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.

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 in bone and tissue near bone-tissue interface from electron beam.

This work has quantitatively studied the variation of dose both within bone and in unit density tissue near bone-tissue interfaces. Dose upstream of a bone-tissue interface is increased because of an increase in the backscattered electrons from the bone. The magnitude of this effect was measured using a thin parallel-plate ionization chamber upstream of a polymethyl methacrylate (PMMA)-hard bone interface. The electron backscatter factor (EBF) increased rapidly with bone thickness until a full EBF was achieved. This occurred at approximately 3.5 mm at 2 MeV and 6 mm at 13.1 MeV. The full EBF at the interface ranged from approximately 1.018 at 13.1 MeV to 1.05 at 2 MeV. It was also observed that the EBF had a dependence on the energy spectrum at the interface. The penetration of the backscattered electrons in the upstream direction of PMMA was also measured. The dose penetration fell off rapidly in the upstream direction of the interface. Dose enhancement to unit density tissue in bone was measured for an electron beam by placing thermoluminescent dosimeters (TLDs) in a PMMA-bone-PMMA phantom. The maximum dose enhancement in bone was approximately 7% of the maximum dose in water. However, the pencil-beam algorithm of Hogstrom et al. predicted an increase of only 1%, primarily owing to the inverse-square correction. Film was also used to measure the dose enhancement in bone. The film plane was aligned either perpendicular or parallel to the central axis of the beam. The film data indicated that the maximum dose enhancement in bone was approximately 8% for the former film alignment (which was similarly predicted by the TLD measurements) and 13% for the latter film alignment. These results confirm that the X ray film is not suitable to be irritated "edge on" in an inhomogeneous phantom without making perturbation corrections resulting from the film acting as a long narrow inhomogeneous cavity within the bone. In addition, the results give the radiotherapist a basis for clinical judgment when electron beams are used to treat lesions behind bone or near bony structures. We feel these data enhance the ability to recognize the shortcomings of the current dose calculation algorithm used clinically.

Evaluation of a total scalp electron irradiation technique.

A dosimetric evaluation of a total scalp electron-beam irradiation technique that uses six stationary fields was performed. The initial treatment plan specified a) that there be a 3-mm gap between abutted fields and b) that the field junctions be shifted 1 cm after 50% of the prescribed dose had been delivered. Dosimetric measurements were made at the scalp surface, scalp-skull interface, and the skull-brain interface in an anthropomorphic head phantom using both film and thermoluminescent dosimeters (TLD-100). The measurements showed that the initial technique yields areas of increased and decreased dose ranging from -50% to +70% in the region of the field junctions. To reduce regions of nonuniform dose, the treatment protocol was changed by eliminating the gap between the coronal borders of abutted fields and by increasing the field shift from 1 cm to 2 cm for all borders. Subsequent measurements showed that these changes in treatment protocol resulted in a significantly more uniform dose to the scalp and decreased variation of doses near field junctions (-10% to +50%).

Dosimetry characteristics of metallic cones for intraoperative radiotherapy.

Dosimetry data were obtained on the first dedicated linear accelerator of its type designed for electron intraoperative radiotherapy (IORT) within an operating room. The linear accelerator uses a high dose rate, 9 Gy.min-1, to reduce the treatment time. Its chrome-plated brass treatment cones, designed with straight ends and 22.5 degrees beveled ends, are not mechanically attached to the collimator head, but are aligned using a laser projection system. Dosimetry measurements were made for each combination of energy (6, 9, 12, 15, and 16 MeV), cone size (diameters range from 5 to 12 cm), and cone type (22.5 degrees beveled or straight). From these data, depth-dose curves, cone output, and air-gap correction factors were generated that allow the calculation of the monitor setting for delivering a prescribed dose at any depth for any irradiation condition (energy, cone, air gap). Isodose data were measured for every cone using film in a solid water phantom. Scatter off the inside wall of the cone resulted in peripheral dose horns near the surface that were energy and cone dependent, being as large as 120%.

Dosimetric evaluation of total scalp irradiation using a lateral electron-photon technique.

PURPOSE: To evaluate the radiation dosimetry of a new technique for total scalp irradiation. METHODS AND MATERIALS: A treatment technique described by Akazawa (1989) has been studied. During each fraction, two electron and two photon fields are treated. While most of the lateral scalp is treated with the electron fields, a rind of scalp close to the midsagittal plane is irradiated by parallel-opposed lateral photon fields. A wax bolus is used to build up skin dose and to protect the brain from electron dose. The dose distribution and dose-volume histograms were evaluated for different field arrangements using a 3-dimensional treatment planning system. After modifying the technique, in-vivo thermoluminescent dosimetry were used to evaluate the dose distributions for the first two patients. RESULTS: To compensate for the lack of dose from the opposed photon field at the junction, the technique was modified using overlapped fields instead of abutting fields. A field overlap of 3 to 4 mm between the electron and photon fields was found optimal. When used with the field junction shift of 1 cm midway through the treatment, this scheme resulted in a dose uniformity of -5% to +15% of the prescribed dose in the region of abutment. Results of the 3-dimensional dose calculation were supported by in-vivo thermoluminescent dosimetry on two patients. CONCLUSION: On the basis of computer dose calculations and in-vivo dosimetry. Akazawa's technique for scalp irradiation can be improved by using a 3 to 4 mm overlap of electron and photon fields. This modified technique is practical and produces clinically acceptable dosimetry.

Technique for verifying treatment fields using portal images with diagnostic quality.

The image quality of portal films for megavoltage photon beams, when using the double-exposure technique, is poor compared to diagnostic quality, X ray images. A technique is described to record on a single film a megavoltage portal image superimposed upon a diagnostic X ray image, which provides the radiotherapist with "diagnostic quality" portal images. The technique uses a commercially available X ray tube mounted on the head of a 60Co unit. The alignment procedure, which uses a leveling device to ensure that the X ray focal spot and 60Co source are at the same location for each exposure, is confirmed by registering on film the image of an alignment marker. An evaluation of film-screen combination showed therapy verification film in a rare earth intensifying screen cassette to be best suited for this technique. The relationship between off-axis dose and the penumbral region of the portal image has been evaluated and should be useful in the interpretation of portal verification film relative to the treatment volume.

Improving the therapeutic ratio of craniospinal irradiation in medulloblastoma.

Radiation therapy delivered to the entire cerebrospinal axis is indicated for a number of pediatric brain tumors, especially medulloblastoma. Improved radiotherapy techniques have changed the near fatal prognosis for children with medulloblastoma to a 50%, 5-year survival. Nevertheless, the treatment results in substantial acute toxicity, and many survivors have serious sequelae. Further improvement in survival with optimal surgery and radiotherapy is not expected unless chemotherapy is added. Refinements in radiotherapy technique, however, can improve the therapeutic ratio of the treatment by lowering its side effects. In the last year children who required craniospinal irradiation at M. D. Anderson Hospital were treated with 6 MV photons to the brain and primary tumor and with 15-17 MeV electrons to the spinal canal. The elective dose to the whole brain was 30 Gy in 17 fractions and 30 Gy in 20 fractions to the spine. The primary tumor received an additional 20-25 Gy. An electron-beam dose distribution was drawn on a computerized tomography (CT) reconstructed sagittal plane. The electron energy was selected so that the 90% isodose line was at least 3 mm anterior to the cord after correction for bone heterogeneity. The treatment was well tolerated in the first five patients. It is projected that the current technique will cause fewer late effects and improve the tolerance to chemotherapy.

Water bolus for electron irradiation of the ear canal.

PURPOSE: To demonstrate that water bolus in the external ear can decrease the dose inhomogeneity caused by auricular surface irregularities when the ear is in an electron-beam field. METHODS AND MATERIALS: Three-dimensional (3D) dose distributions with and without water bolus in the external ear were calculated for a representative patient. The electron dose calculations were made using the Hogstrom pencil beam algorithm as implemented in 3D by Starkschall. To demonstrate the use of water bolus in the ear clinically, the case of a patient with squamous carcinoma of the concha who was treated with electrons is presented. RESULTS: Water bolus markedly lessens the dose heterogeneity caused by the surface irregularities of the ear and the air in the external auditory canal. In the test case, the maximum dose was reduced by 25% using this technique. CONCLUSION: When the ear is in an electron beam field, warm water should be placed in the external auditory canal and concha. This maneuver may reduce the incidence of auricular complications that occur after electron-beam therapy.

Dosimetric evaluation of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction.

The accuracy of a pencil-beam algorithm for electrons employing a two-dimensional heterogeneity correction is demonstrated by comparing calculation with measurement. Ionization measurements have been made in a water phantom for a variety of non-standard geometries. Geometries to demonstrate the effect of an extended treatment distance, a sloping skin surface, and an irregular skin surface have been selected. Additionally, thermoluminescent dosimeters have been used to measure distributions in tissue-substitute phantoms, which were designed from individual patient computerized tomographic scans. Three patient scans have been selected: (1) diffuse hystiocytic lymphoma of the left buccal mucosa and retromolar trigone; (2) squamous cell carcinoma of the nose at the columnella ; and (3) carcinoma of the maxillary antrum. Results demonstrate the algorithm's ability to simultaneously account for the isodose shifting as a result of internal heterogeneities and for sidescatter non-equilibrium caused by lateral discontinuities of the skin surface and internal anatomy. The algorithm is shown to generally be accurate to within +/- 4% in the treatment volume or +/- 4 mm in regions of sharp dose gradients as found in the penumbra and distal edge of the beam. Examples of greater disagreement are shown and their physical interpretation discussed.

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.

Design of metallic electron beam cones for an intraoperative therapy linear accelerator.

A set of circular collimators and treatment cones from 5 to 12 cm diameter has been designed for an intraoperative accelerator (6-18 MeV) that has an optical docking system. Electron beam scattering theory has been used to minimize their weight while minimizing leakage radiation. Both acrylic and brass were evaluated as possible materials; however, because of substantial electron leakage through the lateral cone wall for acrylic, we have concluded that 2 mm thick brass walls are more desirable than acrylic walls. At 18 MeV, isodose measurements beneath the cones showed hot spots as great as 120% for both materials. The placement and dimension of an internal trimmer ring inside the brass cone was studied as a method for reducing the hot spots, and it was found this could only be accomplished at the expense of decreasing coverage of the 90% isodose surface. The effects of 1 degree cone misalignment on the dose distribution has been studied and found to generate changes of less than 5% in the dose and 3 mm in position of the 90% isodose surface. In a study of the contribution of the cone and its matching collimator assembly to x-ray room leakage, it was noted that although the treatment cone had a negligible contribution, the upper annuli of the upper collimator assembly contributed as much as 80% of the leakage at 16 MeV for the 5-cm cone.

Computer-aided design and fabrication of an electron bolus for treatment of the paraspinal muscles.

PURPOSE: Demonstrate the technology for the design, fabrication, and verification of an electron bolus used in the preoperative irradiation of a mesenchymal chondrosarcoma in the paraspinal muscle region (T8-T12), in which the target volume overlay a portion of the spinal cord, both lungs, and the right kidney. METHODS AND MATERIALS: An electron-bolus design algorithm implemented on a three dimensional (3D) radiotherapy treatment planning system designed the bolus to yield a dose distribution that met physician-specified clinical criteria. Electron doses were calculated using a 3D electron pencil-beam dose algorithm. A computer-driven milling machine fabricated the bolus from modeling wax, machining both the patient surface and the beam surface of the bolus. Verification of the bolus fabrication was achieved by repeating the patient's computed tomography (CT) scan with the fabricated bolus in place (directly on the posterior surface of the prone patient) and then recalculating the patient's dose distribution using the 3D radiotherapy treatment planning system. RESULTS: A treatment plan using a 17-MeV posterior electron field with a bolus delivered a superior dose distribution to the patient than did the same plan without a bolus. The bolus plan delivered a slightly increased dose to the target volume as a result of a slightly broader range of doses. There were significant reductions in dose to critical structures (cord, lungs, and kidney) in the bolus plan, as evidenced by dose-volume histograms (DVHs). The patient dose distribution, calculated using CT scan data with the fabricated bolus, showed no significant differences from the planned dose distribution. CONCLUSIONS: A bolus can provide considerable sparing of normal tissues when using a posterior electron beam to irradiate the paraspinal muscles. Bolus design and fabrication using the tools described in this paper are adequate for patient treatment. CT imaging of the patient with the bolus in place followed by calculation of the patient's dose distribution demonstrated a useful method for verification of the bolus design and fabrication process.

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.

Pencil-beam redefinition algorithm for electron dose distributions.

A pencil-beam redefinition algorithm has been developed for the calculation of electron-beam dose distributions on a three-dimensional grid utilizing 3-D inhomogeneity correction. The concept of redefinition was first used for both fixed and arced electron beams by Hogstrom et al. but was limited to a single redefinition. The success of those works stimulated the development of the pencil-beam redefinition algorithm, the aim of which is to solve the dosimetry problems presented by deep inhomogeneities through development of a model that redefines the pencil beams continuously with depth. This type of algorithm was developed independently by Storchi and Huizenga who termed it the "moments method." Such a pencil beam within the patient is characterized by a complex angular distribution, which is approximated by a Gaussian distribution having the same first three moments as the actual distribution. Three physical quantities required for dose calculation and subsequent radiation transport--namely planar fluence, mean direction, and root-mean-square spread about the mean direction--are obtained from these moments. The primary difference between the moments method and the redefinition algorithm is that the latter subdivides the pencil beams into multiple energy bins. The algorithm then becomes a macroscopic method for transporting the complete phase space of the beam and allows the calculation of physical quantities such as fluence, dose, and energy distribution. Comparison of calculated dose distributions with measured dose distributions for a homogeneous water phantom, and for phantoms with inhomogeneities deep relative to the surface, show agreement superior to that achieved with the pencil-beam algorithm of Hogstrom et al. in the penumbral region and beneath the edges of air and bone inhomogeneities. The accuracy of the redefinition algorithm is within 4% and appears sufficient for clinical use, and the algorithm is structured for further expansion of the physical model if required for site-specific treatment planning problems.

Pion in vivo dosimetry using aluminum activation.

The method of aluminum activation to 24Na has been shown feasible as a high-LET, in vivo dosimeter for clinical pion beams at the Clinton P. Anderson Meson Physics Facility in Los Alamos. A 3 X 3 in. phi NaI (Tl) well detector measures the 24Na activity following exposure by windowing the 2.75 MeV photopeak. Calculations of the 24Na activity agree well with experiment if one assumes a production ratio of 0.075 24Na/stopped pi- in aluminum, and an in-flight cross section of 26 mb. The activity is produced primarily by stopping pions although 15-25% of the activity is the result of neutrons. Thus, the induced activation is a good measure of high-LET dose. By comparison with high-LET dose measured by a 7.6 mu silicon detector and a Rossi chamber, the amount of high-LET dose per activation is found to be 1.35 X 10(-6) rad/(24Na/gm Al). A clinical setup has been installed and a sample patient measurement is compared with high-LET dose calculated by treatment planning programs.

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.

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.

Neutron leakage measurements from a medical linear accelerator.

The McCall method has been used to measure neutron leakage from the Mevatron 77, 18- and 15-MV photon beams. Gold foil activation has been used employing a beta counting technique for the 18-MV beam and a gamma counting technique for both the 18- and 15-MV beam. The two counting techniques were used to evaluate their relative merit. The measurements were made at various locations in the patient-treatment plane for different field sizes. The results show that the thermal-neutron dose equivalent contributes only about 1%-2% of the total neutron dose equivalent. At 100 cm, the neutron dose equivalent for the 18-MV beam is approximately six times that of the 15-MV beam, slightly exceeding the 0.1% of the useful beam criteria used by some of the regulatory agencies. In light of the uncertainty in fluence to dose equivalent conversion factors, the increased dose equivalent above 0.1% is insignificant.