NCRP Report no.180-management of exposure to ionizing radiation: NCRP radiation protection guidance for the United States.

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.

Experience in implementing ICRP recommendations: IRPA's perspective on the role of the radiation protection professional.

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.

Measurements of accelerator-produced leakage neutron and photon transmission through concrete.

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.

Neutron fluence and energy spectra around the Varian Clinac 2100C/2300C medical accelerator.

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.

Neutron sources in the Varian Clinac 2100C/2300C medical accelerator calculated by the EGS4 code.

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.

Giant dipole resonance neutron yields produced by electrons as a function of target material and thickness.

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.

Gas bremsstrahlung and associated photon-neutron shielding calculations for electron storage rings.

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.

Tolerances in setup and dosimetric errors in the radiation treatment of breast cancer.

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.

Spectral energy effects in ESR bone dosimetry: photons and electrons.

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.

Higher energy: is it necessary, is it worth the cost for radiation oncology?

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.

Photon beam dosimetry at a blocked beam edge using diffusion approximation.

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.

The v-src oncogene may not be responsible for the increased radioresistance of hematopoietic progenitor cells expressing v-src.

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.

Electron beam modifications for the treatment of superficial malignancies.

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 accuracy at low monitor unit settings.

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.

Study of dose perturbation parameters for eye shielding in megavoltage photon beam therapy.

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.

Validity of transition-zone dosimetry at high atomic number interfaces in megavoltage photon beams.

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.

Determination of primary dose in 60Co gamma beam using a small attenuator.

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 replacement factors for a parallel-plate chamber.

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.

Patient biodistribution of intraperitoneally administered yttrium-90-labeled antibody.

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.

Reconstruction of diagnostic x-ray spectra by numerical analysis of transmission data.

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.