateral breast dose through the use of lead shielding was also investigated. While shielding was observed to have the most profound impact on surface dose, the radiotherapy technique proved to be most important in determining internal dose. Paired wedges or compensators result in the highest contralateral breast doses (nearly 10% of the prescription dose on the medial surface), while use of IMRT or removal of the medial wedge results in significantly lower doses. Aperture-based IMRT results in the lowest internal doses, primarily due to the decrease in the number of monitor units required and the associated reduction in leakage dose. The use of aperture-based IMRT reduced the average dose to the contralateral breast by greater than 50% in comparison to wedges or compensators. Combined use of IMRT and 1/8-inch-thick lead shielding reduced the dose to the interior and surface of the contralateral breast by roughly 60% and 85%, respectively. This reduction may warrant the use of IMRT for younger patients who have a statistically significant risk of contralateral breast cancer associated with breast radiotherapy.

Shielding calculations for gamma stereotactic radiosurgery units are complicated by the fact that the radiation is highly anisotropic. Shielding design for these devices is unique. Although manufacturers will answer questions about the data that they provide for shielding evaluation, they will not perform calculations for customers. More than 237 such units are now installed in centers worldwide. Centers installing a gamma radiosurgery unit find themselves in the position of having to either invent or reinvent a method for performing shielding design. This paper introduces a rigorous and conservative method for barrier design for gamma stereotactic radiosurgery treatment rooms. This method should be useful to centers planning either to install a new unit or to replace an existing unit. The method described here is consistent with the principles outlined in Report No. 151 from the U.S. National Council on Radiation Protection and Measurements. In as little as 1 hour, a simple electronic spreadsheet can be set up, which will provide radiation levels on planes parallel to the barriers and 0.3 m outside the barriers. PACS numbers: 87.53.Ly, 87.56By, 87.52Tr

Purpose: The leaf span for a Varian MLC is 15 cm. Fields larger than this in the leaf motion direction must be split into multiple ports with a “carriage shift” between ports. Leaf position is a function of both carriage position and leaf position within the carriage. The carriage position is dictated by the outermost leaf position, and the carriages do not move during radiation delivery. We have developed a test to assess carriage positioning accuracy by comparing the same abutting MLC-shaped fields both with and without carriage motion between the delivery of the two fields. Methods and Materials: A 14 cm wide × 40 cm long field was split into two 7 cm wide rectangular ports shaped by the MLC. Width is defined in the leaf motion direction. These two fields were delivered using two separate static MLC files, then using a single DMLC file. In the former case, the carriages move while the leaves remain stationary with respect to the carriages. In the latter case, the carriages remain stationary while the leaves move with respect to the carriages. Kodak XV film was taped to the collimator face for both cases and compared. Results: Both films appeared identical upon visual inspection. A quantitative analysis of the profiles was performed using the RIT software system. Comparison of the profiles revealed that the FWHM of the abutting region agreed to within 0.2 mm between the two films. Conclusion: If accuracy of leaf calibration has been demonstrated, differences between these profiles would imply carriage miscalibration. Carriage miscalibration would result in mispositioning of all leaves within the carriage, thereby causing significant dose delivery errors. This carriage calibration test could be performed routinely as part of the QA procedure for MLCs used for IMRT delivery.

Purpose: To compare the surface dose to the ipsilateral breast between compensator and MLC‐based IMRT for breast radiotherapy both with and without simulated respiratory motion. Methods & Materials: An anthropomorphic polystyrene breast phantom mounted on a moving platform was used to simulate the human breast and its respiration‐induced motion during radiotherapy. MOSFET dosimeters were placed on the surface of the phantom at 5 approximately uniformly spaced positions in the central axial plane. Two common IMRT treatment techniques were then used to deliver a uniform dose of 1.8 Gy to the simulated breast both with and without simulated respiratory motion with 1 cm amplitude and a period of 4 seconds. Both techniques used the same parallel‐opposed half‐beam‐blocked tangential beams, but one used bismuth polyethylene compensators to create the modulation while the other used the segmental MLC (SMLC) delivery technique to create the modulation. The segment shapes and weights for the SMLC delivery were created from the fringe lines of the compensator maps so that the plans delivered essentially identical dose distributions to the phantom beyond the build‐up region. Results: Respiratory motion did not significantly alter the surface dose for either technique. All five dosimeters measured lower surface doses from the SMLC plans in comparison to the compensator plans. The average surface dose for SMLC delivery of 127 cGy was 6% lower than the average dose of 135 cGy measured for compensator delivery. Conclusion: The secondary and scattered radiation produced in the compensator increases surface dose. The use of SMLC IMRT results in lower dose to the skin than that from compensator‐based IMRT and this fact should be considered when planning breast IMRT cases.