Dose Distributions In Water Phantom Research Articles

Numerical simulation of electron beam depth dose distribution in water phantom passed through modified plastic materials

The aim of the study is to evaluate the feasibility of using modified plastics for 3D printing of boluses for electron beam therapy. Filaments for 3D printing made from plastic materials with an admixture of copper were studied. Test samples were made from these filaments and investigated. Mathematical models of such products were created based on the obtained data. The thicknesses of the boluses made from each modified material were calculated to achieve the same electron beam depth dose distribution in the water phantom as natural plastic. Using a the developed model of a therapeutic electron beam and newly created models of test object materials, the electron beam depth dose distributions in the water phantom that passed through the materials under study were obtained in bolus geometry. The results of the work demonstrate the potential of using modified plastics for creating forming elements in electron beam therapy. The application of this plastic samples for electron beam shaping with individual configuration allows to modify the irradiation field based on the clinical tasks quickly and efficiently and to increase the effectiveness of electron beam radiotherapy procedures.

Does radiation therapy need more than two photon energies from Linac?

Modern Linacs are equipped with multiple photon energies for radiation therapy, and proper energy is chosen for each case based on tumor characteristics and patient anatomy. The aim of this study is to investigate whether it is necessary to have more than two photons energies. The principle of photon energy synthesis is presented. It is shown that a photon beam of any intermediate energy (Esyn) can be synthesized from a linear combination of a low energy (Elow) and a high energy (Ehigh). The principle is validated on a wide range of scenarios: different intermediate photon energies on the same Linac; between Linacs from the same manufacturer or different manufacturers; open and wedge beams; and extensive photon energies available from published reference data. In addition, 3D dose distributions in water phantom are compared using Gamma analysis. The method is further demonstrated in clinical cases of various tumor sites and multiple treatment modalities. Experimental measurements are performed for IMRT plans and they are analyzed using the standard clinical protocol. The synthesis coefficients vary with energy and field size. The root mean square error (RMSE) is within 1.1% for open and wedge fields. Excellent agreement was observed for British Journal of Radiology (BJR) data with an average RMSE of 0.11%. The 3D Gamma analysis shows a good match for all field sizes in the water phantom and all treatment modalities for the five clinical cases. The minimum gamma passing rate of 95.7% was achieved at 1%/1mm criteria for two measured dose distributions of IMRT plans. A Linac with two photon energies is capable of producing dosimetrically equivalent plans of any energy in-between through the photon energy synthesis, supporting the notion that there is no need to equip more than two photon energies on each Linac. This can significantly reduce the cost of equipment for radiation therapy.

A Dosimetric evaluation of a high dose rate cobalt-60 brachytherapy source using shielded, single and multi-channel cylinder applicators for gynecological cancers.

Vaginal brachytherapy is a standard method for preventing the recurrences of malignancies in the treatment of gynecological cancers. Cylindrical applicators are usually employed in high dose rate cobalt-60 source brachytherapy after abdominal hysterectomy or bilateral salpingo-oophorectomy treatment. The aim of this study is to conduct an assessment of dosimetric properties of single channel, multichannel, and shielded cylinder applicators. This study utilizes a pelvis phantom with three different volumes of bladder and rectum to perform dose measurement around the cylindrical applicators. GAFCHROMIC EBT3 model film was used to measure dose distributions in water phantom around the bladder, rectum, applicator, and CTV. In order to properly compare the performance of the shielded applicator with multichannel and single channel applicators, we consider the both cases of a one-sided CTV and two CTVs placed on both sides of the vagina. Dose-volume histograms analysis shows no statistically significant difference in tumoral region for the two types of CTV between cylindrical applicators. The V120%, V100% and D95% values of target volume were 47.36%. 91.46%, and 3.99Gy for multi-channel cylinder applicator, 60.2%, 95.02%, and 3.99Gy for single channel cylinder applicator, respectively. Statistical analysis reports D0.1cc, D1cc, and D2cc for the multichannel applicator demonstrates a significant dose reduction in bladder compared with single channel. The V120%, V100%, and D95% values of target volume were 55.50%. 97.90%, and 3.91Gy for multi-channel cylinder applicator with four active channels (MCC-4), 41.56%, 92.10%. and 4.13Gy for shielded cylinder applicator, respectively. Statistical analysis reports D0.1cc, D1cc and D2cc for the MCC-4 demonstrates a noticeable dose reduction in bladder and rectum compared with Shielded channel. The results indicate that employing multichannel applicator in vaginal high dose rate cobalt-60 source brachytherapy can reduce the received dose to organs at risk without any significant loss of received dose in CTV, which is contributed to its asymmetric or symmetric dose distribution that depend on which channels are activated.

The simulation of a linear accelerator using Monte Carlo method with EGSnrc Program

The Monte Carlo method is considered to be the most accurate algorithm for dose calculation in radiotherapy. Linear accelerator accurate simulation is required as a prior condition for Monte Carlo dose calculation algorithm. In this study, the 6 MV photon beams from a Siemens Primus Linear Accelerator (LINAC) M5497 at the Dong Nai General Hospital was modelled by using EGSnrc. The BEAMnrc và DOSXYZnrc user code were used for simulation the head of LINAC and the dose distribution in water phantom. The percentage depth dose (PDD) and beam profiles (OCR) were calculated and then compared with the measured ones in order to evaluate the simulation accuracy. Excellent agreement was found between simulations and measurements with an average difference of 1.26 % for PDD, less than 2 % for OCR. In addition, the gamma evaluation method for simulation was also performed using an in-house Matlab code. The percentage gamma passing rate was 100% for PDD and 98.5 % for OCR with 3 % dose difference and 3 mm distance to agreement as acceptance criteria. The result showed that we had successfully simulated LINAC with excellent agreement.

A study on dosimetry accuracy of Strut-Adjust Volume Implant (SAVI) brachytherapy

Introduction: Accelerated Partial Breast Irradiation (APBI) is an effective treatment for breast carcinomas because of its limited number of fractions. Strut-adjusted volume implant (SAVI) is a HDR brachytherapy applicator to deliver the prescription dose with good PTV coverage and acceptable dose to skin, chest wall and organ at risks. Acceptable clinical outcomes depend on accurate dosimetry methods. Currently water is used as the reference dosimetry phantom material in treatment planning systems. The purpose of this study is to consider the corrections on phantom material and geometry in dose distribution of SAVI brachytherapy simulation using MCNP5 Monte Carlo code. Materials and Methods: SAVI with 8 peripheral source channels were considered expanded inside the lumpectomy cavity in breast. Three realistic phantoms were employed to perform the step by step corrections and dose distributions were compared with that of water. Considerations were included using the ICRU-44 defined breast material as the real breast composition and moreover the air-tissue interferences at the skin, lung and the air cavity inside the SAVI were studied. Results: According to the results, the dose differences due to the breast material corrections were less than 1% in the most points of breast and only 2-3% overestimation were observed close to the skin. Breast phantoms containing air cavity have shown up to 2% decreases in regions around it. Finally a phantom with both noted corrections and a curved surface for breast skin, corrected the calculations with about 3-5% underestimations at skin. Conclusion: According to the results of this study, dose distributions in breast phantoms containing different materials does not differ significantly with the dose distribution in water phantom. Conservative approximations can be provided with water phantom and it doesn’t seem necessary that the current treatment planning systems be replaced with the model-based patient specific dosimetry methods.

Abstract ID: 157 Development of Geant4-based patient-specific QA system of gamma knife treatment plans using automated DICOM-RT interface

Gamma knife system which uses 192 small photon beams is widely considered as a very precise modality for intracranial radiosurgery. However, there are still limitations in current treatment planning system (TPS) for accurately estimating the absolute dose in the small volume, or the dose distribution in the inhomogeneous area such as the case of embolization of arteriovenous malformations and the located target volume behind an air cavity [1] . The purpose of this study is to develop quality assurance (QA) system of gamma knife treatment plans based on a patient-specific model constructed by an automated DICOM-RT interface in Geant4. The Leksell Gamma Knife Perfexion was designed in Geant4.10.00 based on the manufacturer’s detailed information. Dose distributions in the spherical water phantom with a diameter of 16 cm were compared between Geant4 and the film measurement for the validation system. For extracting the patient-dependent parameters, automated DICOM-RT interface was developed for constructing the patient-specific model. In-house developed system is able to manually define the physical property of a special volume using the region of interest (ROI) contour data stored in a DICOM file. The dose distributions in the water phantom were compared between Geant4, measurement data and manufacture’s values in terms of full width at half maximum (FWHM). FWHM of Geant4 and measurement were quite well matched with manufacture’s value within 0.3, 0.2 and 0.7 mm in both the x and y direction for 4, 8 and 16 mm, respectively. Furthermore, the dose distributions of Geant4 and commercial TPS were well matched for different beam size combinations for each of the eight sectors. The developed QA system shows the potential to precisely estimate the dose distribution in the patients. In the future, assessment of the absolute dose distribution calculated by Geant4 and commercial TPS for the clinical cases will be performed. Moreover, magnetic resonance (MR) images will be employed in our system for sophisticated modelling the heterogeneities in the patients.

Superficial and peripheral dose in compensator-based FFF beam IMRT.

Flattening filter‐free (FFF) beams produce higher dose rates. Combined with compensator‐based intensity modulated radiotherapy (IMRT) techniques, the dose delivery for each beam can be much shorter compared to the flattened beam MLC‐based or flattened beam compensator‐based IMRT. This ‘snap shot’ IMRT delivery is beneficial to patients for tumor motion management. Due to softer energy, superficial doses in FFF beam treatment are usually higher than those from flattened beams. Due to no flattening filter, thus less photon scattering, peripheral doses are usually lower in FFF beam treatment. However, in compensator‐based IMRT using FFF beams, the compensator is in the beam pathway. Does it introduce beam hardening effects and scattering such that the superficial dose is lower and peripheral dose is higher compared to FFF beam MLC‐based IMRT? This study applied Monte Carlo techniques to investigate the superficial and peripheral doses in compensator‐based IMRT using FFF beams and compared it to the MLC‐based IMRT using FFF beams and flattened beams. Besides varying thicknesses of brass slabs to simulate varying thicknesses of compensators, a simple cone‐shaped compensator was simulated to mimic a clinical application. The dose distribution in water phantom by the cone‐shaped compensator was then simulated by multiple MLC‐defined FFF and flattened beams with varying apertures. After normalization to the maximum dose, Dmax, the superficial and peripheral doses were compared between the FFF beam compensator‐based IMRT and FFF/flattened beam MLC‐based IMRT. The superficial dose at the central 0.5 mm depth was about 1% (of Dmax) lower in the compensator‐based 6 MV FFF (6FFF) IMRT compared to the MLC‐based 6FFF IMRT, and about 8% higher than the flattened 6 MV MLC‐based IMRT dose. At 8 cm off‐axis at depth of central maximum dose, dmax, the peripheral dose between the 6FFF and flattened 6 MV MLC demonstrated similar doses, while the compensator dose was about 1% (of Dmax) higher. Compensators reduce the superficial doses slightly compared to open FFF beams, but increases the peripheral doses due to scatter in the compensator.

SU-F-T-372: Surface and Peripheral Dose in Compensator-Based FFF Beam IMRT

Purpose: Flattening filter free (FFF) beams produce higher dose rates. Combined with compensator IMRT techniques, the dose delivery for each beam can be much shorter compared to the flattened beam MLC-based or compensator-based IMRT. This ‘snap shot’ IMRT delivery is beneficial to patients for tumor motion management. Due to softer energy, surface doses in FFF beam treatment are usually higher than those from flattened beams. Because of less scattering due to no flattening filter, peripheral doses are usually lower in FFF beam treatment. However, in compensator-based IMRT using FFF beams, the compensator is in the beam pathway. Does it introduce beam hardening effects and scattering such that the surface dose is lower and peripheral dose is higher compared to FFF beam MLC-based IMRT? Methods: This study applied Monte Carlo techniques to investigate the surface and peripheral doses in compensator-based IMRT using FFF beams and compared it to the MLC-based IMRT using FFF beams and flattened beams. Besides various thicknesses of copper slabs to simulate various thicknesses of compensators, a simple cone-shaped compensator was simulated to mimic a clinical application. The dose distribution in water phantom by the cone-shaped compensator was then simulated by multiple MLC defined FFF and flattened beams with various openings. After normalized to Dmax, the surface and peripheral dose was compared between the FFF beam compensator-based IMRT and FFF/flattened beam MLC-based IMRT. Results: The surface dose at the central 0.5mm depth was close between the compensator and 6FFF MLC dose distributions, and about 8% (of Dmax) higher than the flattened 6MV MLC dose. At 8cm off axis at dmax, the peripheral dose between the 6FFF and flattened 6MV MLC demonstrated similar doses, while the compensator dose was about 1% higher. Conclusion: Compensator does not reduce the surface doses but slightly increases the peripheral doses due to scatter inside compensator.

Gafchromic EBT3 film dosimetry in electron beams - energy dependence and improved film read-out.

For megavoltage photon radiation, the fundamental dosimetry characteristics of Gafchromic EBT3 film were determined in 60Co gamma ray beam with addition of experimental and Monte Carlo (MC)‐simulated energy dependence of the film for 6 MV photon beam and 6 MeV, 9 MeV, 12 MeV, and 16 MeV electron beams in water phantom. For the film read‐out, two phase correction of scanner sensitivity was applied: a matrix correction for scanning area and dose‐dependent correction by iterative procedure. With these corrections, the uniformity of response can be improved to be within ±50 pixel values (PVs). To improve the read‐out accuracy, a procedure with flipped film orientations was established. With the method, scanner uniformity can be improved further and dust particles, scratches and/or dirt on scanner glass can be detected and eliminated. Responses from red and green channels were averaged for read‐out, which decreased the effect of noise present in values from separate channels. Since the signal level with the blue channel is considerably lower than with other channels, the signal variation due to different perturbation effects increases the noise level so that the blue channel is not recommended to be used for dose determination. However, the blue channel can be used for the detection of emulsion thickness variations for film quality evaluations with unexposed films. With electron beams ranging from 6 MeV to 16 MeV and at reference measurement conditions in water, the energy dependence of the EBT3 film is uniform within 0.5%, with uncertainties close to 1.6% (k=2). Including 6 MV photon beam and the electron beams mentioned, the energy dependence is within 1.1%. No notable differences were found between the experimental and MC‐simulated responses, indicating negligible change in intrinsic energy dependence of the EBT3 film for 6 MV photon beam and 6 MeV–16 MeV electron beams. Based on the dosimetric characteristics of the EBT3 film, the read‐out procedure established, the nearly uniform energy dependence found and the estimated uncertainties, the EBT3 film was concluded to be a suitable 2D dosimeter for measuring electron or mixed photon/electron dose distributions in water phantom. Uncertainties of 3.7% (k=2) for absolute and 2.3% (k=2) for relative dose were estimated.PACS numbers: 87.53.Bn, 87.55.K‐, 87.55.Qr

Measurement and Comparison of Skin Dose Distribution in Water Phantom Exposed to 6 MV Photon Beam

Measurement and Comparison of Skin Dose Distribution in Water Phantom Exposed to 6 MV Photon Beam

SU-E-T-386: A Monte Carlo Dose Calculation Framework for Electron Beams On Varian TrueBeam

Purpose: The design of the linac head is different for TrueBeam than Clinac, and there are differences in measured dose distributions in water phantoms between TrueBeam and Clinac for electron beams. Therefore, MC models for Clinac may not be applied directly to the Truebeam linac. The purpose of this study is to validate a Monte Carlo (MC) dose calculation framework for electron beams on Varian TrueBeam with phase space files provided by Varian. Methods: The particle histories from the phase space file were used as input for the down-stream simulation including jaws, applicators, and water phantom. MC packages BEAMnrc/DOSYXZnrc were used. The down-stream beam components were modeled according to manufacturer specifications and the dose distributions were compared with the measured data of standard cones. The measurements were performed in a water phantom with a p-type electron field diode (diameter 0.2cm) and ion chamber (CC13). Depth dose and orthogonal profiles at depths defined by R{sub 1} {sub 0} {sub 0}, R{sub 5} {sub 0}, Rp were compared. Results: Preliminary results for a 16 MeV phase space and 10x10, 15x15, and 20x20 cm{sup 2} applicator are presented. Simulations were run for a statistical uncertainty of <2% at depth of maximum dose formore » a voxel resolution of 0.5x0.5x0.2cm{sup 2}. Dose and range differences for the PDD profiles were within 2% and 1 mm, respectively. Dose differences within the central 80% of the beam width for the orthogonal profiles at depth of maximum dose were less than 2% for the 10x10, 15x15, and 20x20 cm{sup 2} applicator, respectively. Conclusion: Varian electron phase space files simulations are in agreement with measured commissioning data. These phase space files can be used in the simulation of TrueBeam linacs, and will provide reproducibility across publications. Analyses for all electron energies and standard applicators are under way and results will be included in the presentation.« less

TH‐A‐19A‐05: Modeling Physics Properties and Biologic Effects Induced by Proton and Helium Ions

Purpose:Currently, proton and carbon ions are used for cancer treatment. More recently, other light ions including helium ions have shown interesting physical and biological properties. The purpose of this work is to study the biological and physical properties of helium ions (He‐3) in comparison to protons.Methods:Monte Carlo simulations with FLUKA, GEANT4 and MCNPX were used to calculate proton and He‐3 dose distributions in water phantoms. The energy spectra of proton and He‐3 beams were calculated with high resolution for use in biological models. The repair‐misrepairfixation (RMF) model was subsequently used to calculate the RBE.Results:The proton Bragg curve calculations show good agreement between the three general purpose Monte Carlo codes. In contrast, the He‐3 Bragg curve calculations show disagreement (for the magnitude of the Bragg peak) between FLUKA and the other two Monte Carlo codes. The differences in the magnitude of the Bragg peak are mainly due to the discrepancy in the secondary fragmentation cross sections used by the codes. The RBE for V79 cell lines is about 0.96 and 0.98 at the entrance of proton and He‐3 ions depth dose respectively. The RBE increases to 1.06 and 1.59 at the Bragg peak of proton and He‐3 ions. The results demonstrated that LET, microdosimetric parameters (such as dose‐mean lineal energy) and RBE are nearly constant along the plateau region of Bragg curve, while all parameters increase within the Bragg peak and at the distal edge for both proton and He‐3 ions.Conclusion:The Monte Carlo codes should revise the fragmentation cross sections to more accurately simulate the physical properties of He‐3 ions. The increase in RBE for He‐3 ions is higher than for proton beams at the Bragg peak.

Monte Carlo simulation of 10 MV photon beams and beam profile validations

In radiation therapy, various radiation energies have been used according to the size and depth of tumor for therapeutic purpose. The electron energies are used for superficial treatments, whereas photon energies such as 6MV, 10MV and 18MV energy levels are utilized for the treatment of deeper tumors. The suitability of energy levels for treatment has been determined by dose distributions in water phantom. In this study, beam profiles of clinical linear accelerators for 10MV photon beams in water phantom were obtained by using Monte Carlo simulation program (MCSP) developed. MCSP was carried out taking into account the interactions of photons with matter in MATLAB (The Mathworks, Inc.). Monte Carlo simulation results were calculated for 5×5, 10×10 and 20×20cm2 field sizes at 5, 10 and 20cm depths for the dose profiles. In order to validate the beam profiles, Monte Carlo results were compared with experimental results for 1cm and 10cm depths at all the field sizes. MCSP results have shown a good agreement with experimental results.

SU-D-144-05: Monte Carlo Simulation of a Precision Proton Radiotherapy Platform Designed for Small-Animal Experiments

Purpose: A cyclotron-based proton beam is under development to accommodate the geometric and dosimetric requirements of small-animal research, namely a sharp pristine Bragg peak at ∼8mm depth in tissue. The goal of this project is to develop a Monte Carlo model to support further development of the existing beam, provide data on physical dose in complex tissues or phantoms, and for biological research including microdosimetric analysis of LET and RBE effects. Method: A Monte Carlo simulation of the beam was implemented using the GEANT-4 based TOPAS software, under a beta-test user agreement with the developers. This system allows for creation of geometrical objects, voxel dose scoring within those objects as well as input of DICOM image data. We developed simulations of dose distributions in water phantoms, small ion chambers and in air. We compare these simulations to measured dosimetry data, and predictions of a previously published analytical model of dose deposition named the ‘Proton Energy Loss’ model. Results: The Monte Carlo model with best-fit parameters of incident energy 28.9 MeV and energy spread 1.5% gives less than 2% error in percent depth dose compared to the experimental data. Simulated lateral profiles also match the EBT3-based film measurements to better than 3.5%. Run times for simulations are approximately one hour per million particles. Multiple runs can be processed simultaneously on a 16-core server without noticeable degradation in performance. Conclusion: Monte Carlo simulation of the proton beam provides a foundation for exploring more complex geometrical dose patterns such as a high dose rings or spread-out Bragg peaks. Modeling will be further developed to calculate dose distributions in various experimental arrangements and the complex tissue structures presented in small-animal CT scans. The precision proton radiotherapy platform is expected to enable novel radiobiological research, as have dedicated small-animal X-ray irradiation systems.

Fast, accurate photon beam accelerator modeling using BEAMnrc: A systematic investigation of efficiency enhancing methods and cross‐section data

In this work, an investigation of efficiency enhancing methods and cross-section data in the BEAMnrc Monte Carlo (MC) code system is presented. Additionally, BEAMnrc was compared with VMC++, another special-purpose MC code system that has recently been enhanced for the simulation of the entire treatment head. BEAMnrc and VMC++ were used to simulate a 6 MV photon beam from a Siemens Primus linear accelerator (linac) and phase space (PHSP) files were generated at 100 cm source-to-surface distance for the 10 x 10 and 40 x 40 cm2 field sizes. The BEAMnrc parameters/techniques under investigation were grouped by (i) photon and bremsstrahlung cross sections, (ii) approximate efficiency improving techniques (AEITs), (iii) variance reduction techniques (VRTs), and (iv) a VRT (bremsstrahlung photon splitting) in combination with an AEIT (charged particle range rejection). The BEAMnrc PHSP file obtained without the efficiency enhancing techniques under study or, when not possible, with their default values (e.g., EXACT algorithm for the boundary crossing algorithm) and with the default cross-section data (PEGS4 and Bethe-Heitler) was used as the "base line" for accuracy verification of the PHSP files generated from the different groups described previously. Subsequently, a selection of the PHSP files was used as input for DOSXYZnrc-based water phantom dose calculations, which were verified against measurements. The performance of the different VRTs and AEITs available in BEAMnrc and of VMC++ was specified by the relative efficiency, i.e., by the efficiency of the MC simulation relative to that of the BEAMnrc base-line calculation. The highest relative efficiencies were approximately 935 (approximately 111 min on a single 2.6 GHz processor) and approximately 200 (approximately 45 min on a single processor) for the 10 x 10 field size with 50 million histories and 40 x 40 cm2 field size with 100 million histories, respectively, using the VRT directional bremsstrahlung splitting (DBS) with no electron splitting. When DBS was used with electron splitting and combined with augmented charged particle range rejection, a technique recently introduced in BEAMnrc, relative efficiencies were approximately 420 (approximately 253 min on a single processor) and approximately 175 (approximately 58 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. Calculations of the Siemens Primus treatment head with VMC++ produced relative efficiencies of approximately 1400 (approximately 6 min on a single processor) and approximately 60 (approximately 4 min on a single processor) for the 10 x 10 and 40 x 40 cm2 field sizes, respectively. BEAMnrc PHSP calculations with DBS alone or DBS in combination with charged particle range rejection were more efficient than the other efficiency enhancing techniques used. Using VMC++, accurate simulations of the entire linac treatment head were performed within minutes on a single processor. Noteworthy differences (+/- 1%-3%) in the mean energy, planar fluence, and angular and spectral distributions were observed with the NIST bremsstrahlung cross sections compared with those of Bethe-Heitler (BEAMnrc default bremsstrahlung cross section). However, MC calculated dose distributions in water phantoms (using combinations of VRTs/AEITs and cross-section data) agreed within 2% of measurements. Furthermore, MC calculated dose distributions in a simulated water/air/water phantom, using NIST cross sections, were within 2% agreement with the BEAMnrc Bethe-Heitler default case.

Determination of the initial beam parameters in Monte Carlo linac simulation

For Monte Carlo linac simulations and patient dose calculations, it is important to accurately determine the phase space parameters of the initial electron beam incident on the target. These parameters, such as mean energy and radial intensity distribution, have traditionally been determined by matching the calculated dose distributions with the measured dose distributions through a trial and error process. This process is very time consuming and requires a lot of Monte Carlo simulation experience and computational resources. In this paper, we propose an easy, efficient, and accurate method for the determination of the initial beam parameters. We hypothesize that (1) for one type of linacs, the geometry and material of major components of the treatment head are the same; the only difference is the phase space parameters of the initial electron beam incident on the target, and (2) most linacs belong to a limited number of linac types. For each type of linacs, Monte Carlo treatment planning system (MC-TPS) vendors simulate the treatment head and calculate the three-dimensional (3D) dose distribution in water phantom for a grid of initial beam energies and radii. The simulation results (phase space files and dose distribution files) are then stored in a data library. When a MC-TPS user tries to model their linac which belongs to the same type, a standard set of measured dose data is submitted and compared with the calculated dose distributions to determine the optimal combination of initial beam energy and radius. We have applied this method to the 6 MV beam of a Varian 21EX linac. The linac was simulated using EGSNRC/BEAM code and the dose in water phantom was calculated using EGSNRC/DOSXYZ. We have also studied issues related to the proposed method. Several common cost functions were tested for comparing measured and calculated dose distributions, including chi2, mean absolute error, dose difference at the penumbra edge point, slope of the dose difference of the lateral profile, and the newly proposed Kappaalpha factor (defined as the fraction of the voxels with absolute dose difference less than alpha%). It was found that the use of the slope of the lateral profile difference or the difference of the penumbra edge points may lead to inaccurate determination of the initial beam parameters. We also found that in general the cost function value is very sensitive to the simulation statistical uncertainty, and there is a tradeoff between uncertainty and specificity. Due to the existence of statistical uncertainty in simulated dose distributions, it is practically impossible to determine the best energy/radius combination; we have to accept a group of energy/radius combinations. We have also investigated the minimum required data set for accurate determination of the initial beam parameters. We found that the percent depth dose curves along or only a lateral profile at certain depth for a large field size is not sufficient and the minimum data set should include several lateral profiles at various depths as well as the central axis percent depth dose curve for a large field size.

Estimations of the r.b.e. for pi - and pi + from survival data of T-1 human kidney cells.

SummaryA Monte Carlo programme has been written for computing depth—dose distributions in water phantoms irradiated by positive and negative pion beams. Contributions to the dose from the primary pions and various secondary particles are tabulated. Their contributions have been weighted by a set of r.b.e. values obtained from biological experiments. In particular, estimates of the average r.b.e. for survival of T-1 human kidney cells (at approximately the 10 per cent level) have been carried out as functions of depth along the centre line for typical π− and π+ beams. Isodose curves are plotted for π− beam. Examples of the variations in average r.b.e. with changes in the mean momentum and momentum spread of the beam are also calculated. Comparisons are made with some observed effects produced by pions on biological systems.