Biomedical subjects

J A Antolak

Publications and source records attributed to J A Antolak.

17 recordsLinked to original sources

PubMed2003

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.

Carcinoma, Acinar Cell↗

PubMed2001

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.

Adenocarcinoma↗

PubMed2001

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.

Algorithms↗

PubMed2001

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.

Biophysical Phenomena↗

PubMed2001

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.

Absorptiometry, Photon↗

PubMed2001

Phase I study of concomitant gemcitabine and IMRT for patients with unresectable adenocarcinoma of the pancreatic head.

PURPOSE: We hypothesized that dynamic intensity-modulated radiotherapy (IMRT) would protect normal tissues enough to allow the escalation of either the gemcitabine or radiotherapy dose in unresectable pancreatic cancer patients. METHODS AND MATERIALS: The trial was designed to build on a previous phase I trial that determined the maximum tolerated dose (MTD) of gemcitabine (350 mg/m2) with concurrent radiotherapy (30 Gy/10 fractions). Only patients with unresectable disease based on established criteria were eligible. The plan was to alternate escalating the radiation dose by 3 Gy and the gemcitabine dose by 50 mg/m2. The starting dose of gemcitabine was 350 mg/m2 and 33 Gy/11 fractions of IMRT to the regional lymphatics and primary disease. The NCI Common Toxicity Criteria were used for dose-limiting toxicity (DLT). RESULTS: All three patients in the first cohort treated suffered DLT. Therefore, a second cohort of patients received a lower gemcitabine dose (250 mg/m2). Both patients treated at this dose level experienced DLT. The DLTs were all due to myelosuppression and upper gastrointestinal toxicity. All patients required a gemcitabine dose reduction. Also, four patients required hospital admission for supportive care, while the fifth died of an unrelated cause shortly after completing therapy. The trial was then closed due to excessive toxicity. CONCLUSION: Hypofractionated dynamic IMRT to the primary site and regional lymphatics did not permit escalation of either the radiation or gemcitabine dose. Dynamic IMRT requires further investigation before it can be applied to toxic combinations of chemotherapy and radiation in the upper abdomen.

Adenocarcinoma↗

PubMed2000

Preliminary results of a randomized radiotherapy dose-escalation study comparing 70 Gy with 78 Gy for prostate cancer.

PURPOSE: To determine the effect of radiotherapy dose on prostate cancer patient outcome and biopsy positivity in a phase III trial. PATIENTS AND METHODS: A total of 305 stage T1 through T3 patients were randomized to receive 70 Gy or 78 Gy of external-beam radiotherapy between 1993 and 1998. Of these, 301 were assessable; stratification was based on pretreatment prostate-specific antigen level (PSA). Dose was prescribed to the isocenter at 2 Gy per fraction. All patients underwent planning pelvic computed tomography scan to confirm prostate position. Treatment failure was defined as an increasing PSA on three consecutive follow-up visits or the initiation of salvage treatment. Median follow-up was 40 months. RESULTS: One hundred fifty patients were randomized to the 70-Gy arm and 151 to the 78-Gy arm. The difference in freedom from biochemical and/or disease failure (FFF) rates of 69% and 79% for the 70-Gy and 78-Gy groups, respectively, at 5 years was marginally significant (log-rank P: =.058). Multiple-covariate Cox proportional hazards regression showed that the study randomization was an independent correlate of FFF, along with pretreatment PSA, Gleason score, and stage. The patients who benefited most from the 8-Gy dose escalation were those with a pretreatment PSA of more than 10 ng/mL; 5-year FFF rates were 48% and 75% (P: =.011) for the 70-Gy and 78-Gy arms, respectively. There was no difference between the arms ( approximately 80% 5-year FFF) when the pretreatment PSA was < or = 10 ng/mL. CONCLUSION: A modest dose increase of 8 Gy using conformal radiotherapy resulted in a substantial improvement in prostate cancer FFF rates for patients with a pretreatment PSA of more than 10 ng/mL. These findings document that local persistence of prostate cancer in intermediate- to high-risk patients is a major problem when doses of 70 Gy or less are used.

Disease-Free Survival↗

PubMed1999

Planning target volumes for radiotherapy: how much margin is needed?

PURPOSE: The radiotherapy planning target volume (PTV) encloses the clinical target volume (CTV) with anisotropic margins to account for possible uncertainties in beam alignment, patient positioning, organ motion, and organ deformation. Ideally, the CTV-PTV margin should be determined solely by the magnitudes of the uncertainties involved. In practice, the clinician usually also considers doses to abutting healthy tissues when deciding on the size of the CTV-PTV margin. This study calculates the ideal size of the CTV-PTV margin when only physical position uncertainties are considered. METHODS AND MATERIALS: The position of the CTV for any treatment is assumed to be described by independent Gaussian distributions in each of the three Cartesian directions. Three strategies for choosing a CTV-PTV margin are analyzed. The CTV-PTV margin can be based on: 1. the probability that the CTV is completely enclosed by the PTV; 2. the probability that the projection of the CTV in the beam's eye view (BEV) is completely enclosed by the projection of the PTV in the BEV; and 3. the probability that a point on the edge of the CTV is within the PTV. Cumulative probability distributions are derived for each of the above strategies. RESULTS: Expansion of the CTV by 1 standard deviation (SD) in each direction results in the CTV being entirely enclosed within the PTV 24% of the time; the BEV projection of the CTV is enclosed within the BEV projection of the PTV 39% of the time; and a point on the edge of the CTV is within the PTV 84% of the time. To have the CTV enclosed entirely within the PTV 95% of the time requires a margin of 2.8 SD. For the BEV projection of the CTV to be within the BEV projection of the PTV 95% of the time requires a margin of 2.45 SD. To have any point on the surface of the CTV be within the PTV 95% of the time requires a margin of 1.65 SD. CONCLUSION: In the first two strategies for selecting a margin, the probability of finding the CTV within the PTV is unrelated to dose variations in the CTV. In the third strategy, the specified confidence limit is correlated with the minimum target dose. We recommend that the PTV be calculated from the CTV using a margin of 1.65 SD in each direction. This gives a minimum CTV dose that is greater than 95% of the minimum PTV dose. Additional sparing of adjoining healthy structures should be accomplished by modifying beam portals, rather than adjusting the PTV. Then, the dose distributions more accurately reflect the clinical compromise between treating the tumor and sparing the patient.

Algorithms↗

PubMed1999

Three-dimensional treatment planning for central lymphatic irradiation.

The purpose of this study was to investigate the applicability of 3-dimensional (3D) treatment planning for central lymphatic irradiation (CLI). CLI requires more than 1 course of treatment with large, highly blocked, overlapping beams, and careful planning is required to ensure that such treatments are delivered safely and effectively. Three patients were selected for this study. Each patient completed at least 1 course of radiation therapy for CLI and was scheduled to receive irradiation to an adjacent area with overlapping beams. Two treatment plans were generated for each patient: a standard, 2-dimensional (2D) treatment plan and a 3D treatment plan designed to mimic the standard plan, taking advantage of unique 3D features. The time required to complete the treatment plans and differences in the treatment planning processes were noted. The time required to generate a 3D treatment plan was approximately the same as the time required to generate a standard 2D treatment plan; however, the 3D planning process required less redundancy of data entry than the 2D process. The 3D treatment plan was qualitatively similar to the standard 2D treatment plan; however, differences in beam penumbra and beam junctions were noted, and are most likely due to differences in the dose-calculation models used in these 2 treatment planning systems. Dose-volume histograms (DVHs) were calculated for the spinal cord and were found to be useful to the physicians for quickly and accurately evaluating the presence or absence of hot spots in the junction region. 3D treatment-planning has some advantages over 2D treatment planning for CLI; the main advantage of the 3D treatment plan is that it provides a single plan for each patient with multiple views of the data, including different planar cross-sections and DVHs. For the 2D system, a separate plan was generated for each view, requiring redundant data entry. The quality of the output of the 3D treatment plans is superior to that of 2D treatment plans, but the clinical utility is about the same. Currently, the time required for 2D and 3D treatment plans is similar. However, as dosimetrists become more familiar with 3D treatment planning systems, we expect this familiarity and regularity of use to translate into a significant time advantage.

Humans↗

PubMed1998

Prostate target volume variations during a course of radiotherapy.

PURPOSE: The purpose of this study was to measure the mobility of the clinical target volume (CTV) in prostate radiotherapy with respect to the pelvic anatomy during a course of therapy. These data are needed to properly design the planning target volume (PTV). METHODS AND MATERIALS: Seventeen patients were studied. Each patient underwent computed tomography (CT) scanning for treatment planning purposes. Subsequently, three CT scans were obtained at approximately 2-week intervals during treatment. The prostate, seminal vesicles, bladder, and rectum were outlined on each CT study. The second through the fourth CT studies were aligned with the first study using a rigid body transformation based on the bony anatomy. The transformation was used to compute the center of mass position and bounding box of each organ in the subsequent studies relative to the first study. Differences in the bounding box limits and center of mass positions between the first and subsequent studies were tabulated and correlated with bladder and rectal volume and positional parameters. RESULTS: The mobility of the CTV was characterized by standard deviations of 0.09 cm (left-right), 0.36 cm (cranial-caudal), and 0.41cm (anterior-posterior). Prostate mobility was not significantly correlated with bladder volume. However, the mobility of both the prostate and seminal vesicles was very significantly correlated with rectal volume. Bladder and rectal volumes decreased between the pretreatment CT scan and the first on-treatment CT scan, but were constant for all on-treatment CT scans. CONCLUSION: Margins between the CTV and PTV based on the simple geometric requirement that a point on the edge of the CTV is enclosed by the PTV 95% of the time are 0.7 cm in the lateral and cranial-caudal directions, and 1.1 cm in the anterior-posterior direction. However, minimum dose to the CTV and avoidance of organs at risk are more important considerations when drawing beam apertures. More consistent methods for reproducing prostate position (e.g., empty rectum) and more sophisticated beam aperture optimization are needed to guarantee consistent coverage of the CTV while avoiding organs at risk.

Algorithms↗

PubMed1998

Utilization of thermoluminescent dosimetry in total skin electron beam radiotherapy of mycosis fungoides.

PURPOSE: The purpose of this report is to discuss the utilization of thermoluminescent dosimetry (TLD) in total skin electron beam (TSEB) radiotherapy to: (a) compare patient dose distributions for similar techniques on different machines, (b) confirm beam calibration and monitor unit calculations, (c) provide data for making clinical decisions, and (d) study reasons for variations in individual dose readings. METHODS AND MATERIALS: We report dosimetric results for 72 cases of mycosis fungoides, using similar irradiation techniques on two different linear accelerators. All patients were treated using a modified Stanford 6-field technique. In vivo TLD was done on all patients, and the data for all patients treated on both machines was collected into a database for analysis. Means and standard deviations (SDs) were computed for all locations. Scatter plots of doses vs. height, weight, and obesity index were generated, and correlation coefficients with these variables were computed. RESULTS: The TLD results show that our current TSEB implementation is dosimetrically equivalent to the previous implementation, and that our beam calibration technique and monitor unit calculation is accurate. Correlations with obesity index were significant at several sites. Individual TLD results allow us to customize the boost treatment for each patient, in addition to revealing patient positioning problems and/or systematic variations in dose caused by patient variability. The data agree well with previously published TLD results for similar TSEB techniques. CONCLUSION: TLD is an important part of the treatment planning and quality assurance programs for TSEB, and routine use of TLD measurements for TSEB is recommended.

Electrons↗

PubMed1998

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.

Biophysical Phenomena↗

PubMed1998

Fetal dose estimates for electron-beam treatment to the chest wall of a pregnant patient.

The purpose of this report is to discuss dosimetry and shielding for electron-beam radiotherapy of pregnant patients. Specifically, we have determined fetal dose for a pregnant patient considering electron radiotherapy for a chest wall recurrence of breast cancer. The treatment was simulated using an anthropomorphic phantom, and the measured dose to the unshielded fetus for this plan was 5.3 cGy, a level at which risk to the fetus is uncertain. Therefore abdominal shielding, consisting of 6.6 cm of lead, was used to reduce the dose to the fetus to less than 1.5 cGy, a level considered to be of little risk. We further found that using the lower (instead of upper) variable trimmer bars to define the field edge closest to the fetus resulted in approximately 30% lower dose to the fetus. These results show that it is possible to reduce fetal dose to acceptable limits in electron-beam radiotherapy of the chest wall using the general principles recommended for photon-beam radiotherapy.

Adult↗

PubMed1995

Optimization of pencil beam widths for electron-beam dose calculations.

The pencil beam method of calculating dose distributions for electron-beam radiotherapy has been very useful, however, several limitations in the approach have been recognized. One such limitation is the lack of a mechanism to model range straggling of electrons. For stationary electron-beam calculations, range straggling is incorporated incompletely in the planar-fluence-to-dose conversion factor, which uses measured percentage depth dose curves to force the calculated percentage depth dose to reproduce the measurement. When calculating the dose distribution for an arced beam using a pencil beam algorithm, insufficient modeling of the pencil beams leads to larger errors than when using a stationary beam algorithm. The calculated depth of maximum dose is systematically over-estimated by the pencil beam calculations. We will show that the lack of a way to account for range straggling in the arc-electron pencil beam calculation is primarily responsible for this discrepancy. Methods of incorporating range straggling into the electron pencil beam dose calculation have been presented before, but no data have been shown to support their use for heterogeneous phantoms (patients). This paper presents a similar range-straggling modification, as well as data to show that this model can predict pencil beam width to within 20% for heterogeneous slab phantoms. For stationary electron-beam calculations, the calculated isodose lines follows the measured isodose lines to within 1 mm down to the 10% dose level.(ABSTRACT TRUNCATED AT 250 WORDS)

Algorithms↗

PubMed1992

Optimization of a cord shielding technique for electrons.

Large anterior electron fields are sometimes used to irradiate the neck when treating head & neck tumors. To offer a degree of spinal cord shielding, wax bolus, approximately the width of the vertebral bodies, is placed on the immobilization shell. The thickness of the bolus is adjusted so that the radiological depth of the anterior edge of the vertebral bodies is equal to the R80 depth for the energy used. This approach ignores electron scattering. Using a CT study of a thyroid cancer patient, neck contours were generated at 0.5 cm intervals and entered into the Alberta Treatment Planning system. Internal contours for the trachea and vertebral bodies were added and CT information was used for treatment planning purposes. The bolus outline was added as described above, and the dose calculated using a 3D implementation of the M.D. Anderson (Hogstrom) algorithm. The calculation shows that the simple bolus technique described above is inappropriate. The spinal cord is adequately shielded, but the target volume is not covered by the 80% isodose line. Qualitatively, the results can be explained by the lateral scatter non-equilibrium introduced by the bolus. By iteratively adjusting the shape and thickness of the wax bolus and recalculating the dose distribution, we were able to better fulfill the dose prescription. Comparison with measured data shows reasonable, but not perfect agreement. In conclusion, electron beam treatments must be examined closely to ensure that the treatment goals are met. In some cases, treatment integrity may be compromised by incorrect assumptions regarding the nature of the electron transport and dose deposition.

Humans↗

PubMedDate not supplied

Calculation of the 3-D dose distribution surrounding a 103Pd stent.

PURPOSE: This study was designed to assess the suitability of a 103Pd-implanted stent for use in intravascular brachytherapy. MATERIALS AND METHODS: A stent was modeled as a superposition of 201 identical struts and the EGS4/DOSRZ Monte Carlo code was used to calculate the dose distribution for each strut. To verify the simulation parameters, doses along the transverse axis of a Model 200 103Pd interstitial seed were calculated and compared to those calculated by the TG43 method. RESULTS: Dose profiles within 1 mm of the stent's outer surface were heterogeneous and reflected the stent's structure. For a 2-mm outer-diameter 103Pd-implanted stent, approximately 2.68 x 10(7) Bq were required to deliver 31.5 Gy in 28 days at a distance of 0.5 mm along the perpendicular bisector from the stent's outer surface. The Monte Carlo simulation of the 103Pd seed showed relative doses within 7% of the values calculated by the TG43 method. CONCLUSION: The dosimetry about a 103Pd-implanted stent suggests that the stent is suitable for use in intravascular brachytherapy.

Brachytherapy↗

PubMedDate not supplied

Verification of a two-dimensional pencil beam arc electron dose calculation algorithm.

The dosimetry of arced electron beams is of increasing importance because of the increased capabilities of modern linear accelerators. A practical pencil beam algorithm has been developed for arc electron beams and is capable of using computed tomography information for heterogeneity corrections. For homogeneous phantoms, the maximum dose and bremsstrahlung components are predicted very accurately, that is, within 1% of the maximum dose. However, the depth of maximum dose (treatment depth) is predicted to be deeper than measurement, as much as 0.7 cm deeper. For a heterogeneous lung phantom, the discrepancies are as high as 30%, but the accuracy of dose calculation is consistent with conventional stationary pencil beam algorithms. It was concluded that improvements in the dose prediction are possible with more accurate calculations of the pencil beam widths and the incorporation of range straggling into the algorithm.

Algorithms↗