Collimator Scatter Factors Research Articles

Mathematic modeling of the dose in water for calculating collimator scatter factor of flattening filter and flattening filter free megavoltage photon beams

This study presents an empirical method to model the percent depth dose (PDD) curves of flattening filter (FF) and FF-free (FFF) high-energy photon beams using a home-generated buildup function and tail function (buildup-tail function) in radiation therapy. The modeling parameters n and μ of the buildup-tail function were used to characterize the collimator scatter factor (Sc). The buildup function was a quadratic function in the form of d/ √(d2 + n) with main parameters of d (depth in water) and n, while the tail function was in the form of e−μd and was composed of an exponential function with d and μ. PDD was the product of buildup-tail function and characterized by the buildup-tail function by adjusting the parameters n and μ. The Sc of 6 and 10 MV in FF and FFF beams can be expressed simply by the modeling parameters n and μ. The main parameter n increases when photon energy increases. Its physical meaning expresses the beam hardening of photon energy in PDD. The parameter μ can be treated as an attenuation coefficient in the tail function, decreasing when photon energy increases. The values of n and μ obtained from the fitted buildup-tail function were applied into an analytical formula of Sc,FF = nE(S)0.63 μ E, Sc,FFF = nE(S)4.45 μ E to get the Sc of 6 and 10 MV in FF and FFF photon beams, with nE, μ E, S denoting n and μ at photon energy E of field size S. The calculated Sc was compared with the measured data and showed agreement, finding that the field difference was size within ±1%. This model can be used to parameterize the Sc for some clinical requirements. The modeling parameters n and μ can be used to predict the Sc in either FF or FFF beams for the treatment monitor unit in double-check dose calculation. The technique developed in this study can also be used for systematic or random errors in the quality assurance program, thus improving the clinical dose computation accuracy for patient treatment.

Effect of collimator scatter factors on dose calculation of different breast cancer cases in radiotherapy

Effect of collimator scatter factors on dose calculation of different breast cancer cases in radiotherapy

Empirical modeling of the percent depth dose for megavoltage photon beams.

This study presents an empirical method to model the high-energy photon beam percent depth dose (PDD) curve by using the home-generated buildup function and tail function (buildup-tail function) in radiation therapy. The modeling parameters n and μ of buildup-tail function can be used to characterize the Collimator Scatter Factor (Sc) either in a square field or in the different individual upper jaw and lower jaw setting separately for individual monitor unit check. The PDD curves for four high-energy photon beams were modeled by the buildup and tail function in this study. The buildup function was a quadratic function in the form of [Formula: see text] with the main parameter of d (depth in water) and n, while the tail function was in the form of e-μd and was composed by an exponential function with the main parameter of d and μ. The PDD was the product of buildup and tail function, PDD = [Formula: see text]. The PDD of four-photon energies was characterized by the buildup-tail function by adjusting the parameters n and μ. The Sc of 6 MV and 10 MV can then be expressed simply by the modeling parameters n and μ. The main parameters n increases in buildup-tail function when photon energy increased. The physical meaning of the parameter n expresses the beam hardening of photon energy in PDD. The fitting results of parameters n in the buildup function are 0.17, 0.208, 0.495, 1.2 of four-photon energies, 4 MV, 6 MV, 10 MV, 18 MV, respectively. The parameter μ can be treated as attenuation coefficient in tail function and decreases when photon energy increased. The fitting results of parameters μ in the tail function are 0.065, 0.0515, 0.0458, 0.0422 of four-photon energies, 4 MV, 6 MV, 10 MV, 18 MV, respectively. The values of n and μ obtained from the fitted buildup-tail function were applied into an analytical formula of Sc = nE(S)0.63μE to get the collimator to scatter factor Sc for 6 and 10 MV photon beam, while nE, μE, S denotes n, μ at photon energy E of field size S, respectively. The calculated Sc were compared with the measured data and showed agreement at different field sizes to within ±1.5%. We proposed a model incorporating a two-parameter formula which can improve the fitting accuracy to be better than 1.5% maximum error for describing the PDD in different photon energies used in clinical setting. This model can be used to parameterize the Sc factors for some clinical requirements. The modeling parameters n and μ can be used to predict the Sc in either square field or individual jaws opening asymmetrically for treatment monitor unit double-check in dose calculation. The technique developed in this study can also be used for systematic or random errors in the QA program, thus improves the clinical dose computation accuracy for patient treatment.

Effect of Collimator Scatter Factors on Dose Calculation of Different Breast Cancer Cases in Radiotherapy

Effect of Collimator Scatter Factors on Dose Calculation of Different Breast Cancer Cases in Radiotherapy

Determination of the Small-Field Output Factor for 6 MV Photon Beam Using EGSnrc Monte Carlo.

Accuracy of ionization chamber (IC) to measure the scatter output factor (Scp) of a linear accelerator (linac) is crucial, especially in small field (<4 cm × 4 cm). The common IC volume of 0.6 cc is not adequate for small-field measurement and not all radiotherapy centers can afford to purchase additional IC due to the additional cost. This study aimed to determine the efficiency of the EGSnrc Monte Carlo (MC) to calculate the Scp for various field sizes including small field in Elekta Synergy (Agility multileaf collimator) linac. The BEAMnrc and DOSXYZnrc user codes were used to simulate a 6 MV linac model for various field sizes and calculate the radiation dose output in water phantom. The modeled linac treatment head was validated by comparing the percentage depth dose (PDD), beam profile, and beam quality (Tissue Phantom Ratio (TPR)20,10) with the IC measurement. The validated linac model was simulated to calculate the Scp consisting of collimator scatter factor (Sc) and phantom scatter factor (Sp). The PDD and beam profile of the simulated field sizes were within a good agreement of ±2% compared with the measured data. The TPR20,10 value was 0.675 for field size 10 cm × 10 cm. The Scp, Sc, and Sp simulated values were close to the IC measurement within ±2% difference. The simulation for Sc and Sp in 3 cm × 3 cm field size was calculated to be 0.955 and 0.884, respectively. In conclusion, this study validated the efficiency of the MC simulation as a promising tool for the Scp calculation including small-field size for linac.

The use of Pantherpix pixel detector in radiotherapy QA.

There are various different detectors, which can be used for radiotherapy measurements, and more are about to be adopted. Hybrid pixel detectors (HPD) have been originally developed for the high energy physics. However, over the last few years they also expanded in the medical physics. Novel 2D detector Pantherpix is a HPD designed specifically for the radiotherapy. In this article, its properties are characterised and an assessment of its use in radiotherapy photon beams is provided. Properties such as response stability, response linearity, angular dependence and energy dependence were studied. In order to prove sufficient clinical quality for relative dosimetry, further measurements were undertaken (i.e. dose profiles and collimator scatter factors). Acquired results were compared with ion chamber and gafchromic film results. Namely the applicability of PhPix for cobalt beam therapy, which is still widely used (and will be used in near future) in economically less developed countries, is considered.

COLLIMATOR EXCHANGE EFFECT OF MEGAVOLTAGE PHOTON BEAMS AND ITS IMPACT ON CLINICAL DOSIMETRY

AIM To determine the Collimator Exchange Effect (CEE) for telecobalt unit (Bhabhatron -II TAW) and Linear accelerator unit (Varian Trilogy). MATERIALS AND METHOD The study was carried out in Bhabhatron-II TAW Telecobalt machine and Varian Trilogy Linear Accelerator. The study was done to find the collimator scatter factor (S¬c) for rectangular fields at 5 and 10gm/cm2 depths using indigenously designed mini phantom. Three sets of electrometer reading were noted for the irradiation of each field size and the average was taken for calculation. Sc values for different rectangular fields were then calculated from these data sets and the CEE at two depths (5 and 10 gm/cm2) for the 6MV and 15MV photon and Co-60 gamma beam were calculated. RESULTS The values of Sc obtained for the rectangular fields as alternatively defined by X &amp; Y jaws are different for high energy photon beams indicating CEE. The maximum percentage difference between the Sc of the corresponding collimator settings for the Bhabhatron-II TAW unit for depths 5 and 10gm/cm2 were found to be 0.42% and 0.5% respectively. Sc values for 6MV and 15MV photon beams were found to be higher when Y-jaw (upper jaw) acts as the longer side of the rectangular field. The maximum percentage difference between the Sc values of the corresponding collimator settings for 6MV at depths 5 and 10gm/cm2 were 2.74% and 2.87% respectively whereas for 15 MV the differences were 3% and 2.99%. CONCLUSION The CEE of Cobalt Teletherapy units can be ignored in clinical dosimetry. However, the CEE of Linear Accelerators having energies 6MV &amp; 15MV should be taken into consideration. A two dimensional table of Sc should be generated for rectangular fields during MU calculations in Linear Accelerators. For TPS based calculations, the generated data should be incorporated during beam modeling for accurate dose delivery in clinical dosimetry.

Investigation of Estimation Accuracy of Phantom Scatter Factor by Clarkson Method Considering Depth in MLC Irregular Field

In monitor unit (MU) independent verification by calculation for irregular field (MLC field) using multileaf collimator in X-ray therapy, it has become common to use collimator scatter factor (Sc) and phantom scatter factor (Sp) instead of total scatter factor (Sc, p). It is usually expressed as Sc, p (A)=Sc (A)×Sp (A), and the field size A is considered but the depth d is not. Sc is data of in-air output, and measure with a mini-phantom at constant depth to remove electron contamination. On the other hand, Sp is obtained from measurement data of Sc, p and Sc, and can be expressed as Sc, p (d, A)=Sc (constant depth, A)×Sp (d, A) at an arbitrary depth d, thus Sp depends on the depth of Sc, p. Therefore, Sp needs to consider depth. In addition, a linear accelerator equipped with the tertiary MLC has two field sizes, that are collimator field by upper and lower collimators and MLC field by tertiary MLC below them. In MU independent verification by calculation, it is often used that the estimated value of Sp obtained by converting MLC field to equivalent square field and referring to data of Sp in square field. To convert the MLC field to equivalent square field, a conversion formula from sector radius r to equivalent square field L by Clarkson's sector integration (Clarkson method) is used. In this study, using 24 types of MLC fields to evaluate estimation accuracy due to the difference of conversion formula in Clarkson method, we estimated value of Sp using r=0.5611L of B-Clarkson method and using r=0.5580L of A-Clarkson method. And the difference with the measured value of Sp obtained by measuring Sc, p and Sc in the same MLC fields was compared. While, to evaluate estimation accuracy due to the different depths using these Clarkson methods, the difference between estimated value and measured value of Sp similarly obtained at depth of 5, 10 and 15 cm was compared. As results, estimated value of Sp using A-Clarkson method than using B-Clarkson method was close to measured value, and it was the same trend at depth of 5, 10 and 15 cm. Therefore, it was suggested that estimation accuracy of Sp by A-Clarkson method is higher than B-Clarkson method when verifying beams with different depths in MU independent verification by calculation for MLC field.

Interpretation of In-air Output Ratio of Wedged Fields in Different Measurement Conditions.

Background:The collimator scatter factor (Sc) is one of the most important parameters in monitor unit (MU) calculation. There are several factors that impact Sc values, including head structures, backscatter in dose monitoring chambers, and wedges. The objective of this study was to investigate the variation of Sc with different buildup cap materials, wall thickness of buildup caps, source-to-chamber distances (SCDs), ionization chambers, and wedge angles in 6 MV photon beam.Methods:In this study, copper and Perspex buildup caps were made with two different thicknesses for each buildup cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK dosimeter with a sensitive volume of 0.120 cm3 and Farmer-type ion chamber with a sensitive volume of 0.65 cm3. In all measurements, buildup caps and ionization chambers were positioned such as to stand vertically to the beam central axis. It was also investigated the effect of internal wedge with different angles (30° and 60°) different SCDs on Sc.Results:It was found in large field sizes, Sc values in Perspex buildup cap were higher than copper. Different SCDs and type of ion chamber and wall thickness of buildup caps had no significant influence on Sc values. The presence of wedge influenced Sc values significantly. Variation of Sc in wedged fields compared to open fields had a maximum deviation of 0.9% and 6.8% in 30° and 60° wedge angles, respectively.Conclusion:It was found that the presence of wedges had a significant influence on Sc and increases with wedge angles. As such, it should be taken into account in manual MU calculations.

Advantages and disadvantages of removed flattening filter Linac: A Monte Carlo study on full quantitative dosimetric data of 18 MV-Varian Linac

Introduction: During intensity modulated radiation therapy (IMRT) technique, theoretically, presence of flattening filter (FF) across the beamline of clinical linear accelerator (linac) is not essential to obtain uniform lateral profiles due to intensity modulation of photon beams by multileaf collimators (MLCs). The aim of this study was to investigate the dosimetrical properties of 18 MV photon Beam-Varian linac with and without FF. Materials and Methods: All dose measurements were performed on 18 MV, FF Mode-Varian 2100C/D linac. The FF and flattening filter free (FFF) modes of linac were modeled by MCNPX 2.4. code. The photon and contaminant electrons spectra, dose rate, present depth doses (PDD), lateral dose profiles, total and collimator scatter factors and out of field doses were calculated and compared with and without FF. Results: Removing the FF increased the photon and contaminant electron fluences by factors of 5.48 and 3.94 for a 5 × 5 cm2 field size, respectively. The surface dose increased up to 155%. The flatness of dose profile was disturbed and deteriorated with increase of field size. Despite the dependence of the total scattering factor on field size, the variation of collimator scattering factors was neglected. The out-of-field dose decreased about 81.5 % for a 5 × 5 cm2 field size. Conclusion: Removing FF from the linac’s head increases the dose rate and decreases the out-of-field dose, but the increased skin dose and deteriorated flatness of lateral dose profile are the main

Beam characteristic of P.E collimators Add-on multileaf collimator

Introduction: Multileaf collimators (MLCs) are computer-controlled devices which are Useful and cost-effective tools for conformal therapy and intensity modulation radiotherapy (IMRT). Nowadays MLCs are important and standard configuration of the new medical linear accelerators which are substitute of conventional Blocks and has eliminated the difficulties of working with blocks. MLCs are available from many manufacturers and their designs differ in the way they couple with conventional jaws, number and width of leaves and in their physical and dosimetric characteristics, well understanding on the mechanics and dosimetric characteristics of MLC is the basic task of medical physicist. Materials and Methods: In this study some mechanical and dosimetric characteristics of a P.E. Collimators Add-On MLC which is installed on VARIAN CLINAC 2100 C/D are presented. Scatter factors (Sc, Sp, Scp), central axis depth dose and beam profiles were measured before and after installation of MLC. mechanical and Leaf Leakages for 6MV and 18MV beams were measured with Gafchromic EBT3 films. Results: radiation and light fields were match below the 2mm acceptable level based on AAPM reports. Average leaf leakage was noted 1.79% for 6MV and 1.98% for 18MV beams, respectively. No significant differences were found in beam profiles, collimator (Sc), phantom (Sp) and total (Sc, p) scatter factors after MLC installation. The percent depth dose data for all fields and both beam energies were within 1.6 % of the original data. Conclusion: Based on the Mechanical and dosimetric results which have been achieved from our different standard tests, we found no significant differences between before and after MLC installation Values on Clinac 2100 C/D Varian LINAC.

Collimator scatter factor: Monte Carlo and in-air measurements approaches

BackgroundLinac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.MethodsBeams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm2 were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions.Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.ResultsThe depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.ConclusionA better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.

Utilizing a diagnostic kV imaging system for x-ray psoralen activated cancer therapy (X-PACT)

X-ray psoralen activated cancer therapy (X-PACT) is a new therapeutic approach that has been shown to induce tumor cell apoptosis and cytotoxicity in vitro, and slow tumor growth in BALB/c mice with syngeneic 4T1 tumors. X-PACT is accomplished by injection of co-incubated psoralen and phosphors; the phosphors emit UV to activate psoralen, and are activated by an external kV source. Here we describe application of a kV x-ray source mounted on board a medical linear accelerator for X-PACT in preparation for a phase I clinical trial of X-PACT for spontaneous tumors in pet dogs. We commissioned a 80 kVp beam at 50–80 cm from the source with varied blade settings to achieve rectangular collimated beams. Commissioning included dosimetry measurements, developing a formalism for absolute dose calculation in water, FLUKA Monte Carlo based planning and evaluation, and verification measurements. Dosimetry measurements included AAPM TG-61 absolute dose calibration, depth dose curves, backscatter and collimator scatter factors, heel effect, and leakage. Reasonable agreement was achieved between measurement and Monte Carlo, and between calculated dose and verification measurements. Finally, we demonstrate the X-PACT treatment process for an example dog with a 3–5 cc left hip sarcoma located at a 2 cm depth. The absolute dose formalism indicated 21 pulses of 160 mAs were required to deliver the prescription dose of 0.6 Gy to 2.8 cm depth. The dose distribution was calculated with the Monte Carlo planning tool and visualized at 1 × 1 × 2 mm3 spatial resolution. A dose enhancement in the hip bone of up to 4.5 Gy was observed. This work demonstrates that X-PACT is feasible utilizing diagnostic kV sources such as those mounted on a clinical linear accelerator, and reports commissioning and treatment planning data and formalism respectively.

Independent dose verification for brain stereotactic radiotherapy using different add-on micro multi-leaf collimators.

An add-on micro multi-leaf collimator (mMLC) is used for stereotactic radiosurgery (SRS) and brain stereotactic radiotherapy (SRT), in which rotational radiotherapy may make more complex and time-consuming. We performed a retrospective study of an independent dose calculation verification for brain SRS and SRT in two institutions to show the accuracy of the verification system and propose a tolerance value for the verification. Several comparisons of static plans and patients' plans were conducted using a phantom measurement, and patients' plans using the patients' own computed tomography image. We evaluated the accuracy of the Clarkson-based dose calculation based on either the equivalent square field formed by the mMLC or by the collimator jaws to determine the collimator scatter factor (S c). The results for the static plans showed good agreement (<1%), except when we used a 1cm2 field size (<4%). The phantom measurements for the patients' plans showed deviations of 0.1±2.3 and 1.2±1.6% (2 SD) for the treatment planning system and the verification system, respectively. The patients' plans showed a deviation of 2.0±2.1% (2 SD). Depending on the mMLC system, the S c was calculated using the equivalent field size formed by the mMLC. In this study, we suggest a tolerance level for the brain SRS and SRT of 2-3.5%. However, beam modeling in the treatment planning system would affect the deviation. The S c should be computed according to the size of the collimator fitted to the MLC.

Dosimetric characterization of a novel dual-energy medical linear accelerator without a flattening filter

Objective To study the dosimetric characteristics of flattened and flattening filter free (FFF) beams with 6 MV and 10 MV photon energy using a novel dual-energy medical linear accelerator (Elekta Versa HDTM), to identify the dosimetric characteristics and advantages of FFF beams, and to provide a basis for their clinical application. Methods The percentage depth dose (PDD), profiles/dose rate of off-axis ratio (OAR), field size, penumbral width, dose out of the fields, collimator scatter factor (Sc), and total scatter factor (Sc, p) were compared between flattened and FFF beams. Results (1) After beam energy matching, the FFF beams had the same beam energy with the flattened beams. The matching error of PDD at a depth of 10 cm was less than 1% between fields.(2) The FFF beams had a smaller variation in dose rate of OAR with the depth than the flattened beams.(3) The FFF beams had smaller variations in field size and penumbral width than the flattened beams. Moreover, the penumbral width of the FFF beams increased with the increasing field size or depth. The FFF beams had a lower dose out of the fields than the flattened beams.(4) The FFF beams had smaller variations in Sc and Sc, p with the field size and depth than the flattened beams. Conclusions Removal of the flattening filter can substantially improve the dose rate, shorten radiotherapy time, and reduce leakage and scattering of the head. The dosimetric advantages make the FFF beams appropriate for clinical treatment. Key words: Linear accelerator; Flattening filter; Flattening filter free

An Analytical Method to Calculate Phantom Scatter Factor for Photon Beam Accelerators.

IntroductionOne of the important input factors in the commissioning of the radiotherapy treatment planning systems is the phantom scatter factor (Sp) which requires the same collimator opening for all radiation fields. In this study, we have proposed an analytical method to overcome this issue.MethodsThe measurements were performed using Siemens Primus Plus with photon energy 6 MV for field sizes from 5×5cm2 to 40×40cm2. Phantom scatter factor was measured through the division of total scatter output factors (Scp), and collimator scatter factor (Sc).ResultsThe mean percent difference between the measured and calculated Sp was 1.00% and -3.11% for 5×5, 40×40 cm2 field size respectively.ConclusionThis method is applicable especially for small fields used in IMRT which, measuring collimator scatter factor is not reliable due to the lateral electron disequilibrium.

Impact of Jaw Position Accuracy on Dose Variation in Volumetric Modulated Arc Therapy Using Jaw Tracking.

The technique of jaw tracking can be used in volumetric modulated arc therapy (VMAT) and intensity modulated radiation therapy (IMRT). In this technique, jaw tracks apertures of the multi leaf collimator (MLC) during irradiation. It is reported that dose variation is occurred by the changing accuracy of MLC position in VMAT and IMRT. Though jaw moves dynamically in the irradiation by using this technique, the influence of the jaw position accuracy on dose variation is not examined. The purpose of this study is to verify the influence of jaw position accuracy on dose variation in VMAT using jaw tracking. We appended intentional errors on jaw position in digital image communication in medicine-radiation therapy (DICOM-RT) plans created using jaw tracking technique. These plans were irradiated on the phantom that was inserted ion chamber, and we measured dose variation by changing the intentional error. The dose variation in planning target volume (PTV) was enlarged by increasing the error, and agreed with the variation of a collimator scatter factor within 0.03%. In clinical case of oropharyngeal cancer, the maximum dose variations in parotid gland were 0.179% and 1.23% when the errors were 1 mm and 10 mm, respectively. Dose variation in parotid gland was larger than the variation in PTV and spinal cord because of increasing MLC transmission. The dose variation caused by jaw position error was smaller than it caused by MLC position error. So, we can keep the dose error slightly that is related to jaw position error in VMAT by maintaining jaw position accuracy correctly.

Validation and clinical implementation of commercial secondary check software with heterogeneity corrections

AimTo validate and implement PTW diamond secondary check software (SCS) in a routine clinical use. BackgroundThe secondary independent monitor unit or dose calculation verifications have led to a significant increase in the workflow associated with QA treatments. Modelling, validation and commissioning are necessary steps thereby making it a useful tool for QA. Materials and methodsPTW Diamond SCS is capable of calculating VMAT fields, based on modified Clarkson integration, accounting for multi-leaf collimators (MLC) transmission and measured collimator scatter factors. Validation for heterogeneity corrections is made using circular phantom with inserts of various density materials. 150 VMAT plans were compared using (i) plans calculated in homogeneous cylindrical phantom and (ii) VMAT plans calculated with heterogeneity corrections using electron density values for each organ. ResultsDiamond SCS calculated dose for homogeneous cylindrical phantom resulted in average deviation of (0.1±2.14%) with Eclipse TPS calculated dose and (−2.0±1.66%) with absolute measured dose. PTW's OCTAVIUS-4D phantom with 729 ion chamber detector array measurements agreed well with Eclipse TPS calculated dose showing an average deviation of (−1.69±1.56%). Diamond SCS dose calculations were performed with heterogeneity corrections for 124 VMAT plans with isocentre at a region above −350HU. The overall MU variations between Diamond SCS and TPS Acuros-XB algorithms were within ±5%. ConclusionHence, the Diamond SCS can be used as an additional tool along with phantom measurements for patient specific quality assurance of VMAT plans with heterogeneity corrections having isocentre at a region above −350HU.

SU-G-TeP1-08: LINAC Head Geometry Modeling for Cyber Knife System

Purpose: Knowledge of the LINAC head information is critical for model based dose calculation algorithms. However, the geometries are difficult to measure precisely. The purpose of this study is to develop linac head models for Cyber Knife system (CKS). Methods: For CKS, the commissioning data were measured in water at 800mm SAD. The measured full width at half maximum (FWHM) for each cone was found greater than the nominal value, this was further confirmed by additional film measurement in air. Diameter correction, cone shift and source shift models (DCM, CSM and SSM) are proposed to account for the differences. In DCM, a cone-specific correction is applied. For CSM and SSM, a single shift is applied to the cone or source physical position. All three models were validated with an in-house developed pencil beam dose calculation algorithm, and further evaluated by the collimator scatter factor (Sc) correction. Results: The mean square error (MSE) between nominal diameter and the FWHM derived from commissioning data and in-air measurement are 0.54mm and 0.44mm, with the discrepancy increasing with cone size. Optimal shift for CSM and SSM is found to be 9mm upward and 18mm downward, respectively. The MSE in FWHM is reduced to 0.04mm and 0.14mm for DCM and CSM (SSM). Both DCM and CSM result in the same set of Sc values. Combining all cones at SAD 600–1000mm, the average deviation from 1 in Sc of DCM (CSM) and SSM is 2.6% and 2.2%, and reduced to 0.9% and 0.7% for the cones with diameter greater than 15mm. Conclusion: We developed three geometrical models for CKS. All models can handle the discrepancy between vendor specifications and commissioning data. And SSM has the best performance for Sc correction. The study also validated that a point source can be used in CKS dose calculation algorithms.

SU‐C‐204‐06: Monte Carlo Dose Calculation for Kilovoltage X‐Ray‐Psoralen Activated Cancer Therapy (X‐PACT): Preliminary Results

Purpose:X‐PACT is an experimental cancer therapy where kV x‐rays are used to photo‐activate anti‐cancer therapeutics through phosphor intermediaries (phosphors that absorb x‐rays and re‐radiate as UV light). Clinical trials in pet dogs are currently underway (NC State College of Veterinary Medicine) and an essential component is the ability to model the kV dose in these dogs. Here we report the commissioning and characterization of a Monte Carlo (MC) treatment planning simulation tool to calculate X‐PACT radiation doses in canine trials.Methods:FLUKA multi‐particle MC simulation package was used to simulate a standard X‐PACT radiation treatment beam of 80kVp with the Varian OBI x‐ray source geometry. The beam quality was verified by comparing measured and simulated attenuation of the beam by various thicknesses of aluminum (2–4.6 mm) under narrow beam conditions (HVL). The beam parameters at commissioning were then corroborated using MC, characterized and verified with empirically collected commissioning data, including: percent depth dose curves (PDD), back‐scatter factors (BSF), collimator scatter factor(s), and heel effect, etc. All simulations were conducted for N=30M histories at M=100 iterations.Results:HVL and PDD simulation data agreed with an average percent error of 2.42%±0.33 and 6.03%±1.58, respectively. The mean square error (MSE) values for HVL and PDD (0.07% and 0.50%) were low, as expected; however, longer simulations are required to validate convergence to the expected values. Qualitatively, pre‐ and post‐filtration source spectra matched well with 80kVp references generated via SPEKTR software. Further validation of commissioning data simulation is underway in preparation for first‐time 3D dose calculations with canine CBCT data.Conclusion:We have prepared a Monte Carlo simulation capable of accurate dose calculation for use with ongoing X‐PACT canine clinical trials. Preliminary results show good agreement with measured data and hold promise for accurate quantification of dose for this novel psoralen X‐ray therapy.Funding Support, Disclosures, &amp; Conflict of Interest: The Monte Carlo simulation work was not funded; Drs. Adamson &amp; Oldham have received funding from Immunolight LLC for X‐PACT research.