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Within a few months of discovery, X-rays were being used worldwide for diagnosis and within a year or two for therapy. It became clear very quickly that while there were immense benefits, there were significant associated hazards, not only for the patients, but also for the operators of the equipment. Simple radiation protection measures were implemented within a decade or two and radiation protection for physicians and other operators has continued to evolve over the last century driven by cycles of widening uses, new technologies, realization of previously unidentified effects, development of recommendations and regulations, along with the rise of related societies and professional organizations. Today, the continue acceleration of medical radiation uses in diagnostic imaging and in therapeutic modalities not imagined at the turn of this century, such as positron emission tomography, calls for constant vigilance and flexibility to provide adequate protection for the growing numbers of medical radiation workers.

NCRP Report No. 180, 'Management of Exposure to Ionizing Radiation: Radiation Protection Guidance for the United States (2018)' was developed by Council Committee 1. The report builds and expands upon previous recommendations of NCRP and ICRP, covering exposure to radiation and radioactive materials for five exposure categories: occupational, public, medical, emergency workers, and nonhuman biota. Actions to add, increase, reduce or remove a source of exposure to humans require justification. Optimisation of protection universally applies, taking into account societal, economic, and environmental factors; addressing all hazards, and striving for continuous improvement when it is reasonable to do so. Numeric protection criteria for management of dose to an individual for a given exposure situation are provided, and differ in some respects from ICRP. A specific numeric criterion is suitable to be designated as a regulatory dose limit only when the source of exposure is stable, characterised, and the responsible organisation has established an appropriate radiation control program in advance of source introduction. Medical exposure includes patients, comforters and caregivers of a patient, and voluntary participants in biomedical research. Emergency workers are a new exposure category; their exposure is treated separately from occupational, public or medical exposure, and numeric criteria are provided for deterministic and stochastic effects. For nonhuman biota, the focus is on population maintenance of the affected species, and a guideline is provided for when additional assessment may be necessary. In addition, the recommendations emphasise that: ethical principles support decision-making; stakeholder engagement is necessary in deciding suitable management of their radiation exposure; and a strong safety culture is intrinsic to effective radiation protection programs.

This summary of the 2014 Annual Meeting of the National Council on Radiation Protection and Measurement (NCRP) captures the opportunities presented during the Warren K. Sinclair Keynote Address, the Lauriston S. Taylor Lecture, and the six scientific sessions including the subsequent questions and answers. It captures the important issues that emerge in these opportunities and discusses the challenges that they bring to radiation protection. These opportunities arise in the basic sciences; in operational areas such as emerging technologies, preparing for the improbable but possible event, industry and medicine; and in education, communication and policy. The challenges include identifying the most important aspects of radiation protection and measurement, prioritizing them in accordance with the NCRP mission, and gaining support for the activities of the NCRP to address these issues in the fulfillment of its charter.

The International Radiation Protection Association (IRPA) has a membership of approximately 17,000 individuals who are members of 48 national societies in 60 countries worldwide. As such, IRPA's vision is to be recognised as the international voice of the radiation protection professional. This article will discuss elements of the outcome of the 12th International Congress of IRPA ('Focus on the future'), objectives and current activities of IRPA, criteria and priorities for the engagement of IRPA with international organisations, current IRPA initiatives in the areas of radiation protection culture and certification/qualification of radiation protection experts, planning for the 13th International Congress of IRPA, comments on the implementation of recent recommendations of the International Commission on Radiological Protection (ICRP), and suggestions about IRPA and ICRP collaboration in their implementation. IRPA recognises that ICRP is the international body to determine policy and to make recommendations for protection against ionising radiation, and IRPA is in a position to participate in and facilitate the implementation of those recommendations., (Copyright © 2012. Published by Elsevier Ltd.)

Global concerns over energy supply and climate change have given rise to an increase in uranium prospecting, mining and extraction. The changing world economy is spreading the use of advanced nuclear and radiation-related technologies to many parts of the world, giving rise to global initiatives on nuclear energy and operation of nuclear fuel cycle facilities. The emerging global nuclear safety regime promotes and encourages high standards of radiation safety worldwide. These developments call for increasing capacity and capabilities in radiation protection expertise and continue to present both challenges and opportunities to the International Radiation Protection Association (IRPA), an association of 46 societies representing 58 countries with an individual membership of approximately 17,000. IRPA's objectives include: (1) assisting the development of competent radiation protection programs; (2) fostering the exchange of scientific and technical information through its international and regional congresses; (3) promoting the scientific and professional recognition of the radiation protection expert; and (4) supporting continuing education programs at each IRPA congress. IRPA has adopted a Code of Ethics and Guiding Principles for the Conduct of Stakeholder Engagement. Recently work began to develop guidance for maintaining and improving current levels of radiation protection and transferring this culture to future radiation protection professionals. These IRPA projects are developed through the Associate Society Forum discussions that are held at each IRPA international and regional congress. Finally, IRPA maintains a close working relationship with various international organizations and is also represented on the Inter-Agency Committee on Radiation Safety., (Copyright © 2010 Health Physics Society)

The current recommendations of the National Council on Radiation Protection and Measurements (NCRP) relative to ionizing radiation are based on radiation protection principles that developed historically as information about radiation effects on human populations became available. Because the NCRP Charter states that the NCRP will cooperate with the International Commission on Radiological Protection (ICRP), the basic principles and recommendations for radiation protection of the NCRP are closely coupled with those of the ICRP. Thus, the fundamental principles of justification, optimization, and dose limitation as initially stated in ICRP Publication 26 have been adopted and applied by the NCRP in its recommendations. ICRP and NCRP recommendations on dose limitation for the general public and for occupationally exposed individuals are based on the same analyses of radiation risk, and, while similar, there are differences reflecting the aspects of radiation application and exposure circumstances unique to the United States. The NCRP has recently extended its guidance to address exposure to individuals engaged in space activities. Several reports have been issued or are in preparation to provide recommendations on dose limitation and the development of radiation safety programs to apply the radiation protection principles in space activities. The biological basis for these recommendations is provided in these and accompanying NCRP reports. Recommendations for the application of basic radiation protection principles have been made in many reports over the years. Those that are most current appear in approximately 50 reports published in the last 15 y. These address radiation safety practices in industrial and medical institutions, control of radionuclides in the environment, protection of the public, and assessment of radiation risk. Some of the aspects of these recommendations will be discussed. Current recommendations related to radiation safety practice are based on the principles and dose limits specified in Report No. 116. The limits are based on estimates of the risk of fatal cancer and an assessment of the risk that should be tolerated by workers who are occupationally exposed and by the general public. These levels of risk are related to other risks that individuals accept in their lives. Looking to the future, one might consider other directions that the NCRP could take in developing radiation safety recommendations that are still based upon the stated principles, such as relating dose to loss of life expectancy instead of fatal cancer risk. It may also be that the principles of justification, optimization, and dose limitation should be reconsidered. For example, the NCRP may make recommendations about the relationship of radiation dose to various biological effects or outcomes and the resulting estimates of risk, but not specify dose limits. This would relieve the NCRP of the necessity to speculate about acceptable risks. One can also imagine that the principle of justification could be applied not only to the introduction of a new source of radiation, but also to the removal of an existing source of radiation, i.e., the idea of justifying decontamination efforts. It is clear that as we move into the 21st century there will be a continuing need for the NCRP to identify the principles upon which radiation protection is to be based and to provide guidance on the application of those principles for the many beneficial uses of radiation and radioactive materials in society.

Optimum shielding of the radiation from particle accelerators requires knowledge of the attenuation characteristics of the shielding material. The most common material for shielding this radiation is concrete, which can be made using various materials of different densities as aggregates. These different concrete mixes can have very different attenuation characteristics. Information about the attenuation of leakage photons and neutrons in ordinary and heavy concrete is, however, very limited. To increase our knowledge and understanding of the radiation attenuation in concrete of various compositions, we have performed measurements of the transmission of leakage radiation, photons and neutrons, from a Varian Clinac 2100C medical linear accelerator operating at maximum electron energies of 6 and 18 MeV. We have also calculated, using Monte Carlo techniques, the leakage neutron spectra and its transmission through concrete. The results of these measurements and calculations extend the information currently available for designing shielding for medical electron accelerators. Photon transmission characteristics depend more on the manufacturer of the concrete than on the atomic composition. A possible cause for this effect is a non-uniform distribution of the high-density aggregate, typically iron, in the concrete matrix. Errors in estimated transmission of photons can exceed a factor of three, depending on barrier thickness, if attenuation in high-density concrete is simply scaled from that of normal density concrete. We found that neutron transmission through the high-density concretes can be estimated most reasonably and conservatively by using the linear tenth-value layer of normal concrete if specific values of the tenth-value layer of the high-density concrete are not known. The reason for this is that the neutron transmission depends primarily on the hydrogen content of the concrete, which does not significantly depend on concrete density. Errors of factors of two to more than ten, depending on barrier thickness, in the estimated transmission of neutrons through high-density concrete can be made if the attenuation is scaled by density from normal concrete.

We have simulated the head geometry of a Varian Clinac 2100C/2300C medical accelerator in a Monte Carlo calculation to produce photoneutrons and transport them through the head shielding into a typical therapy room (modeled by a test cell at Varian Associates). The fast neutron leakage fluence and energy spectra have been calculated at 7 positions around the linac head for typical beam operation at 10, 15, 18 and 20 MV. The results of these calculations have been compared with limited measurements made using the same model accelerator operating in a Varian test cell. Calculations were also made for the fluence and energy spectra outside the head with no surrounding concrete walls, floor or ceiling to eliminate the effects of scattering from concrete. Comparisons were also made with calculations using a much simplified head geometry. The results indicate that the calculations using the complex head geometry compare, within the uncertainties, with the measurements. The simple head geometry leads to differences of a factor of 2 from the complex geometry. Results of these calculations can be used to calculate fast neutron transmission through various shielding configurations and through labyrinths.

The photoneutron yields produced in different components of the medical accelerator heads evaluated in these studies (24-MV Clinac 2500 and a Clinac 2100C/2300C running in the 10-MV, 15-MV, 18-MV and 20-MV modes) were calculated by the EGS4 Monte Carlo code using a modified version of the Combinatorial Geometry of MORSE-CG. Actual component dimensions and materials (i.e., targets, collimators, flattening filters, jaws and shielding for specific accelerator heads) were used in the geometric simulations. Calculated relative neutron yields in different components of a 24-MV Clinac 2500 were compared with the published measured data, and were found to agree to within +/-30%. Total neutron yields produced in the Clinac 2100/2300, as a function of primary electron energy and field size, are presented. A simplified Clinac 2100/2300C geometry is presented to calculate neutron yields, which were compared with those calculated by using the fully-described geometry.

This paper characterizes the functional dependence of the giant dipole resonance neutron yield produced by electrons in terms of the atomic number (Z) and thickness (T) of the target. The yields were calculated by integrating, over the photon energy, the product of the differential photon track length and published photoneutron cross sections. The EGS4 Monte Carlo code and analytical formulas were used to calculate the differential photon track length. In thick targets, the Giant Dipole Resonance neutron yield approaches a saturation value as target thickness T increases to 10 radiation lengths. A formula, 8 x 10(-6) x (Z1/2 + 0.12 Z3/2 - 0.001 Z5/2) n electron-1 MeV-1, developed from EGS4 calculations, estimates thick-target neutron yields for incident electron energies Eo above 50 MeV. Giant dipole resonance neutron yields, calculated by several analytic formulas for the differential photon track length, are compared with EGS4 calculations. Modifications to the analytic formulas are suggested. A scaling function is derived to estimate, from the thick-target formula, neutron yields produced in thin targets.

The EGS4 electron-photon Monte Carlo code has been used to study the characteristics of the bremsstrahlung x rays generated from the interaction of circulating electrons with the residual gas in accelerator storage rings. Gas bremsstrahlung dose rates are given for various opening angles as a function of the electron beam energy ranging from 0.5-10 GeV. Photon and neutron dose rates, generated from various devices struck by gas bremsstrahlung in a synchrotron radiation beamline, are also presented along with the photon spectral and transmission results. The EGS4-predicted results are found to be in basic agreement with the measurements made at the Stanford Synchrotron Radiation Laboratory. Figures, equations, and a simple method useful for the photon-neutron shielding design for beamlines are provided.

Purpose: Treatment failure in radiation therapy, as well as unexpected complications, can be associated with set up changes or variations that can cause deviations from the prescribed radiation dose distribution both inside and outside the target volume. The effect of various deviations from the planned setup on the delivery of the prescribed radiation dose to the desired treatment volume was studied., Methods and Materials: Adding a second simulation was investigated as means of minimizing setup changes on treatment. The first simulation was used for planning the treatment and the second simulation was essentially a mock treatment. Dosimetric evaluations based on dose volume histograms were analyzed for each deviation in the setup., Results: In 95% of the patients, the frequency of the changes in the setup parameters between the second simulation and the treatment setup were reduced significantly from the changes that occurred between the first simulation and the second simulation. The changes in isocenter coordinates up to +/- 1.0 cm have minimal effects (+/- 2%) on the dose distributions. Gantry angle variations up to +/- 4 degrees produce a change of less than +/- 5% in the dose distribution within the target volume. However, this angular variation resulted in additional tissue irradiation outside of the desired treatment field (about 10 cm3 for a large patient). A gantry angle variation of +/- 6 degrees can change the volume of tissue that receives the prescribed dose by at least +/- 10%. In addition, such a change can increase the volume of tissue outside the desired treatment field that is irradiated., Conclusion: It is concluded that individually, deviations in one of the parameters from the planned setup of +/- 1.0 cm in isocenter position and +/- 4 degrees in gantry angle do not produce significant deviations from the planned dose distribution. However, a significant change in dose distribution is observed if the setup parameters are concurrently changed. A second simulation may minimize the deviations of the treatment setup from the planned setup and maximize the precision in dose delivery to the target volume.

The spectral energy-dependence of the radiation-induced ESR signal has been studied in ovine cortical bone. Crushed bone samples were irradiated using photon beams with effective energies in the range from 0.06 to 6 MeV, and electron beams with mean energies in the range from 2 to 10 MeV. The photon and electron data were normalized to a dose to bone of 50 Gy and the results are reported as response relative to the ESR signal for photon irradiation at 1.25 MeV (60Co). The photon irradiation results show that the ESR response is greatest at low energies with a relative value of 1.2 at 0.06 MeV. The relative response decreases, as the energy increases, to approximately 0.85 in the region of 2 to 3 MeV. These variations in the relative ESR responses are significantly less than the ESR energy-dependent responses reported in the literature for human tooth enamel and synthetic hydroxyapatite. An explanation for this difference is offered. For electron beam irradiations, the ESR signal is fairly constant with energy, and approximately equal to that at a photon energy of 1.25 MeV. Implications of these results are discussed.

The physical characteristics of the interactions of megavoltage photons and electrons with matter provide distinct advantages, relative to low-energy (orthovoltage) x rays, that lead to better radiation dose distributions in patients. Use of these high-energy radiations has resulted in better patient care, which has been reflected in improved radiation treatment outcome in recent years. But, as the desire for higher energy radiation beams increases, it becomes important to determine whether the physical characteristics that make megavoltage beams beneficial continue to provide a net advantage. It is demonstrated that, in fact, there is an energy range from 4 to 15 MV for photons and 4 to 20 MeV for electrons that is optimally suited for the treatment of cancer in humans. Radiation beams that exceed these maximum energies were found to add no advantage. This is because the costs (price of unit, installation, maintenance, shielding for neutron and photons) are not justified by either improved physical characteristics of the radiation (penetration, skin sparing, dose distribution) or treatment outcome. In fact, for photon beams some physical characteristics result in less desirable dose distributions, less accurate dosimetry, and increased safety problems as the energy increases for example, increasingly diffuse beam edges, loss of electron equilibrium, uncertainty in dose perturbations at interfaces, increased neutron contamination, and potential for higher personnel dose. The special features that make electron beams useful at lower energies, for example, skin sparing and small penetration, are lost at high energies. These physical factors are analyzed together with the economic factors related to radiation therapy patient care using megavoltage beams.

A simple analytical model is presented for the transport of secondary electrons at a photon beam edge using the energy averaged solution of the Boltzmann equation, originally developed for beta-ray dosimetry at a plane interface. Dose at a point under a block is assumed to be due to secondary electrons and the scattered photons generated from the primary photon beam. The diffusion approximation is used for the secondary electron transport at a virtual plane interface created by the block. The dose from the scattered photon component is treated as decaying exponentially with distance from the beam edge. Comparisons made with the model and measurements are in general agreement for high energy accelerator beams.

Infection of the IL-3-dependent, myeloid progenitor cell line 32D cl 3 with murine retroviruses that contain either the wild-type or a temperature-sensitive mutant v-src can render these cells growth-factor independent. These cells also became resistant to gamma irradiation administered at the low-dose rate of 0.05 Gy/min, which is used clinically. The v-src-dependent nature of resistance to gamma irradiation was examined by studying four clones of 32D cl 3 cells that had been infected with a retrovirus carrying the tsLA31A mutant of v-src. The tyrosine-specific kinase activity of this mutant is dramatically reduced at the nonpermissive temperature of 39 degrees C. Cells transformed by v-src and grown at either 34 or 39 degrees C, in the presence or absence of IL-3, demonstrated a significantly higher D0 compared to parental cells examined under identical conditions. In addition, expression of v-src abrogated the synergistic killing effect of heat and gamma irradiation. The D0 of parental 32D cl 3 cells kept at 39 degrees C after gamma irradiation was reduced significantly compared to the D0 of these cells kept at 34 degrees C. This contrasts with data from 32D cl 3 cells infected with either the wild-type v-src or the temperature-sensitive mutant, neither exhibited a synergistic effect in the D0 at either 34 or 39 degrees C. Therefore, while continuous expression of a v-src gene product is required for maintenance of the growth-factor-independent state, v-src does not appear to be responsible for the increased gamma-radiation resistance of these cells at low dose rate.

For the treatment of superficial tumors, the surface dose should be high; unfortunately, because of pronounced dose buildup in low energy electron beams, their efficacy for such treatment is reduced. Electron beams can be modified by placing a low atomic number material called a beam spoiler in the beam. In general, the surface dose is a function of electron energy, source to surface distance, field size, thickness of beam spoiler, distance of beam spoiler from surface, atomic number of beam spoiler, and angle of the beam. The effects of these parameters are evaluated with respect to surface dose, bremsstrahlung dose, and field size changes for small fields at standard SSD and electron energies from 6 to 17 MeV. It was found that the use of a beam spoiler can generally increase the surface dose to values exceeding 90% of the maximum buildup value while maintaining a bremsstrahlung dose less than 3%. Changes in field size related to the placement of the beam spoiler were considerable in some cases.

Dosimetric accuracies at low monitor units are evaluated for linear accelerators from various manufacturers. A large error is observed in the majority of the accelerators. The error can be positive or negative. Although the error can exceed 20% for the first few monitor units, it is usually less than 5% when more than 10 monitor units are delivered. When low doses are required proper precautions should be taken for dosimetric accuracy including the beam energy, beam flatness and dose per monitor unit.

Shielding blocks are frequently used to minimize dose and shield sensitive organs in radiation therapy. The blocks, which are made of high atomic number materials, produce significant dose perturbations in megavoltage photon beams. The effects of these perturbations are studied with special interest in the eye shielding in the treatment of head and neck malignancies. Optimum parameters for the treatment are suggested.

A measurement technique previously proposed for determining dose from primary radiation has been tested using 60Co gamma rays. It is shown that the dose from primary radiation is reliably determined for field sizes of 10 X 10 and 20 X 20 cm2 at depths of 0.5, 5, and 10 cm in water. With further development this technique may be useful for verifying dose from primary radiation that may be calculated using a variety of methods.

Measurement of dose or dose perturbation factors at high atomic number interfaces are usually performed with a thin-window parallel-plate ion chamber. In a transition region, under nonequilibrium conditions, accuracy of ion chamber readings for the dose measurements has often been questioned. This paper critically analyzes the factors (stopping power ratio and charge collection) for the dose measurements at interfaces. Monte Carlo simulations were performed to investigate the secondary electron spectrum produced by photon beams and to calculate the stopping power ratios at the point of measurement. The validity of dose measurements was studied for the photon beams in the range of Co-60 gamma rays to 24-MV x rays at bone and lead interfaces with polystyrene, using thermoluminescent dosimeters, extrapolation chamber and several types of commercially available parallel-plate ion chambers. It is observed that for energies greater than 10 MV most parallel-plate chambers can be used to measure dose accurately. At lower energies, however significant differences between measured doses with different detectors were noticed. It is suggested that at high-Z interfaces and lower energies, the dose measurements should be performed with ultrathin-window parallel-plate ion chambers or extrapolation chambers.

We present comparisons of calculated and measured dose equivalent rates outside the shielding and in the entry mazes of 2 medical linac facilities producing x rays at 6 and 15 MV. Calculations were made using the NCRP recommendations for estimating transmission of radiation through a shielding wall and scattering of radiation in a maze. We found that, for walls made of high-density concrete, the x-ray dose rate outside the shield was estimated within 50% if transmission factors measured in the appropriate high-density concrete were used. The dose rate was overestimated by a factor of 2-4 when transmission factors for normal concrete were scaled using a density ratio. Dose equivalent rates calculated for x rays and neutrons in the entry mazes agreed within a factor of 2 with the measurements.

The adherent stromal layer in long-term bone marrow cultures (LTBMC) provides the cellular environment necessary for the in vitro proliferation and differentiation of pluripotential hematopoietic stem cells. The role of humoral hematopoietic growth factors, colony-stimulating factors (CSF) in the regulation of hematopoietic cell production in this system is poorly understood. We have recently isolated and cloned an adherent cell line, D2XRII, derived from murine LTBMC. Plateau phase 25 cm2 cultures of 2 X 10(6) D2XRII cells in 8.0 ml produced CSF-1 (M-CSF) at around 100-150 units/0.1 ml medium. Following X-irradiation there was a dose-dependent decrease in the production of CSF-1 to a plateau of 50% of control levels at 10,000 rad. Higher doses did not produce a further decrease. The X-ray dose reducing CSF-1 production to 50% was 100-fold above the lethal dose as measured by clonagenic survival following trypsinization and replating. Trypsinized replated viable adherent but nondividing X-irradiated D2XRII cells were maintained for up to 8 weeks after irradiation and demonstrated continuous production of CSF-1. The data indicate significant divergence of two biologic effects of X-irradiation on plateau-phase marrow stromal cells: physiologic function of adherence and CSF-1 production, versus proliferative integrity. This divergence of effects may be very relevant to understanding the mechanism of X-irradiation-associated marrow suppression and leukemogenesis.

Measurements were made in air on the central axis of radiation beams from linear accelerators operating in the energy range from 4 to 18 MV, to determine the magnitude and source of head scattered radiation. Machines of several manufacturers were studied. The data indicate that, except for one unique collimator design, head scatter originates primarily in the flattening filter and is relatively independent of energy and machine.

We have measured the replacement correction factors (Prepl) for a PTW/Markus parallel plate chamber at mean incident electron energies of 3.1, 4.4, 8.9, 13.0, 16.3, and 18.8 MeV. The factors are significantly different from unity at low energies.

The "quality of radiation" for a high energy x-ray beam can be specified by its attenuation curve in a selected material. The inverse Laplace transform of the attenuation curve can be used as an approximate indication of the energy spectrum of the beam. Existing mathematical procedures for this purpose have been evaluated and were found to poorly represent measured transmission data for 4 MV x-rays from a linear accelerator. The transmission data between 1 and 0.002 could be fitted within the experimental uncertainty by expressing the logarithmic transmission as a second order polynomial of attenuator thickness. The inverse Laplace transform them becomes a Gaussian function of the attenuation coefficient. This new version of "attenuation analysis" provides a practical method for specification of the quality of the radiation in this energy range.

A comparison has been made of central axis percent depth dose and absorbed dose in electron beams of 7.8 and 10.2 MeV, measured with devices of differing geometry and construction. Flat and cylindrical ionization chambers have been used as well as thin thermoluminescent dosimeters. The ionization chambers had walls of air equivalent or tissue equivalent plastic. Results indicate that central axis depth dose measurements are independent of measuring device. No significant difference was found among the various ionization chambers with air equivalent walls in the determination of absorbed dose. The dose determined by the tissue-equivalent wall chamber was about 3% higher than the dose determined by the other ionization chambers. Dose maximum on the central axis in water is about 4% greater than when this same quantity is calculated from data measured in polystyrene.

A 300 kvp orthovoltage machine has been permanently installed in an operating room for delivering intraoperative radiation therapy (IORT). A historical review of orthovoltage IORT and our present approach are described. The preliminary experience with 38 patients treated with orthovoltage IORT indicates that this technique is feasible, has low acute morbidity, and can be useful for palliation. "Radical" radiation therapy consisting of IORT "boost" treatment combined with external beam was used in 24 patients with primary or recurrent cancer. Local failure in 27 patients treated with IORT +/- external beam radiation therapy was 56%, but varied from 11% (1/9) for patients with resected disease to 78% (14/18) for patients with unresected disease. Complications occurred in nine patients (24%) and have been acceptable. There are 17 patients alive and six are NED, with follow-up of 4-18 months. There appears to be a role for orthovoltage IORT especially when combined with surgical resection for local control of advanced cancer arising in the abdomen where the use of high doses of external radiation therapy are hazardous.

Photon and electron transmission measurements were made for silicone elastomer and water, used in mammary prostheses. Photon energies used ranged from 100 kVp, 1.0-mm AI HVL, to 8 MV, and electron energies ranged from 3 to 11 MeV. Transmission of photons above 150 keV and electrons above 2 MeV through the silicone and water is identical, as expected from calculations of mass attenuation coefficients and mass stopping power.

Radiation dose to organs outside the radiotherapy treatment field can be significant and therefore is of clinical interest. We have made measurements of dose at distances up to 70 cm from the central axes of 5 X 5, 15 X 15 and 25 X 25 cm radiation fields of 300 kVp, 4 MV and 8 MV X rays, and 60Co gamma rays, at the surface and at depths in water of 5 and 10 cm. Contributions to the total secondary radiation dose from water scatter, machine (collimator) scatter and leakage radiation have been separated. We have found that the component of dose from water scatter can be described by a simple exponential function of distance from the central axis of the radiation field for all energies and field sizes. Machine scatter contributes 20 to 40% of the total secondary dose depending on machine, field size and distance from the field. Leakage radiation contributes very little dose, but becomes the dominant component at distances beyond 60 cm from the central axis. Estimates of the risk of second tumors in long term survivors indicate a small incremental increase above the natural incidence rate based on information from the 1980 BEIR Committee report.

Although 90Y is one of the best radionuclides for radioimmunotherapeutic applications, the lack of gamma rays in its decay complicates the estimation of radiation dose since its biodistribution cannot be accurately determined by external imaging. A limited clinical trial has been conducted with tracer doses (1 mCi) of 90Y in five patients who then received second-look surgery such that tissue samples were obtained for accurate radioactivity quantitation by in vitro counting. The anti-ovarian antibody OC-125 as the F(ab')2 fragment was coupled with diethylenetriaminepentaacetic acid, radiolabeled with 90Y and administered intraperitoneally to patients with suspected or documented ovarian cancer. Size exclusion and ion exchange high performance liquid chromatography analysis of patient ascitic fluid and serum samples showed no evidence of radiolabel instability although a high molecular weight species (presumably immune complex) was observed in three patients. Total urinary excretion of radioactivity prior to surgery averaged 7% of the administered radioactivity while at surgery the mean organ accumulation was 8% of the administered radioactivity in serum, 10% in liver, 7% in bone marrow, and 19% in bone with large patient to patient variation. The mean tumor/normal tissue radioactivity ratio varied between 3 and 25. On the assumption that the above radioactivity levels were achieved immediately following administration, that the radioactivity remained in situ until decayed and that the dimensions of tumor were sufficient to completely attenuate the emissions of 90Y, the dose to tumor for a 1-mCi administration would be approximately 50 rad with normal tissues receiving approximately 8 rad.

This study concerns the accuracy and precision of the IAEA/WHO LiF TLD system used in intercomparison by mail of absorbed doses from 60Co gamma-radiation and 4-25 MV x rays. The system employs 160 mg LiF powder in polystyrene capsules, which are placed at 5 or 7 cm depth in water and irradiated to doses close to 200 rad (2.00 Gy). The dosimeters are mailed to the IAEA Dosimetry Laboratory and read out under conditions to minimize variatons in instrument sensitivity. The precision of the readout technique, using 3 capsules per irradiation and the readout of 5 aliquots per capsule, is characterized by 0.2% standard deviation of the resulting mean. Since random errors during the irradiation are added, the detectable systematic descrepancy in dose delivery, at the 95% confidence level, is +/- 2% for 60Co gamma and +/- 3% for high-energy x rays.

A 300 kVp orthovoltage therapy machine has been installed in an operating room for intraoperative radiation therapy (IORT). A description is presented of the physical aspects of the treatment system including applicator design, radiation field data, dosimetry, and radiation shielding of the operating room.

Numerical reconstruction of x-ray spectra from narrow-beam transmission data was tested with published spectra in the range from 45 to 100 kVp. Transmission curves were calculated from the spectra to simulate measured data. Spectra were reconstructed from these transmission curves with use of an iterative numerical analysis. Comparison of the calculated spectra with the original spectra shows good agreement, including the tungsten characteristic x rays. This demonstrates the potential usefulness of measured transmission data for deducing x-ray spectra in the diagnostic energy range.

Radiation dose outside the radiotherapy treatment field can be significant and therefore is of clinical interest in estimating organ doses. In a previous paper we reported the results of measurements made using unmodified radiation fields. We have extended this study to include the effects of wedge filters and blocks. For a given dose on the central axis of a radiation field, wedges can cause a factor of 2 to 4 increase in dose at any point outside the field compared with the dose when no wedge is used. Adding blocks to a treatment field can cause an increase in dose at points outside the field, but the effect is much smaller than the effect of a wedge, and generally less than a factor of 2. From the results of these measurements, doses to selected organs outside the field for specified treatment geometries were estimated, and the potential for reducing these organ doses by additional shielding was assessed.

An instrument for daily beam checks of medical accelerators is described. Using three silicon diodes, the instrument provides information about calibration, beam symmetry, flatness, and energy variation for both photon and electron beams. Operating characteristics of the instrument and the electronic circuit design permit simple and reliable measurements.

Dose and integral dose from bremsstrahlung in a 10-MeV electron beam were measured for irradiation of large areas with 120 degrees-arc rotational fields. The maximum bremsstrahlung dose ranged from 2% to 7% of the maximum electron dose for the different beam arrangements, while the integral dose showed the same range of variation. The concomitant bremsstrahlung beam should be collimated by the x-ray photon collimators and the use of narrow field rotations avoided.

Transmission data for 4-MV bremsstrahlung beams have been measured with a combination of lead and aluminum attenuators. From these data, the original energy spectra have been reconstructed using an iterative least-squares technique, previously evaluated by simulation studies. The spectra on the central axis for three similar 4-MV linear accelerators indicated no significant differences. When studying the spectra at 5 degrees and 9 degrees off the central axis, that at 9 degrees showed the expected increase of low-energy photons. All these spectra showed a maximum photon energy of 4.5 +/- 0.2 MeV. When the magnetron power was reduced, the spectrum on the central axis shifted to lower energies and the maximum photon energy decreased to 3.5 +/- 0.2 MeV. The result of this experimental study confirms the conclusions from the previous stimulation, that the numerical technique for analysis of transmission data can accurately represent 4-MV bremsstrahlung spectra and detect differences in energy distribution with changes in machine tuning and position in the radiation field for a 4-MV bremsstrahlung beam.

The use of transmission measurements with combinations of attenuating materials has been investigated as a method of determining the x-ray energy spectrum from a 4-MV linear accelerator. Simulation studies have been made with three 4-MV x-ray spectra of slightly different shapes. From each of these spectra, the narrow beam transmission curve was calculated, using lead to attenuate the beam for the first several points. Aluminum was then added to attenuate the beam to 0.001 of its initial intensity. A computer program using an iterative least-squares technique has been developed to analyze the transmission data and determine the spectrum by applying some physically reasonable sensible constraints. The calculated transmission values agreed with the simulated data within the estimated experimental uncertainties, and it is concluded that the technique is reliable for the spectral reconstruction and is sensitive to small spectral changes.

A "gradient correction factor" and an "electron fluence correction factor" are used in the AAPM Task Group 21 dosimetry protocol, the product of the two being the "replacement correction factor." The separation of two physical processes, implied by the concepts, is theoretically unfounded. The proof of this statement rests on Fano's theorem, the conditions for which constitute a case without variations in fluence or its directional and energy distributions. Deviations from these conditions represent gradients in the radiation field, affect the response of a measurement cavity, and do so by perturbing the fluence of electrons that deposit energy in the cavity. Thus, in beams of both electrons and photons, corrections are required when fluence variations exist in the vicinity of the cavity. The rationale for an electron fluence correction based on "in-scattering" and "obliquity" is weak, since the effects only occur when gradients are present. The choice of values for the two correction factors at the depth of maximum dose are particularly inconsistent with the actual characteristics of the radiation field.

A Philips RT-305 X-ray therapy unit was found to produce beams of considerable asymmetry. This has been corrected by use of a small wedge-shaped symmetry filter, placed close to the X-ray tube. The symmetry filter has only a small effect on depth doses and half-value layers.