Evaluating the accuracy of a three-term pencil beam algorithm in heterogeneous media.

The goal of this work was to evaluate the accuracy of our in-house analytical dose calculation code against MCNPX data in heterogeneous phantoms. The analytical model utilizes a pencil beam model based on Fermi-Eyges theory to account for multiple Coulomb scattering and a least-squares fit to Monte Carlo data to account for nonelastic nuclear interactions as well as any remaining, uncharacterized scatter (the 'nuclear halo'). The model characterized dose accurately (up to 1% of maximum dose in broad fields (4 × 4 cm2 and 10 × 10 cm2) and up to 0.01% in a narrow field (0.1 × 0.1 cm2) fit to MCNPX data). The accuracy of the model was benchmarked in three types of stylized phantoms: (1) homogeneous, (2) laterally infinite slab heterogeneities, and (3) laterally finite slab heterogeneities. Results from homogeneous phantoms and laterally infinite slab heterogeneities showed high levels of accuracy (>98% of points within 2% or 0.1 cm distance-to-agreement (DTA)). However, because range straggling and secondary particle production were not included in our model, central-axis dose differences of 2-4% were observed in laterally infinite slab heterogeneities when compared to Monte Carlo dose. In the presence of laterally finite slab heterogeneities, the analytical model resulted in lower pass rates (>96% of points within 2% or 0.1 cm DTA), which was attributed to the use of the central-axis approximation.

SU-E-T-101: Dosimetry Intercomparison for a Synchrotron-Produced Monochromatic X-Ray Beam.

PURPOSE: This study performed a dosimetry intercomparison for synchrotron-produced monochromatic x-ray beams. Ion chamber depth-dose measurements in a polymethylmethacrylate (PMMA) phantom were compared with the product of MCNP5 Monte Carlo calculations of dose per fluence and measured incident fluence at 25 and 35 keV. The ion chamber measurements are being used to calibrate dose output for cell irradiations designed to investigate photoactivated Auger electron therapy at the LSU Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. METHODS: Monochromatic beams of 25 and 35 keV were generated on the tomography beamline at CAMD. A cylindrical, air-equivalent ion chamber was used to measure the ionization created in a 10×10×10-cm3 PMMA phantom at depths of 0.6 - 7.7 cm. AAPM TG-61 protocol was applied to convert measured ionization into dose. MCNP5 simulations of the irradiation geometry were performed to determine the dose deposition per photon fluence in the phantom. Photon fluence was determined using a NaI detector to make scattering measurements of the beam from a polyethylene target at angles 15 - 60 degrees. Differential Compton and Rayleigh scattering cross sections were used to derive the incident fluence. RESULTS: At 35 keV dose measurements for equal exposures determined using the MCNP5-fluence results underestimated those of the ion chamber by 1.8 - 4.8% for PMMA depths from 0.6 - 7.7 cm, respectively. At 25 keV there was an overestimate of 6.6 - 1.9%. CONCLUSIONS: These results show that TG-61 ion chamber dosimetry, used to calibrate the dose output for the cell irradiations, is accurate within approximately 7% for beam energies 25-35 keV. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

SU-E-T-155: Dose Response Curve of EBT2 and EBT3 Radiochromic Films to a Synchrotron-Produced Monochromatic X-Ray Beam.

PURPOSE: This work investigates the dose-response curves of Gafchromic EBT2 and EBT3 radiochromic films using synchrotron-produced monochromatic x-ray beams. These dosimeters are being utilized for dose verification in photoactivated Auger electron therapy at the LSU Center for Advanced Microstructures and Devices (CAMD) synchrotron facility. METHODS: Monochromatic beams of 25, 30 and 35 keV were generated on the tomography beamline at CAMD. Ion chamber depth-dose measurements were used to calculate the dose delivered to films irradiated simultaneously at depths from 0.7 - 8.5 cm in a 10×10×10-cms polymethylmethacrylate phantom. AAPM TG-61 protocol was applied to convert measured ionization into dose. Calibrations of films at 4 MV were obtained for comparison using a Clinac 21 EX radiotherapy accelerator at Mary Bird Perkins Cancer Center. Films were digitized using an Epson 1680 Professional flatbed scanner and analyzed using the optical density (OD) derived from the red channel. RESULTS: For EBT2 film the average sensitivity (OD/dose) at 50, 100, and 200 cGy relative to that for 4-MV x- rays was 1.07, 1.20, and 1.23 for 25, 30, and 35 keV, respectively. For EBT3 film the average sensitivity was within 3 % of unity for all three monochromatic beams. CONCLUSIONS: EBT2 film sensitivity shows strong energy dependence over an energy range of 25 keV - 4 MV. EBT3 film shows weak energy dependence, indicating that it would be the better dosimeter for Auger electron therapy. This research was supported by contract W81XWH-10-1-0005 awarded by The U.S. Army Research Acquisition Activity, 820 Chandler Street, Fort Detrick, MD 21702-5014. This report does not necessarily reflect the position or policy of the Government, and no official endorsement should be inferred.

Dual scattering foil design for poly-energetic electron beams.

The laser wakefield acceleration (LWFA) mechanism can accelerate electrons to energies within the 6-20 MeV range desired for therapy application. However, the energy spectrum of LWFA-generated electrons is broad, on the order of tens of MeV. Using existing laser technology, the therapeutic beam might require a significant energy spread to achieve clinically acceptable dose rates. The purpose of this work was to test the assumption that a scattering foil system designed for a mono-energetic beam would be suitable for a poly-energetic beam with a significant energy spread. Dual scattering foil systems were designed for mono-energetic beams using an existing analytical formalism based on Gaussian multiple-Coulomb scattering theory. The design criterion was to create a flat beam that would be suitable for fields up to 25 x 25 cm2 at 100 cm from the primary scattering foil. Radial planar fluence profiles for poly-energetic beams with energy spreads ranging from 0.5 MeV to 6.5 MeV were calculated using two methods: (a) analytically by summing beam profiles for a range of mono-energetic beams through the scattering foil system, and (b) by Monte Carlo using the EGS/BEAM code. The analytic calculations facilitated fine adjustments to the foil design, and the Monte Carlo calculations enabled us to verify the results of the analytic calculation and to determine the phase-space characteristics of the broadened beam. Results showed that the flatness of the scattered beam is fairly insensitive to the width of the input energy spectrum. Also, results showed that dose calculated by the analytical and Monte Carlo methods agreed very well in the central portion of the beam. Outside the useable field area, the differences between the analytical and Monte Carlo results were small but significant, possibly due to the small angle approximation. However, these did not affect the conclusion that a scattering foil system designed for a mono-energetic beam will be suitable for a poly-energetic beam with the same central energy. Further studies of the dosimetric properties of LWFA-generated electron beams will be done using Monte Carlo methods.

Dose properties of x-ray beams produced by laser-wakefield-accelerated electrons.

Given that laser wakefield acceleration (LWFA) has been demonstrated experimentally to accelerate electron beams to energies beyond 25 MeV, it is reasonable to assess the ability of existing LWFA technology to compete with conventional radiofrequency linear accelerators in producing electron and x-ray beams for external-beam radiotherapy. We present calculations of the dose distributions (off-axis dose profiles and central-axis depth dose) and dose rates of x-ray beams that can be produced from electron beams that are generated using state-of-the-art LWFA. Subsets of an LWFA electron energy distribution were propagated through the treatment head elements (presuming an existing design for an x-ray production target and flattening filter) implemented within the EGSnrc Monte Carlo code. Three x-ray energy configurations (6 MV, 10 MV and 18 MV) were studied, and the energy width deltaE of the electron-beam subsets varied from 0.5 MeV to 12.5 MeV. As deltaE increased from 0.5 MeV to 4.5 MeV, we found that the off-axis and central-axis dose profiles for x-rays were minimally affected (to within about 3%), a result slightly different from prior calculations of electron beams broadened by scattering foils. For deltaE of the order of 12 MeV, the effect on the off-axis profile was of the order of 10%, but the central-axis depth dose was affected by less than 2% for depths in excess of about 5 cm beyond d(max). Although increasing deltaE beyond 6.5 MeV increased the dose rate at d(max) by more than 10 times, the absolute dose rates were about 3 orders of magnitude below those observed for LWFA-based electron beams at comparable energies. For a practical LWFA-based x-ray device, the beam current must be increased by about 4-5 orders of magnitude.

Dose properties of a laser accelerated electron beam and prospects for clinical application.

Laser wakefield acceleration (LWFA) technology has evolved to where it should be evaluated for its potential as a future competitor to existing technology that produces electron and x-ray beams. The purpose of the present work is to investigate the dosimetric properties of an electron beam that should be achievable using existing LWFA technology, and to document the necessary improvements to make radiotherapy application for LWFA viable. This paper first qualitatively reviews the fundamental principles of LWFA and describes a potential design for a 30 cm accelerator chamber containing a gas target. Electron beam energy spectra, upon which our dose calculations are based, were obtained from a uniform energy distribution and from two-dimensional particle-in-cell (2D PIC) simulations. The 2D PIC simulation parameters are consistent with those reported by a previous LWFA experiment. According to the 2D PIC simulations, only approximately 0.3% of the LWFA electrons are emitted with an energy greater than 1 MeV. We studied only the high-energy electrons to determine their potential for clinical electron beams of central energy from 9 to 21 MeV. Each electron beam was broadened and flattened by designing a dual scattering foil system to produce a uniform beam (103%>off-axis ratio>95%) over a 25 x 25 cm2 field. An energy window (deltaE) ranging from 0.5 to 6.5 MeV was selected to study central-axis depth dose, beam flatness, and dose rate. Dose was calculated in water at a 100 cm source-to-surface distance using the EGS/BEAM Monte Carlo algorithm. Calculations showed that the beam flatness was fairly insensitive to deltaE. However, since the falloff of the depth-dose curve (R10-R90) and the dose rate both increase with deltaE, a tradeoff between minimizing (R10-R90) and maximizing dose rate is implied. If deltaE is constrained so that R10-R90 is within 0.5 cm of its value for a monoenergetic beam, the maximum practical dose rate based on 2D PIC is approximately 0.1 Gy min(-1) for a 9 MeV beam and 0.03 Gy min(-1) for a 15 MeV beam. It was concluded that current LWFA technology should allow a table-top terawatt (T3) laser to produce therapeutic electron beams that have acceptable flatness, penetration, and falloff of depth dose; however, the dose rate is still 1%-3% of that which would be acceptable, especially for higher-energy electron beams. Further progress in laser technology, e.g., increasing the pulse repetition rate or number of high energy electrons generated per pulse, is necessary to give dose rates acceptable for electron beams. Future measurements confirming dosimetric calculations are required to substantiate our results. In addition to achieving adequate dose rate, significant engineering developments are needed for this technology to compete with current electron acceleration technology. Also, the functional benefits of LWFA electron beams require further study and evaluation.

Utilization of custom electron bolus in head and neck radiotherapy.

Conventional methods of treating superficial head and neck tumors, such as the wedge pair technique or the use of multiple electron fields of varying energies, can result in excellent tumor control. However, in some cases, these techniques irradiate healthy tissue unnecessarily and/or create hot and cold spots in junction regions, particularly in patients with complex surface contour modification or varying planning target volume (PTV) thickness. The objective of this work is to demonstrate how bolus electron conformal therapy can be used for these patients. Two patients treated using this technique are presented. The first patient was diagnosed with malignant fibrous histiocytoma involving the right ear concha and was treated with 12-MeV electrons. The second patient was diagnosed with acinic cell carcinoma of the left parotid gland and was treated with 20-MeV electrons after having undergone a complete parotidectomy. Each patient's bolus was designed using bolus design tools implemented in an in-house treatment-planning system (TPS). The bolus was fabricated using a computer-controlled milling machine. As part of the quality assurance process to ensure proper fabrication and placement of the bolus, the patients underwent a second computed tomography (CT) scan with the bolus in place. Using that data, the final dose distribution was computed using the Philips Pinnacle(3) TPS (Philips Medical Systems, Andover, MA). Results showed that the 90% isodose surface conformed well to the PTV and that the dose to critical structures such as cord, brain, and lung was well below tolerance limits. Both patients showed no evidence of disease six months post-radiotherapy. In conclusion, electron bolus conformal therapy is a viable option for treating head and neck tumors, particularly patients having a variable thickness PTV or surface anatomy with surgical defects.

A custom three-dimensional electron bolus technique for optimization of postmastectomy irradiation.

PURPOSE: Postmastectomy irradiation (PMI) is a technically complex treatment requiring consideration of the primary tumor location, possible risk of internal mammary node involvement, varying chest wall thicknesses secondary to surgical defects or body habitus, and risk of damaging normal underlying structures. In this report, we describe the application of a customized three-dimensional (3D) electron bolus technique for delivering PMI. METHODS AND MATERIALS: A customized electron bolus was designed using a 3D planning system. Computed tomography (CT) images of each patient were obtained in treatment position and the volume to be treated was identified. The distal surface of the wax bolus matched the skin surface, and the proximal surface was designed to conform to the 90% isodose surface to the distal surface of the planning target volume (PTV). Dose was calculated with a pencil-beam algorithm correcting for patient heterogeneity. The bolus was then fabricated from modeling wax using a computer-controlled milling device. To aid in quality assurance, CT images with the bolus in place were generated and the dose distribution was computed using these images. RESULTS: This technique optimized the dose distribution while minimizing irradiation of normal tissues. The use of a single anterior field eliminated field junction sites. Two patients who benefited from this option are described: one with altered chest wall geometry (congenital pectus excavatum), and one with recurrent disease in the medial chest wall and internal mammary chain (IMC) area. CONCLUSION: The use of custom 3D electron bolus for PMI is an effective method for optimizing dose delivery. The radiation dose distribution is highly conformal, dose heterogeneity is reduced compared to standard techniques in certain suboptimal settings, and excellent immediate outcome is obtained.

Electron pencil-beam redefinition algorithm dose calculations in the presence of heterogeneities.

The electron pencil-beam redefinition algorithm (PBRA) is currently being refined and evaluated for clinical use. The purpose of this work was to evaluate the accuracy of PBRA-calculated dose in the presence of heterogeneities and to benchmark PBRA dose accuracy for future improvements to the algorithm. The PBRA was evaluated using a measured electron beam dose algorithm verification data set developed at The University of Texas M. D. Anderson Cancer Center. The data set consists of measurements made using 9 and 20 MeV beams in a water phantom with air gaps, internal air and bone heterogeneities, and irregular surfaces. Refinements to the PBRA have enhanced the speed of the dose calculations by a factor of approximately 7 compared to speeds previously reported in published data; a 20 MeV, 15 x 15 cm2 field electron-beam dose distribution took approximately 10 minutes to calculate. The PBRA showed better than 4% accuracy in most experiments. However, experiments involving the low-energy (9 MeV) electron beam and irregular surfaces showed dose differences as great as 22%, in albeit a small fractional region. The geometries used in this study, particularly those in the irregular surface experiments, were extreme in the sense that they are not seen clinically. A more appropriate clinical evaluation in the future will involve comparisons to Monte Carlo generated patient dose distributions using actual computed tomography scan data. The present data also serve as a benchmark against which future enhancements to the PBRA can be evaluated.

Correlation between lung fibrosis and radiation therapy dose after concurrent radiation therapy and chemotherapy for limited small cell lung cancer.

PURPOSE: To evaluate the relationship between physician-identified radiographic fibrosis, lung tissue physical density change, and radiation dose after concurrent radiation therapy and chemotherapy for limited small cell lung cancer. MATERIALS AND METHODS: Fibrosis volumes of different severity levels were delineated on computed tomography (CT) images obtained at 1-year follow-up of 21 patients with complete response to concurrent radiation therapy and chemotherapy for limited small cell lung carcinoma. Delivered treatments were reconstructed with a three-dimensional treatment planning system and geometrically registered to the follow-up CT images. Tissue physical density change and radiation dose were computed for each voxel within each fibrosis volume and within normal lung. Patient responses were grouped per radiation and chemotherapy protocol. RESULTS: A significant correlation was noted between fibrosis grade and tissue physical density change and fibrosis grade. For doses less than 30 Gy, the probability of observing fibrosis was less than 2% with conventional fractionation and less than 4% with accelerated fractionation. Physical lung density change also showed a threshold of 30-35 Gy. For doses of 30-55 Gy and cisplatin and etoposide (PE) chemotherapy, fibrosis probability was 2.0 times greater for accelerated fractionation compared with conventional fractionation (P < .005) and was correlated to increasing dose for both fractionation schedules. CONCLUSION: Lung tissue physical density changes correlated well with fibrosis incidence, and both increased with increasing dose greater than a threshold of 30-35 Gy. With concurrent PE chemotherapy, fibrosis probability was twice as great with accelerated fractionation as with once-daily fractionation.

Modelling pencil-beam divergence with the electron pencil-beam redefinition algorithm.

The electron pencil-beam redefinition algorithm (PBRA), which is used to calculate electron beam dose distributions, assumes that the virtual source of each pencil beam is identical to that of the broad beam incident on the patient. In the present work, a virtual source specific for each pencil beam is modelled by including the source distance as a pencil-beam parameter to be redefined with depth. To incorporate a variable pencil-beam source distance parameter, the transport equation was reformulated to explicitly model divergence resulting in the algorithm divPBRA. Allowing the virtual source position to vary with individual pencil beams is expected to better model the effects of heterogeneities on local electron fluence divergence (or convergence). Selected experiments from a measured data set developed at The University of Texas M D Anderson Cancer Center were used to evaluate the accuracy of the dose calculated using divPBRA. Results of the calculation showed that the theory accurately predicted the virtual source position in regions of side-scatter equilibrium and predicted reasonable virtual source positions in regions lacking side-scatter equilibrium (i.e. penumbra and in the vicinity and shadow of internal heterogeneities). Results of the evaluation showed the dose accuracy of divPBRA to be marginally better to that of PBRA, except in regions of extremely sharp dose perturbations, where the divPBRA calculations were significantly greater than the measured data. Dose calculations using divPBRA took 45% longer than those using PBRA. Therefore, we concluded that divPBRA offers no significant advantage over PBRA for the purposes of clinical treatment planning. However, the results were promising and divPBRA might prove useful if further modelling were to include large-angle scattering, low-energy delta rays and brehmsstrahlung.

Verification of tangential breast treatment dose calculations in a commercial 3D treatment planning system.

The accuracy of the photon convolution/superposition dose algorithm employed in a commercial radiation treatment planning system was evaluated for conditions simulating tangential breast treatment. A breast phantom was fabricated from machineable wax and placed on the chest wall of an anthropomorphic phantom. Radiographic film was used to measure the dose distribution at the axial midplane of the breast phantom. Subsequently, thermoluminescent dosimeters (TLDs) were used to measure the dose at four points within the midplane to validate the accuracy of the film dosimetry. Film measurements were compared with calculations performed using the treatment planning system for four types of treatment: optimized wedged beams at 6 and 18 MV and two-dimensional compensated beams at 6 and 18 MV. Both the film- and TLD-measured doses had a precision of approximately 0.6%. The film-measured doses were approximately 1.5% lower than the TLD-measured doses, ranging from 0-3% at 6 MV and 0.5-1% at 18 MV. Such results placed a high level of confidence in the accuracy and precision of the film data. The measured and calculated doses agreed to within +/-3% for both the film and TLD measurements throughout the midplane exclusive of areas not having charged particle equilibrium. Good agreement was not expected within these regions due to the limitations in both film dosimetry and the dose-calculation algorithm. These results indicated that the treatment planning system calculates doses at the midplane with clinically acceptable accuracy in conditions simulating tangential breast treatment.

A measured data set for evaluating electron-beam dose algorithms.

The purpose of this work was to develop an electron-beam dose algorithm verification data set of high precision and accuracy. Phantom geometries and treatment-beam configurations used in this study were similar to those in a subset of the verification data set produced by the Electron Collaborative Working Group (ECWG). Measurement techniques and quality-control measures were utilized in developing the data set to minimize systematic errors inherent in the ECWG data set. All measurements were made in water with p-type diode detectors and using a Wellhöfer dosimetry system. The 9 and 20 MeV, 15 x 15 cm2 beams from a single linear accelerator composed the treatment beams. Measurements were made in water at 100 and 110 cm source-to-surface distances. Irregular surface measurements included a "stepped surface" and a "nose-shaped surface." Internal heterogeneity measurements were made for bone and air cavities in differing orientations. Confidence in the accuracy of the measured data set was reinforced by a comparison with Monte Carlo (MC)-calculated dose distributions. The MC-calculated dose distributions were generated using the OMEGA/BEAM code to explicitly model the accelerator and phantom geometries of the measured data set. The precision of the measured data, estimated from multiple measurements, was better than 0.5% in regions of low-dose gradients. In general, the agreement between the measured data and the MC-calculated data was within 2%. The quality of the data set was superior to that of the ECWG data set, and should allow for a more accurate evaluation of an electron beam dose algorithm. The data set will be made publicly available from the Department of Radiation Physics at The University of Texas M. D. Anderson Cancer Center.

The effect of scattering foil parameters on electron-beam Monte Carlo calculations.

The sensitivity of electron-beam Monte Carlo dose calculations to scattering foil geometrical parameters is described. A method for resolving discrepancies between Monte Carlo calculation and measured data in a systematic manner is also described. As part of a project to investigate the utility of Monte Carlo methods for calculating data required for commissioning electron beams, a large discrepancy between measured and calculated 20 MeV cross-beam profiles for the largest field size was found. It was hypothesized that the discrepancy was due to incorrect input data and that better agreement between calculation and measurement could be achieved with small changes in the scattering foil system geometry. Four parameters describing the foil system were varied individually until better agreement between calculation and measurement was achieved, and the percentage change in the parameter was tabulated as an indication of the sensitivity of the model to that parameter. The accelerator model for the 20 MeV electron beam was most sensitive to the distance between the scattering foils and to a slightly lesser extent, to the width of the shaped secondary scattering foil. Changes to the primary or secondary foil thickness also significantly modified the falloff and bremsstrahlung component of depth dose, which was unacceptable for the present case. Therefore, the distance between the two scattering foils was changed in our calculations, which the manufacturer later confirmed was indeed the case. For 6 and 12 MeV electron beams, the change was not nearly as significant. It was concluded that Monte Carlo calculations for higher-energy beams and larger field sizes are most sensitive to the geometric configuration of the scattering foil system and should therefore be calculated first to help verify the accuracy of the geometric information.

A comparison of 18-MV and 6-MV treatment plans using 3D dose calculation with and without heterogeneity correction.

Homogeneity of the dose distribution in irradiation of the intact breast for stage I and II cancers is an important factor, particularly for larger breasts. In the present work, we have studied dose homogeneity for 6- and 18-MV treatment plans in 10 patients, typically with larger breasts. For each patient, 6 3-dimensional (3D) dose distributions were calculated using patient computed tomography data and the ADAC Pinnacle3 treatment planning system. First, a dose distribution was calculated, assuming the patient was water, with the 6-MV beam parameters used to treat the patient. Second, the calculation was repeated using the actual patient anatomy. Comparison of these 2 distributions showed how patient heterogeneity affected dose. Third, individual beam weights were optimized, and the dose calculation was repeated. Each of these 3 dose calculations was repeated at 18 MV. Results showed that: (1) at 6 MV, the ratio of mean dose in the target volume calculated with heterogeneity considerations to that without was 1.014 +/- 0.006, and the ratio of the standard deviation of dose in the target volume was 0.919 +/- 0.042; (2) at 18 MV, the ratio of mean dose to the target volume calculated with heterogeneity considerations to that without was 1.001 +/- 0.005, and the ratio of the standard deviation of dose in the target volume was 1.15 +/- 0.09; and (3) the dose homogeneity, measured by the standard deviation of the dose distribution in the target volume, was 25% less for the 18-MV treatment plan for patients with breast volumes greater than 1600 cm3. We conclude that: (1) 3D, heterogeneity-corrected dose calculation is necessary to fairly evaluate any advantage of 18 MV over 6 MV; (2) excluding the dose buildup region, 18 MV produces a significantly more homogeneous dose distribution for breast volumes greater than 1600 cm3; and (3) when prescribing dose using heterogeneity-corrected dose distributions, dose prescriptions should be increased by 1.5% at 6 MV, but no increase is needed for 18 MV.

Effect of using an initial polyenergetic spectrum with the pencil-beam redefinition algorithm for electron-dose calculations in water.

This work compares the accuracy of dose distributions computed using an incident polyenergetic (PE) spectrum and a monoenergetic (ME) spectrum in the electron pencil-beam redefinition algorithm (PBRA). It also compares the times required to compute PE and ME dose distributions. This has been accomplished by comparing PBRA calculated dose distributions with measured dose distributions in water from the National Cancer Institute electron collaborative working group (ECWG) data set. Comparisons are made at 9 and 20 MeV for the 15 x 15 cm2 and 6 x 6 cm2 fields at 100- and 110-cm SSD. The incident PE spectrum is determined by a process that best matches the weighted sum of monoenergetic PBRA calculated central-axis depth doses, each calculated with the energy correction factor, C(E), equal to unity, to the ECWG measured depth dose for the 15 x 15 cm2 field at 100-cm SSD. C(E) is determined by a least square fit to central-axis depth dose for the PE PBRA. Results show that both the PE and ME PBRA accurately calculate central-axis depth dose at 100-cm SSD for the 6 x 6 cm2 and 15 x 15 cm2 field sizes and also at 110-cm SSD for the 15 x 15 cm2 field size. In the penumbral region, the PE PBRA calculation is significantly more accurate than the ME PBRA for all measurement conditions. Both the PE and ME PBRA exhibit significant dose errors (> 4%) outside the penumbra at shallow depths for the 6 x 6 cm2 and 15 x 15 cm2 fields at 100-cm SSD and inside the penumbra at shallow depths for the 6 x 6 cm2 field size at 110-cm SSD. These errors are attributed to the fact that the PBRA does not model collimator scatter in the incident beam. Calculation times for the PE PBRA are approximately 70%-140% greater than those for the ME PBRA. We conclude that the PE PBRA is significantly more accurate than the ME PBRA, and we believe that the increase in time for the PE PBRA will not significantly impact the clinical utility of the PBRA.

Multiple scattering theory for total skin electron beam design.

The purpose of this manuscript is to describe a method for designing a broad beam of electrons suitable for total skin electron irradiation (TSEI). A theoretical model of a TSEI beam from a linear accelerator with a dual scattering system has been developed. The model uses Fermi-Eyges theory to predict the planar fluence of the electron beam after it has passed through various materials between the source and the treatment plane, which includes scattering foils, monitor chamber, air, and a plastic diffusing plate. Unique to this model is its accounting for removal of the tails of the electron beam profile as it passes through the primary x-ray jaws. A method for calculating the planar fluence profile for an obliquely incident beam is also described. Off-axis beam profiles and percentage depth doses are measured with ion chambers, film, and thermoluminescent dosimeters (TLD). The measured data show that the theoretical model can accurately predict beam energy and planar fluence of the electron beam at normal and oblique incidence. The agreement at oblique angles is not quite as good but is sufficiently accurate to be of predictive value when deciding on the optimal angles for the clinical TSEI beams. The advantage of our calculational approach for designing a TSEI beam is that many different beam configurations can be tested without having to perform time-consuming measurements. Suboptimal configurations can be quickly dismissed, and the predicted optimal solution should be very close to satisfying the clinical specifications.

Dosimetric evaluation of lead and tungsten eye shields in electron beam treatment.

PURPOSE: The purpose of this study is to report that commercially available eye shields (designed for orthovoltage x-rays) are inadequate to protect the ocular structures from penetrating electrons for electron beam energies equal to or greater than 6 MeV. Therefore, a prototype medium size tungsten eye shield was designed and fabricated. The advantages of the tungsten eye shield over lead are discussed. METHODS AND MATERIALS: Electron beams (6-9 MeV) are often used to irradiate eyelid tumors to curative doses. Eye shields can be placed under the eyelids to protect the globe. Film and thermoluminescent dosimeters (TLDs) were used within a specially constructed polystyrene eye phantom to determine the effectiveness of various commercially available internal eye shields (designed for orthovoltage x-rays). The same procedures were used to evaluate a prototype medium size tungsten eye shield (2.8 mm thick), which was designed and fabricated for protection of the globe from penetrating electrons for electron beam energy equal to 9 MeV. A mini-TLD was used to measure the dose enhancement due to electrons backscattered off the tungsten eye shield, both with or without a dental acrylic coating that is required to reduce discomfort, permit sterilization of the shield, and reduce the dose contribution from backscattered electrons. RESULTS: Transmission of a 6 MeV electron beam through a 1.7 mm thick lead eye shield was found to be 50% on the surface (cornea) of the phantom and 27% at a depth of 6 mm (lens). The thickness of lead required to stop 6-9 MeV electron beams is impractical. In place of lead, a prototype medium size tungsten eye shield was made. For 6 to 9 MeV electrons, the doses measured on the surface (cornea) and at 6 mm (lens) and 21 mm (retina) depths were all less than 5% of the maximum dose of the open field (4 x 4 cm). Electrons backscattered off a tungsten eye shield without acrylic coating increased the lid dose from 85 to 123% at 6 MeV and 87 to 119% at 9 MeV. For the tungsten eye shield coated with 2-3 mm of dental acrylic, the lid dose was increased from 85 to 98.5% at 6 MeV and 86 to 106% at 9 MeV. CONCLUSION: Commercially available eye shields were evaluated and found to be clearly inadequate to protect the ocular structures for electron beam energies equal to or greater than 6 MeV. A tungsten eye shield has been found to provide adequate protection for electrons up to 9 MeV. The increase in lid dose due to electrons backscattered off the tungsten eye shield should be considered in the dose prescription. A minimum thickness of 2 mm dental acrylic on the beam entrance surface of the tungsten eye shield was found to reduce the backscattered electron effect to acceptable levels.