Physical Wedge Research Articles

Contralateral breast doses measured by film dosimetry: tangential techniques and an optimized IMRT technique

The contralateral breast (CLB) doses for three tangential techniques were characterized by using a female thorax phantom and GafChromic EBT film. Dose calculations by the pencil beam and collapsed cone algorithms were included for comparison. The film dosimetry reveals a highly inhomogeneous dose distribution within the CLB, and skin doses due to the medial fields that are several times higher than the interior dose. These phenomena are not correctly reproduced by the calculation algorithms. All tangential techniques were found to give a mean CLB dose of approximately 0.5 Gy. All wedged fields resulted in higher CLB doses than the corresponding open fields, and the lateral open fields resulted in higher CLB doses than the medial open fields. More than a twofold increase in the mean CLB dose from the medial open field was observed for a 90° change of the collimator orientation. Replacing the physical wedge with a virtual wedge reduced the mean dose to the CLB by 35% and 16% for the medial and lateral fields, respectively. Lead shielding reduced the skin dose for a tangential technique by approximately 50%, but the mean CLB dose was only reduced by approximately 11%. Finally, a technique based on open medial fields in combination with several IMRT fields is proposed as a technique for minimizing the CLB dose. With and without lead shielding, the mean CLB dose using this technique was found to be 0.20 and 0.27 Gy, respectively.

SU-FF-T-642: Evaluation of 3-D Treatment Plans Using Physical and Motorised Enhanced Dynamic Wedges

Purpose: To analyze the treatment plans generated using physical and enhanced dynamic wedges for head and neck patients from Eclipse treatment planning system. Materials and Methods: The physical wedge filters of angles of 15°, 30°, 45° and 60° are used. In Enhanced Dynamic Wedge technique no external beam modifier is used to create wedge dose profiles, instead wedged isodose profiles is created by the sweeping action of the jaw from open to closed position while the beam is on. The dose rate and jaw speed are also varied during the treatment, which is the function of selected energy, field size and wedge angle. The single STT called Golden STT (GSTT) is used for 60° wedge angle and other wedge effects are obtained by mixing open and wedge field intensities. The conformal treatment plans of 20 head and neck patients were used in this study. Plans are generated using both physical and enhanced dynamic wedges. The dose was normalized to 100% at isocenter. The plans were compared using Dose-Volume histogram (DVH) tool. Results and Discussion: The treatment plans generated shows that the enhanced dynamic wedge plans were comparable with that of the physical wedge plan and the DVH analysis showed that the critical organ sparing is slightly better with enhanced dynamic wedge. The maximum dose within the target volume is higher in physical wedge plan. The number of monitor units to deliver a particular dose with EDW field is less than that of PW field due to change in wedge factor, which could probably reduce the scatter dose to structures off the field. Enhanced dynamic wedges eliminate the beam hardening effect, and also it is possible to obtain an asymmetric wedge fields with EDW. The dose and jaw position control accuracy statistics are displayed and saved to dynalog files after treatment.

SU‐FF‐T‐281: Variation of Beam Characteristics for Physical and Enhanced Dynamic Wedge From a Dual Energy Accelerator

Purpose: The aim of this study was to compare few of the dosimetric characteristics of a physical and enhanced dynamic wedge from a dual energy (6 and 18 MV) linear accelerator. Materials and Methods: This study used the Varian Clinac‐DHX (S.No.3172) linear accelerator which is equipped with three different type of wedges. To compare the change in beam quality with physical and enhanced dynamic wedge half value layer (HVL) was measured using a 0.14 cc ion chamber covered with brass build up cap positioned at 2 meter distance from the source and the distance from the target to absorber distance was maintained as 1 meter. Transmission measurements were made for varying thickness of white polystyrene phantom placed on the couch. The surface dose with physical and enhanced dynamic wedges were measured using a parallel plate chamber at 85 and 100 cm source to surface distance (SSD), for 6 and 18 MV photons. Results: The HVL was estimated for 6 and 18 MV photons along the central axis and at off‐axis points for the field size of 20 × 20 cm2 with 45° upper physical wedge and compared with that of the enhanced dynamic wedge (EDW). For physical wedged field, at heel edge side HVL value was high compared with the measured that of EDW. It was noticed that, the HVL variation across the beam was significantly high for 6 MV X‐rays than for 18 MV X‐rays. The measured surface dose was found to be more at 85 cm FSD for all the fields. The lower energy photons (6 MV) were found to result in higher surface dose than the higher energy photons (18 MV). Conclusion: The three wedge systems produce significantly different surface doses that should be considered in properly choosing a wedge system for clinical use.

Study of 2D ion chamber array for angular response and QA of dynamic MLC and pretreatment IMRT plans

Aim To study of 2 Dimensional ion chamber array for angular response and its utility for quality assurance of dynamic multileaf collimator and pretreatment intensity modulated radiotherapy plans. Materials and Methods The MLC QA test patterns and IMRT plans were executed on 2D ion chamber array having 1020 vented pixel ionization chambers. The dynamic MLC QA test patterns were chair test, x–wedge, pyramid, open swipe field, garden fence and picket fence. Performance of Dynamic wedges was compared with physical wedges. For IMRT verification, five patients with localized prostate carcinoma were planned using dynamic IMRT technique. Angular response of MatriXX was measured by exposing the system from different gantry angles. Results Dynamic MLC QA tests such as chair, x-wedge, pyramid, and open swipe field were successfully verified. MatriXX was not able to recognize the bar pattern of picket test and garden fence test. The response of MatriXX gradually decreases from 0° to 180° angles and it was 7.7% less at 180° angle. The dynamic wedge profiles were matching with corresponding physical wedge profiles. For pretreatment IMRT QA, the average dose difference between planned and measured dose was 1.26% with standard deviation of 1.06. Conclusion I'mRT MatriXX can be used for routine dynamic MLC and IMRT pretreatment QA but care should be taken while taking measurements in penumbra region because of its limited spatial resolution.

Poster - Thurs Eve-21: Experience with the Velocity(TM) pre-commissioning services.

As the first Canadian users of the Velocity™ program offered by Siemens, we would like to share our experience with the program. The Velocity program involves the measurement of the commissioning data by an independent Physics consulting company at the factory test cell. The data collected was used to model the treatment beams in our planning system in parallel with the linac delivery and installation. Beam models and a complete data book were generated for two photon energies including Virtual Wedge, physical wedge, and IMRT, and 6 electron energies at 100 and 110 cm SSD. Our final beam models are essentially the Velocity models with some minor modifications to customize the fit to our liking. Our experience with the Velocity program was very positive; the data collection was professional and efficient. It allowed us to proceed with confidence in our beam data and modeling and to spend more time on other aspects of opening a new clinic. With the assistance of the program we were able to open a three-linac clinic with Image-Guided IMRT within 4.5 months of machine delivery.

SU‐GG‐T‐249: Analysis of Scatter Dose On a Contralateral Breast as a Function of Surface Dose and Treatment Depth

Purpose: To characterize scatter dose, as a function of depth and off‐axis distance, on the contralateral breast (CB), during treatment of ipsilateral breast cancer using 6 and 18MV photon beams, utilizing wedge techniques and IMRT. Method and Materials: A 30×30×30 solid water phantom was utilized to simulate dose delivery to a right ipsilateral breast. Both surface dose measurements and planar dose distributions at depths (d = 1cm and d = 2cm) were analyzed, as well as dose distributions along the transverse axis of the phantom perpendicular to the beam. Data was taken using EDR2 film, thermal‐luminescent dosimeters, and MOSFET detectors. A Varian 2100CD linear accelerator was used. Measurements were performed for the following arrangements: open field (8×18cm2), 30 and 45° physical wedge, 30 and 45° Enhanced Dynamic Wedge (EDW), and control points (CP) used in IMRT. Results: Analysis of the data shows that for the hard wedges used, a significant increase in scatter dose to the CB during treatment by 5–7% and 12–15% for the 6MV and 18MV respectively is present. At a depth of 1 cm (6MV), data indicates that for a position in the CB, scatter dose is approximately 7% of the central axis dose of the open beam. The relative increase of 2% occurred with the use of EDW. No increase in scatter dose was measured for the CP case. Conclusion: This study seems to indicate that it may be possible to quantify scatter dose through a mathematical function, based on beam energy, depth of treatment, and treatment modality. Application of such a function may be of benefit in improving treatment plans and providing guidelines with regard to dose delivery and potential shielding of CB during whole or partial breast radiotherapy.

SU‐GG‐T‐488: Dosimetric Parameter Comparison of the Electronic Tissue Compensator Technique with the Conventional Physical Wedge Technique for the Whole Breast Treatment

Purpose: To compare a dosimetric benefit between electronic tissue compensator (ETC) technique and the conventional physical wedge (PW) technique for a whole breast (WB) tangential field irradiation treatment. Method and Materials: 17 breast cases, with chest wall separation 16 to 26 cm at normalization point slice, were included in this study. Eclipse Planning System (V7.3) was used to generate a plan. The plan remained with a same beam setup, beam weighting, normalization point, and energy (6MV or 6MV mixed with 18MV) in both technique for each case. The breast tissue was outlined as a planning target volume (PTV). For a PTV dose coverage information, PTV encompassed by 95% isodose line (V95) and PTV received the minimum dose (Dmin) were studied. To examine a high dose region, a PTV received at least 105% prescription dose (V105) was computed. To study dose inhomogeneity (DI) in the PTV, a ratio of the difference of Dmax and Dmin to the prescription dose was estimated. In additional, a total MU from each plan was recorded to not only measure the linac beam on time, but also as a factor for the machine scatter and leakage evaluation. Results: By comparing with the PW plan, (1) The PTV coverage, V95, showed quite similar (difference within 0.5%) in both techniques and Dmin was lower on a percentage of 1.2±9.1 from ETC plan; (2) The hotspot, V105, was reduced by a percentage of 2.9±2.2 in ETC plan. (3) DI was decreasing by a percentage of 1.3±12.3 in ETC plan. The total MU reduced by about 8.9±17.1 percent in ETC plan. Conclusion: ETC technique presented a dosimetric benefit for the WB tangential field treatment. And these benefits were subjected to case‐by‐case study. A careful comparison is needed for the treatment plan selection.

Study and evaluation of the Siemens virtual wedge factor: dosimetric monitor system and variable field effects

In the year 1997 Siemens introduced the virtual wedge in its accelerators. The idea was that a dose profile similar to that of a physical wedge can be obtained by moving one of the accelerator jaws at a constant speed while the dose rate is changing. This work explores the observed behaviour of virtual wedge factors. A model is suggested which takes into account that at any point in time, when the jaw moves, the dose at a point of interest in the phantom is not only due to the direct beam. It also depends on the scattered radiation in the phantom, the head scatter and the behaviour of the monitoring system of the accelerator. Measurements are performed in a Siemens Primus accelerator and compared to the model predictions. It is shown that the model agrees reasonably well with measurements spanning a wide range of conditions. A strong dependence of virtual wedge factors on the dosimetric board has been confirmed and an explanation has been given on how the balance between different contributions is responsible for virtual wedge factors values.

Experimental evaluation of the accuracy of skin dose calculation for a commercial treatment planning system

The present work uses the Eclipse treatment planning system (TPS) to investigate the accuracy of skin dose calculations. Micro‐MOSFETs (metal oxide semiconductor field effect transistors) were used to measure skin dose for a range of irradiation conditions (open fields, physical wedges, dynamic wedges, various source‐to‐surface distances) for 6‐MV and 10‐MV beams, and the results were compared with the calculated mean dose to a “skin” structure 2 mm thick for semi‐cylindrical phantoms (representative of a neck or breast). Agreement between the calculated and measured skin dose values was better than ±20% for 95% of all measured points (6‐MV and 10‐MV X‐ray spectra alike). For a fixed geometry, the TPS correctly calculated relative changes in dose, showing that minimization of skin dose in intensity‐modulated radiation therapy will be effective in Eclipse.PACS numbers: 87.53.Bn, 87.53.Dq, 87.66.Pm, 87.66.Xa

Optimization of dose distribution with multi-leaf collimator using field-in-field technique for parallel opposing tangential beams of breast cancers

3 Dimensional Conformal Radiotherapy (3D-CRT) planning software helps in displaying the 3D dose distribution at different levels in the planned target volume (PTV). Physical or dynamic wedges are commonly applied to obtain homogeneous dose distribution in the PTV. Despite all these planning efforts, there are about 10% increased dose hot spots encountered in final plans. To overcome the effect of formation of hot spots, a manual forward planning method has been used. In this method, two more beams with multi-leaf collimator (MLC) of different weights are added in addition to medial and lateral wedged tangent beams. Fifteen patient treatment plans were taken up to check and compare the validity of using additional MLC fields to achieve better homogeneity in dose distributions. The resultant dose distributions with and without presence of MLC were compared objectively. The dose volume histogram (DVH) of each plan for the PTV was evaluated. The 3D dose distributions and homogeneity index (HI) values were compared. The 3D dose maximum values were reduced by 4% to 7%, and hot spots assumed point size. Optimizations of 3D-CRT plans with MLC fields improved the homogeneity and conformability of dose distribution in the PTV. This paper outlines a method of obtaining optimal 3D dose distribution within the PTV in the 3D-CRT planning of breast cases.

Theoretical considerations of static and dynamic characteristics of air foil thrust bearing with tilt and slip flow

The thrust pad of the rotor is used to sustain the axial force generated due to the pressure difference between the compressor and turbine sides of turbomachinery such as gas turbines, compressors, and turbochargers. Furthermore, this thrust pad has a role to maintain and determines the attitude of the rotor. In a real system, it also helps reinforce the stiffness and damping of the journal bearing. This study was performed for the purpose of analyzing the characteristics of the air foil thrust bearing. The model for the air foil thrust bearing used in this study is composed of two parts: one is an inclined plane, which plays a role in increasing the load carrying capacity using the physical wedge effect, and the other is a flat plane. This study mainly consists of three parts. First, the static characteristics were obtained over the region of the thin air film using the finite-difference method (FDM) and the bump foil characteristics using the finite-element method (FEM). Second, the analysis of the dynamic characteristics was conducted by perturbation method. For more exact calculation, the rarefaction gas coefficients perturbed about the pressure and film thickness were taken into consideration. At last, the static and dynamic characteristics of the tilting condition of the thrust pad were obtained. Furthermore, the load carrying capacity and torque were calculated for both tilting and nontilting conditions. From this study, several results were presented: (1) the stiffness and damping of the bump foil under the condition of the various bump parameters, (2) the load carrying capacity and bearing torque at the tilting state, (3) the bearing performance for various bearing parameters, and (4) the effects considering the rarefaction gas coefficients.

Segmental MLC is superior to dynamic MLC for IMRT delivery

Segmental MLC is superior to dynamic MLC for IMRT delivery

Monte Carlo simulation of the Varian Clinac 600C accelerator dynamic and physical wedges

The present paper describes the study done on the dosimetric characteristics of the Varian Clinac 600C dynamic wedges (DW) and their comparison with the physical wedges (PW) in terms of the differences affecting the dose distributions, beam spectra, energy fluence and angular distributions. The geometry of the 4 MV photon beam and the dose distributions in a water phantom were simulated with GEANT3 and DPM Monte Carlo code systems. The DW was modelled through the constant movement of the upper jaws. The depth dose distributions and lateral profiles for the DW, PW and open fields were measured and compared with the Monte Carlo simulations and the global agreement was found to be within 3%. It was also found that the effects of a DW on beam spectral and angular distributions are much less significant than those produced by a PW. For example, in our study we found out that the 45°PW, when compared with the corresponding open field, can introduce a 30% increase in the mean photon energy due to the beam hardening effect and that it can also introduce a 4.5% dose reduction in the build-up region because of the reduction of the contaminated electrons by the PW. For the DW neither this mean-energy increase nor such dose reduction was found. The PW, when compared to the DW, significantly alters the photon-beam spectrum and these dosimetric differences are significant and further investigation must be performed to quantify the impact in clinical use of these beams.

SU‐FF‐T‐430: Two Beam Segment‐Based Intensity Modulated Radiation Therapy for Solid Tumours of the Brain

We present a novel segment‐based intensity‐modulated radiation therapy (s‐IMRT) technique that utilizes a single lateral and single vertex beam for the treatment of solid brain tumours that eliminates the necessity of employing physical wedges traditionally employed for such treatments. The two beam s‐IMRT method potentially allows for streamlined treatment planning and improved dose calculation accuracy while minimizing quality assurance workload compared to conventional multi‐beam IMRT treatments. The beam segments for lateral beams were generated based on the isodose distributions of a 100% weighted vertex beam while the vertex beam were created from the isodose distribution of a 100% weighted lateral beam. Segment weight optimization was performed using the inverse planning tool of the Pinnacle 7.6 treatment planning software. Segment‐IMRT plans were compared to “best‐case” wedge‐based plans to evaluate differences in dose uniformity, target coverage, and normal tissue sparing. Five patients with solid tumours of the brain were planned for radiation therapy. Overall, the s‐IMRT approach resulted in similar dose distributions as the wedge‐based plans. However, the overall treatment planning time was significantly reduced using s‐IMRT (∼ 60 min. compared to ∼ 150 min.). With the addition of script‐based automated segment generation and optimized inverse planning parameters it is believed that treatment planning time may be further reduced to ∼ 30 min. Furthermore, it is anticipated that the elimination of physical wedges will expedite patient set‐up time for treatments while improving dosimetric accuracy.

Comparison of Contralateral Breast Dose for Various Tangential Field Techniques in Clinical Radiotherapy

Contralateral breast (CLB) cancer is a rare but serious concern in radiotherapy. In this study, the CLB dose was measured using MOSFET dosimeter in 49 patients who underwent breast conservation surgery treated by different radiotherapy tangential field techniques, which included enhanced dynamic wedge (EDW), physical wedge, and intensity modulated radiation therapy (IMRT). The mean percent of the prescribed dose received by the contralateral areola in treatment technique using physical wedge (Cobalt), physical wedge (Linac), EDW, and IMRT were 4.27% (SD: 0.65), 3.61% (SD: 0.60), 3.38% (SD: 0.58), and 1.65% (SD: 0.24), respectively. There was a 29% CLB dose reduction at 3 cm from the medial tangential field border with IMRT compared to other wedged tangential field techniques. The study shows that the CLB dose could be reduced with IMRT or reducing or avoiding the medial wedge in conventional tangential field planning for breast cancer.

A study on contralateral breast surface dose for various tangential field techniques and the impact of set-up error on this dose

The risk of inducing contralateral breast (CLB) cancer in patients undergoing tangential field irradiation for the treatment of breast cancer is a serious concern in radiation oncology. A bilateral breast phantom made of wax attached onto the Alderson Rando phantom was used for studying the CLB dose for techniques using physical wedges, EDWs, IMRT and open fields. The skin dose to the CLB was measured at four different points (3 cm from the medial border of the tangential field (P1), nipple (P3), axilla (P4), midpoint between P3 and P1 (P2)). The highest measured dose occurred at P1 with the 60 degrees physical wedges; it was 15.3% of the dose at isocentre. Similarly, the dose measured at P3 (nipple) with 60 degrees physical wedges was 1.90 times higher than the dose with 60 degrees EDWs. The dose at P1 for IMRT (7.8%) was almost the same as that for the open field (8.7%). The skin dose measured at the nipple was 2.1 - 10.9 % of the isocentre dose. The highest CLB doses were contributed by medial wedged fields. The dose to the CLB can be reduced by using IMRT or avoiding wedging the medial tangential fields. A set-up error in the longitudinal direction has little impact on the CLB dose. Set-up errors > 1 cm in the vertical and lateral directions have significant impact on the CLB dose.

Experimental validation of the Eclipse AAA algorithm

The present study evaluates the performance of a newly released photon‐beam dose calculation algorithm that is incorporated into an established treatment planning system (TPS). We compared the analytical anisotropic algorithm (AAA) factory‐commissioned with “golden beam data” for Varian linear accelerators with measurements performed at two institutions using 6‐MV and 15‐MV beams. The TG‐53 evaluation regions and criteria were used to evaluate profiles measured in a water phantom for a wide variety of clinically relevant beam geometries. The total scatter factor (TSF) for each of these geometries was also measured and compared against the results from the AAA.At one institute, TLD measurements were performed at several points in the neck and thoracic regions of a Rando phantom; at the other institution, ion chamber measurements were performed in a CIRS inhomogeneous phantom. The phantoms were both imaged using computed tomography (CT), and the dose was calculated using the AAA at corresponding detector locations. Evaluation of measured relative dose profiles revealed that 97%, 99%, 97%, and 100% of points at one institute and 96%, 88%, 89%, and 100% of points at the other institution passed TG‐53 evaluation criteria in the outer beam, penumbra, inner beam, and buildup regions respectively. Poorer results in the inner beam regions at one institute are attributed to the mismatch of the measured profiles at shallow depths with the “golden beam data.”For validation of monitor unit (MU) calculations, the mean difference between measured and calculated TSFs was less than 0.5%; test cases involving physical wedges had, in general, differences of more than 1%. The mean difference between point measurements performed in inhomogeneous phantoms and Eclipse was 2.1% (5.3% maximum) and all differences were within TG‐53 guidelines of 7%. By intent, the methods and evaluation techniques were similar to those in a previous investigation involving another convolution–superposition photon‐beam dose calculation algorithm in another TPS, so that the current work permitted an independent comparison between the two algorithms for which results have been provided.PACS number: 87.53.Dq

Testing of the analytical anisotropic algorithm for photon dose calculation

The analytical anisotropic algorithm (AAA) was implemented in the Eclipse (Varian Medical Systems) treatment planning system to replace the single pencil beam (SPB) algorithm for the calculation of dose distributions for photon beams. AAA was developed to improve the dose calculation accuracy, especially in heterogeneous media. The total dose deposition is calculated as the superposition of the dose deposited by two photon sources (primary and secondary) and by an electron contamination source. The photon dose is calculated as a three-dimensional convolution of Monte-Carlo precalculated scatter kernels, scaled according to the electron density matrix. For the configuration of AAA, an optimization algorithm determines the parameters characterizing the multiple source model by optimizing the agreement between the calculated and measured depth dose curves and profiles for the basic beam data. We have combined the acceptance tests obtained in three different departments for 6, 15, and 18 MV photon beams. The accuracy of AAA was tested for different field sizes (symmetric and asymmetric) for open fields, wedged fields, and static and dynamic multileaf collimation fields. Depth dose behavior at different source-to-phantom distances was investigated. Measurements were performed on homogeneous, water equivalent phantoms, on simple phantoms containing cork inhomogeneities, and on the thorax of an anthropomorphic phantom. Comparisons were made among measurements, AAA, and SPB calculations. The optimization procedure for the configuration of the algorithm was successful in reproducing the basic beam data with an overall accuracy of 3%, 1 mm in the build-up region, and 1%, 1 mm elsewhere. Testing of the algorithm in more clinical setups showed comparable results for depth dose curves, profiles, and monitor units of symmetric open and wedged beams below dmax. The electron contamination model was found to be suboptimal to model the dose around dmax, especially for physical wedges at smaller source to phantom distances. For the asymmetric field verification, absolute dose difference of up to 4% were observed for the most extreme asymmetries. Compared to the SPB, the penumbra modeling is considerably improved (1%, 1 mm). At the interface between solid water and cork, profiles show a better agreement with AAA. Depth dose curves in the cork are substantially better with AAA than with SPB. Improvements are more pronounced for 18 MV than for 6 MV. Point dose measurements in the thoracic phantom are mostly within 5%. In general, we can conclude that, compared to SPB, AAA improves the accuracy of dose calculations. Particular progress was made with respect to the penumbra and low dose regions. In heterogeneous materials, improvements are substantial and more pronounced for high (18 MV) than for low (6 MV) energies.

The effects of intrafraction motion on dose homogeneity in a breast phantom with physical wedges, enhanced dynamic wedges, and ssIMRT

This study attempts to compare how breathing motion affects intact-breast cancer patients between three different treatment techniques and to determine the degree of improvement on dose homogeneity when implementing gating therapy. A breast phantom and respiratory simulator were designed to simulate respiratory motion to a first-order approximation. Film was used as a dosimeter, and static dosimetry data were used as a control for comparison. Three velocities of the breast phantom were studied, and gating therapy was introduced for each data set. Dose area histograms (DAHs) were calculated for a breast and a "lung" planning target area (PTA), and Normalized Agreement Test (NAT) indices were calculated in reference to the static case. Deviations from the static case were highest if the collimator speed was of the same magnitude as the speed of the target. In general, gating therapy improved dose uniformity to the breast PTA by up to 14% and reduced dose to the "lung" PTA by up to 24%. With step-and-shoot intensity-modulated radiation therapy (ssIMRT), gating the beam may compromise dose coverage of the breast PTA if the timing interval of the gate is too large. Gating the beam decreased NAT indices by 9 for physical wedges, by 16 for enhanced dynamic wedges, and by 6 for ssIMRT. Both the phantom and respiratory simulator are adequate for showing differences in dose distributions for all three treatment modalities. Gating therapy improves dose homogeneity to the PTAs and decreases the dose delivered to areas below the posterior border of the beams.

TH‐E‐224C‐03: Implementation of Monte Carlo Code VMC++ for Photon Beam Treatment Planning

Purpose: To implement the VMC++ Monte Carlo code for radiotherapy treatment planning of photon beams. The implementation includes the sampling of particles from a multiple‐source model, modeling of accessories, and commissioning of the model based on beam data measurements. Method and Materials: In this work, the radiation output from a linear accelerator was modeled using a multiple‐source model with separate sub‐sources for primary radiation, extra‐focal radiation and electron contamination. This approach enables the commissioning of an individual accelerator using a previously developed procedure, which determines the model parameters by minimizing deviations between measurements and superposition dose calculations. The same source model was used both for the superposition and for the Monte Carlo based dose calculation method. Physical wedges, MLCs, blocks and compensators were modeled by transporting particles sampled from the source model through a detailed geometrical model of the specific accessory. The MLC model accounts for tongue‐and‐groove, divergent leaf alignment, air cavities between adjacent leaves, and rounded leaf tips. Results: The accuracy of the developed algorithm was studied by comparing calculations to measurements for open fields, wedged fields, and irregular MLC apertures for 6 MV and 18 MV beam energies. The agreement between measurements and calculations was within statistical uncertainties for all of the studied cases. The calculation of a typical treatment plan takes from minutes to few hours, depending on the required statistical accuracy. Conclusion: It has been demonstrated that VMC++ with an optimized multiple‐source model and particle transport through accessories results in accurate dose distributions in a wide range of conditions. The developed algorithm has acceptable calculation speed, and has large potential for improving dose calculation accuracy in complex situations and heterogeneous cases compared to traditional dose calculation algorithms. Conflict of Interest: Research sponsored by Varian Medical Systems Inc.