Treatment Planning System Dose Calculation Research Articles

Proton range verification in inhomogeneous tissue: Treatment planning system vs. measurement vs. Monte Carlo simulation.

In particle radiotherapy, range uncertainty is an important issue that needs to be overcome. Because high-dose conformality can be achieved using a particle beam, a small uncertainty can affect tumor control or cause normal-tissue complications. From this perspective, the treatment planning system (TPS) must be accurate. However, there is a well-known inaccuracy regarding dose computation in heterogeneous media. This means that verifying the uncertainty level is one of the prerequisites for TPS commissioning. We evaluated the range accuracy of the dose computation algorithm implemented in a commercial TPS, and Monte Carlo (MC) simulation against measurement using a CT calibration phantom. A treatment plan was produced for eight different materials plugged into a phantom, and two-dimensional doses were measured using a chamber array. The measurement setup and beam delivery were simulated by MC code. For an infinite solid water phantom, the gamma passing rate between the measurement and TPS was 97.7%, and that between the measurement and MC was 96.5%. However, gamma passing rates between the measurement and TPS were 49.4% for the lung and 67.8% for bone, and between the measurement and MC were 85.6% for the lung and 100.0% for bone tissue. For adipose, breast, brain, liver, and bone mineral, the gamma passing rates computed by TPS were 91.7%, 90.6%, 81.7%, 85.6%, and 85.6%, respectively. The gamma passing rates for MC for adipose, breast, brain, liver, and bone mineral were 100.0%, 97.2%, 95.0%, 98.9%, and 97.8%, respectively. In conclusion, the described procedure successfully evaluated the allowable range uncertainty for TPS commissioning. The TPS dose calculation is inefficient in heterogeneous media with large differences in density, such as lung or bone tissue. Therefore, the limitations of TPS in heterogeneous media should be understood and applied in clinical practice.

A simple model for transit dosimetry based on a water equivalent EPID.

The aim of this study was to demonstrate a new model for implementing a transit dosimetry system as a means of invivo dose verification with a water equivalent electronic portal imaging device (WE-EPID) and a conventional treatment planning system (TPS). A standard amorphous silicon (a-Si) EPID was modified to a WE-EPID configuration by replacing the metal-plate/phosphor screen situated above the photodiode detector with a 3cm thick water equivalent plastic x ray converter material. A clinical TPS was used to calculate dose to the WE-EPID in its conventional EPID position behind the phantom/patient. The "extended phantom" concept was used to facilitate dose calculation at the EPID position, which is outside the CT field of view (FOV). The CT images were manipulated from 512×512 into 1024×1024 and padded pixels were assigned the density of air before importing to the TPS. The virtual WE-EPID was added as an RT structure of water density at the EPID plane. The accuracy of TPS dose calculations at the EPID plane in transit geometry was first evaluated for different field sizes and thickness of object in the beam by comparison with the dose measured using a 2D ion chamber array (ICA) and the WE-EPID. Following basic dose response tests, clinical fields including direct single fields (open and wedged) and modulated fields (integrated or control point by control point doses for VMAT) were measured for 6 MV photons with varying of solid water thickness or an anthropomorphic phantom present in beam. The EPID images were corrected for dark signal and pixel sensitivity and converted to dose using a single dose calibration factor. The 2D dose evaluation was conducted using 3%/3 and 2%/2mm gamma-index criteria. The measured dose-response with the ICA and WE-EPID for all basic dose-response tests agreed with TPS dose calculations to within 1.5%. The maximum difference in dose profiles for the largest measured field size of 25×25cm2 was 2.5%. Gamma evaluation showed at least 94% (3%/3mm criteria) and 90% (2%/2mm) agreement in both integrated and control-point doses for all clinical fields acquired by the WE-EPID and ICA when compared with TPS-calculated portal dose images. A new approach to transit dose verification has been demonstrated utilizing a water equivalent EPID and a commercial TPS. The accuracy of dose calculations at the EPID plane using a commercial TPS beam model was experimentally confirmed. The model proposed in this study provides an accurate method to directly verify doses delivered during treatment without the additional uncertainties inherent in modelling the complex dose-response of standard EPIDs.

Monaco and film dosimetry of 3D CRT, IMRT and VMAT cases in a realistic pelvic prosthetic phantom

The dosimetry of patients with metallic hip implants during irradiation of pelvic lesions is challenging due to dose distortions caused by implants. This work presents a dosimetric comparison of various multi-field photon-beam dose distributions in the presence of unilateral hip titanium prosthesis (UHTiP) embedded in a unique pelvic phantom made out of water-equivalent nylon slices. The impact of the UHTiP on the accuracy of dose calculations from a Monaco TPS (treatment planning system) using the X-ray voxel Monte Carlo (XVMC) algorithm was benchmarked against measured dose data using Gafchromic EBT3 film. Multi-field beam arrangements including a 4-field box, 5-field 3DCRT (three-dimensional conformal radiation therapy), 6-field IMRT (intensity modulated radiation therapy) and a single-arc VMAT (volumetric modulated arc therapy) plan were set up for 6MV and 15MV beams. These plans were generated for the pelvic phantom that contains the prosthesis with film inserted. Compared to Monaco TPS dose calculations, film measurements showed enhanced dose in the prosthesis which was not predicted by Monaco due to its limitation in relative density assignment. The enhanced prosthesis dose increased with increase in beam energy and decreased with the complexity of the treatment plans, with VMAT giving the least escalated dose. The dose increased between 5% and 19% for 6MV and between 6% and 21% for 15MV. A gamma index analysis showed that 70–92% of dose points (excluding the prosthesis) were within 3% discrepancy. Increasing the number of treatment fields increases target dose coverage and improves the agreement between film and Monaco. When the relative electron density (RED) in the prosthesis was varied between 3.72 and 15 the dose discrepancy between film and Monaco increased from 30% to 57% for 6MV and from 30% to 50% for 15MV. The study indicates that beam weights for fields that pass through the prosthesis should be minimised and its RED must be correct for accurate dose calculation on Monaco.

15. EPID-based pretreatment quality assurance: Dosimetric evaluation of application software

Introduction With the prevalence of gantry-fixed and rotational IMRT, pretreatment dosimetric Quality Assurance (QA) is a good practice. The pretreatment control is a dosimetric measurement under the treatment device which allows the virtual information transmitted from the Treatment Planning System (TPS) to be linked to the actual dose delivery. As known for a few years, the Electronic Portal Imaging Device (EPID) could potentially be used for dosimetric and routine machine QA purposes, other than patient setup verification which is its intended initial purpose. Based on EPID acquisitions, software dedicated to patient specific QA allow comparing, in dosimetric terms, the portal image acquired under the treatment machine with a predicted image computed under ideal conditions. In this work, we evaluate an independent and automatic solution integrating both an absolute dose conversion model of the portal images, and a prediction model computing theoretical 2D dose distributions from the TPS Dicom RT Plan file. The purpose of the study is to describe the relevance of the prediction model, with reference to TPS calculations, and the conversion model compared to a true dose measurement. Methods Predictive 2D dose images and dose converted portal images are computed with the ThinkQA/EpiBeam solution (Dosisoft S.A.). By convention, these two images are expressed in absolute dose at isocentre level under 5 cm depth of water. Reference dose calculations and dose measurements are performed, in homogenous phantom in the same conventional conditions, respectively by the TPS Isogray (Dosisoft S.A.) or Monaco (Elekta) and with the Matrixx (IBA). EPID images are acquired with the iViewGT system (Elekta). The evaluation is carried out with a 6 MV photon beam delivered by an Elekta Synergy treatment unit and includes static beams, IMRT and VMAT plans. Dosimetric comparisons are performed according to 2D gamma-index agreement (GAI) values. Results Similar dose distributions are obtained with the prediction model and both TPS. GAI are greater than 95% with restrictive gamma-index criteria (2% dose, 2 mm distanceto-agreement) for dose region receiving more than 10% of the maximum dose. Conversion model returns pertinent experimental dose values compared to the Matrixx measurements. GAI of about 90% (3%, 3 mm) or 95% (3%, 4 mm) are achievable according to the poor spatial resolution (7 mm) of the Matrixx device. Conclusions The pretreatment software solution studied returns accurate predictive and converted dose images and therefore, a relevant comparison between TPS dose calculation and beam delivery. Automatic computation of the predictive image by the system frees the users from the need of creating specific QA plans in the TPS.

Evaluation of Effect of Different Computed Tomography Scanning Protocols on Hounsfield Unit and Its Impact on Dose Calculation by Treatment Planning System

Introduction: In radiotherapy treatment planning system (TPS), basic input is the data from computed tomography (CT) scan, which takes into account the effect of inhomogeneities in dose calculations. Measurement of CT numbers may be affected by scanner-specific parameters. Therefore, it is important to verify the effect of different CT scanning protocols on Hounsfield unit (HU) and its impact on dose calculation. This study was carried out to analyse the effect of different tube voltages on HU for various tissue substitutes in phantom and their dosimetric impact on dose calculation in TPS due to variation in HU–relative electron density (RED) calibration curves. Materials and Methods: HU for different density materials was obtained from CT images of the phantom acquired at various tube voltages. HU-RED calibration curves were drawn from CT images with various tissue substitutes acquired at different tube voltages used to quantify the error in dose calculation for different algorithms. Doses were calculated on CT images acquired at 120 kVp and by applying CT number to RED curve obtained from 80, 100, 120, and 140 kVp voltages. Results: No significant variation was observed in HU of different density materials for various kVp values. Doses calculated with applying different HU-RED calibration curves were well within 1%. Conclusion: Variation in doses calculated by algorithms with various HU-RED calibration curves was found to be well within 1%. Therefore, it can be concluded that clinical practice of using the standard HU-RED calibration curve by a 120 kVp CT acquisition technique is viable.

Evaluation of dose calculations accuracy of a commercial treatment planning system for the head and neck region in radiotherapy.

The objective was to quantify dose calculation accuracy of TiGRT TPS for head and neck region in radiotherapy. In radiotherapy of head and neck cancers, treatment planning is difficult, due to the complex shape of target volumes and also to spare critical and normal structures. These organs are often very near to the target volumes and have low tolerance to radiation. In this regard, dose calculation accuracy of treatment planning system (TPS) must be high enough. Thermoluminescent dosimeter-100 (TLD-100) chips were used within RANDO phantom for dose measurement. TiGRT TPS was also applied for dose calculation. Finally, difference between measured doses (Dmeas) and calculated doses (Dcalc) was obtained to quantify the dose calculation accuracy of the TPS at head and neck region. For in-field regions, in some points, the TiGRT TPS overestimated the dose compared to the measurements and for other points underestimated the dose. For outside field regions, the TiGRT TPS underestimated the dose compared to the measurements. For most points, the difference values between Dcalc and Dmeas for the in-field and outside field regions were less than 5% and 40%, respectively. Due to the sensitive structures to radiation in the head and neck region, the dose calculation accuracy of TPSs should be sufficient. According to the results of this study, it is concluded that the accuracy of dose calculation of TiGRT TPS is enough for in-field and out of field regions.

Development of a flattening filter free multiple source model for use as an independent, Monte Carlo, dose calculation, quality assurance tool for clinical trials.

The Imaging and Radiation Oncology Core-Houston (IROC-H) Quality Assurance Center (formerly the Radiological Physics Center) has reported varying levels of compliance from their anthropomorphic phantom auditing program. IROC-H studies have suggested that one source of disagreement between institution submitted calculated doses and measurement is the accuracy of the institution's treatment planning system dose calculations and heterogeneity corrections used. In order to audit this step of the radiation therapy treatment process, an independent dose calculation tool is needed. Monte Carlo multiple source models for Varian flattening filter free (FFF) 6 MV and FFF 10 MV therapeutic x-ray beams were commissioned based on central axis depth dose data from a 10 × 10 cm2 field size and dose profiles for a 40 × 40 cm2 field size. The models were validated against open-field measurements in a water tank for field sizes ranging from 3 × 3 cm2 to 40 × 40 cm2 . The models were then benchmarked against IROC-H's anthropomorphic head and neck phantom and lung phantom measurements. Validation results, assessed with a ±2%/2 mm gamma criterion, showed average agreement of 99.9% and 99.0% for central axis depth dose data for FFF 6 MV and FFF 10 MV models, respectively. Dose profile agreement using the same evaluation technique averaged 97.8% and 97.9% for the respective models. Phantom benchmarking comparisons were evaluated with a ±3%/2 mm gamma criterion, and agreement averaged 90.1% and 90.8% for the respective models. Multiple source models for Varian FFF 6 MV and FFF 10 MV beams have been developed, validated, and benchmarked for inclusion in an independent dose calculation quality assurance tool for use in clinical trial audits.

Clinical Experience with Machine Log File Software for Volumetric-Modulated Arc Therapy Techniques

Mobius FX, an add-on software module from Mobius Medical Systems” for intensity-modulated radiation therapy (IMRT) quality assurance (QA), uses linac treatment logs to efficiently calculate and verify the 3D dose delivered to patients. An advantage of the Mobius FX module is that it does not require device positioning. In this study, we compared the Mobius FX with another QA option, ArcCheck, as well as with the treatment planning system (TPS) using 30 volumetric-modulated arc therapy (VMAT) plans planned and delivered on a Varian TrueBeam linac. The plans, which involved 6 and 10 MV and consisted of 2 to 3 arcs per plan, were selected to provide a clinically relevant sample. The average gamma value for all plans between Mobius FX and the TPS was 99.96% for the criterion of 3%-3 mm and 98.80% for the criterion of 2%-2 mm. Very similar results were found when comparing Mobius FX and the TPS dose calculations with those acquired by traditional methods (i.e., ArcCheck). As the gamma criterion of the analysis was narrowed, discrepancies between Mobius FX and traditional methods appeared. Profile analysis showed the production of comparable results when using the Mobius FX method or traditional QA methods. In conclusion, the Mobius FX method for pretreatment of patient-specific QA is capable of producing results similar to those obtained by traditional methods.

Out-of-field in vivo dosimetry using TLD in SABR for primary kidney cancer involving mixed photon fields

PurposeTo assess out-of-field dose using three different variants of LiF thermoluminescence dosimeters (TLD) for ten patients who underwent stereotactic ablative body radiotherapy (SABR) for primary renal cell carcinoma (RCC) and compare with treatment planning system (TPS) dose calculations. Methods and materialsThermoluminescent dosimeter (TLD) measurements were conducted at 20, 30, 40 and 50cm from isocentre on ten patients undergoing SABR for primary RCC. Three types of high-sensitivity LiF:Mg,Cu,P TLD material with different 6Li/7Li isotope ratios were used. Patient plans were calculated using Eclipse Anisotropic Analytical Algorithm (AAA) for clinical evaluation and recalculated using Pencil Beam Convolution (PBC) algorithm for comparison. ResultsBoth AAA and PBC showed diminished accuracy for photon doses at increasing distance out-of-field. At 50cm, measured photon dose was 0.3cGy normalised to a 10Gy prescription on average with only small variation across all patients. This is likely due to the leakage component of the out-of-field dose. The 6Li-enriched TLD materials showed increased signal attributable to additional neutron contribution. ConclusionLiF:Mg,Cu,P TLD containing 6Li is sensitive enough to measure out-of-field dose 50cm from isocentre however will over-estimate the photon component of out-of-field dose in high energy treatments due to the presence of thermal neutrons. 7Li enriched materials which are insensitive to neutrons are therefore required for accurate photon dosimetry. Neutron signal has been shown here to increase with MUs and is higher for patients treated using certain non coplanar beam arrangements. Further work is required to convert this additional neutron signal to dose.

The dosimetric comparison of Truebeam and Cyberknife treatment planning systems dose calculation accuracy for brain SRS treatment on Randophantom

Purpose Brain stereotactic radiosurgery (SRS) involves the use of precisely directed, single session radiation to create a desired radiobiologic response within the brain target with acceptable minimal effects on surrounding structures or tissues. In this study, the dosimetric comparison of target and OAR doses in Truebeam 6 FFF and Cyberknife M6 machines were made. Methods Treatment planning were done using 2 full arc VMAT technique with 6 FFF beam on the CT scan of Randophantom simulating the treatment of sterotactic treatments for one brain metastasis. The dose distribution were calculated using Eclipse treatment planning system (TPS) with Acuros 13 algorithm. In addition treatment planning for same target volume were also done for Cyberknife M6 machine using Multiplan TPS with Monte Carlo algorithm. Target and brainstem, chiasm, optic nerve, lense median doses were measured using OSL dosimeters. The measured and calculated doses were compared. Results The calibration of OSL dosimetry were done at first. Target dose measurements made using the OSL dosimetry for both Cyberknife and Truebeam within the measurement uncertainty of 3.1%. The max differences between OSL measured and Eclipse calculated OARs doses were 3.3%,4.5%,5.6% and 6.2% for brainstem, chiasm, optic nerve, lense doses respectively. For cyberknife treatment, the max differences between OSL measured and Monte Carlo calculated OARs doses were 0.9%,1.2%,1.6% and 2.2% for brainstem, chiasm, optic nerve, lense doses respectively. Conclusion The study shows that dosimetrically comparable plans are achievable Cyberknife and Linac. However, a better conformity, target coverage, less OAR dose and insteeper dose fall off away from the target is achieved with Cyberknife. Monte carlo calculation algorithm predicts the OARs doses more accurately than Acuros algorithm.

Dosimetric evaluation of the dose calculation accuracy of different algorithms for two different treatment techniques during whole breast irradiation

Purpose In this study, dosimetric comparison of field in field (FIF) and intensity modulated radiation therapy (IMRT) techniques used for treatment of whole breast radiotherapy (WBRT) were made. The dosimetric accuracy of treatment planning system (TPS) for Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB) algorithms in predicting PTV and OAR doses was also investigated. Methods Two different treatment planning techniques of left-sided breast cancer were generated for rando phantom. FIF and IMRT plans were compared for doses in PTV and OAR volumes including ipsilateral lung, heart, left ascending coronary artery, contralateral lung and the contralateral breast. PTV and OARs doses and homogeneity and conformality indexes were compared between two techniques. The accuracy of TPS dose calculation algorithms was tested by comparing PTV and OAR doses measured by thermoluminescent dosimetry with the dose calculated by the TPS using AAA and AXB for both techniques. Results IMRT plans had better conformality and homogeneity indexes than FIF technique and it spared OARs better than FIF. While both algorithms overestimated PTV doses they underestimated all OAR doses. For IMRT plan, PTV doses, overestimation up to 2.5% was seen with AAA algorithm but it decreased to 1.8% when AXB algorithm was used. Based on the results of the anthropomorphic measurements for OAR doses, underestimation greater than 7% is possible by the AAA. The results from the AXB are much better than the AAA algorithm. However, underestimations of 4.8% were found in some of the points even for AXB. For FIF plan, similar trend was seen for PTV and OARs doses in both algorithm. Conclusions When using the Eclipse TPS for breast cancer, AXB the should be used instead of the AAA algorithm, bearing in mind that the AXB may still underestimate all OAR doses.

Dosimetric accuracy of the cone-beam CT-based treatment planning of the Vero system: a phantom study.

We report an investigation on the accuracy of dose calculation based on the cone‐beam computed tomography (CBCT) images of the nonbowtie filter kV imaging system of the Vero linear accelerator. Different sets of materials and tube voltages were employed to generate the Hounsfield unit lookup tables (HLUTs) for both CBCT and fan‐beam CT (FBCT) systems. The HLUTs were then implemented for the dose calculation in a treatment planning system (TPS). Dosimetric evaluation was carried out on an in‐house‐developed cube phantom that consists of water‐equivalent slabs and inhomogeneity inserts. Two independent dosimeters positioned in the cube phantom were used in this study for point‐dose and two‐dimensional (2D) dose distribution measurements. The differences of HLUTs from various materials and tube voltages in both CT systems resulted in differences in dose calculation accuracy. We found that the higher the tube voltage used to obtain CT images, the better the point‐dose calculation and the gamma passing rate of the 2D dose distribution agree to the values determined in the TPS. Moreover, the insert materials that are not tissue‐equivalent led to higher dose‐calculation inaccuracy. There were negligible differences in dosimetric evaluation between the CBCT‐ and FBCT‐based treatment planning if the HLUTs were generated using the tissue‐equivalent materials. In this study, the CBCT images of the Vero system from a complex inhomogeneity phantom can be applied for the TPS dose calculation if the system is calibrated using tissue‐equivalent materials scanned at high tube voltage (i.e., 120 kV).PACS number(s): 87.55.de, 87.56.Fc, 87.57.qp

SU‐F‐T‐406: Verification of Total Body Irradiation Commissioned MU Lookup Table Accuracy Using Treatment Planning System for Wide Range of Patient Sizes

Purpose:To verify the accuracy of total body irradiation (TBI) measurement commissioning data using the treatment planning system (TPS) for a wide range of patient separations.Methods:Our institution conducts TBI treatments with an 18MV photon beam at 380cm extended SSD using an AP/PA technique. Currently, the monitor units (MU) per field for patient treatments are determined using a lookup table generated from TMR measurements in a water phantom (75 × 41 × 30.5 cm3). The dose prescribed to an umbilicus midline point at spine level is determined based on patient separation, dose/ field and dose rate/MU. One‐dimensional heterogeneous dose calculations from Pinnacle TPS were validated with thermoluminescent dosimeters (TLD) placed in an average adult anthropomorphic phantom and also in‐vivo on four patients with large separations. Subsequently, twelve patients with various separations (17–47cm) were retrospectively analyzed. Computed tomography (CT) scans were acquired in the left and right decubitus positions from vertex to knee. A treatment plan for each patient was generated. The ratio of the lookup table MU to the heterogeneous TPS MU was compared.Results:TLD Measurements in the anthropomorphic phantom and large TBI patients agreed with Pinnacle calculated dose within 2.8% and 2%, respectively. The heterogeneous calculation compared to the lookup table agreed within 8.1% (ratio range: 1.014–1.081). A trend of reduced accuracy was observed when patient separation increases.Conclusion:The TPS dose calculation accuracy was confirmed by TLD measurements, showing that Pinnacle can model the extended SSD dose without commissioning a special beam model for the extended SSD geometry. The difference between the lookup table and TPS calculation potentially comes from lack of scatter during commissioning when compared to extreme patient sizes. The observed trend suggests the need for development of a correction factor between the lookup table and TPS dose calculations.

SU‐F‐SPS‐04: Dosimetric Evaluation of the Dose Calculation Accuracy of Different Algorithms for Two Different Treatment Techniques During Whole Breast Irradiation

Purpose:In this study, dosimetric comparison of field in field (FIF) and intensity modulated radiation therapy (IMRT) techniques used for treatment of whole breast radiotherapy (WBRT) were made. The dosimetric accuracy of treatment planning system (TPS) for Anisotropic Analytical Algorithm (AAA) and Acuros XB (AXB) algorithms in predicting PTV and OAR doses was also investigated.Methods:Two different treatment planning techniques of left‐sided breast cancer were generated for rando phantom. FIF and IMRT plans were compared for doses in PTV and OAR volumes including ipsilateral lung, heart, left ascending coronary artery, contralateral lung and the contralateral breast. PTV and OARs doses and homogeneity and conformality indexes were compared between two techniques. The accuracy of TPS dose calculation algorithms was tested by comparing PTV and OAR doses measured by thermoluminescent dosimetry with the dose calculated by the TPS using AAA and AXB for both techniques.Results:IMRT plans had better conformality and homogeneity indexes than FIF technique and it spared OARs better than FIF. While both algorithms overestimated PTV doses they underestimated all OAR doses. For IMRT plan, PTV doses, overestimation up to 2.5 % was seen with AAA algorithm but it decreased to 1.8 % when AXB algorithm was used. Based on the results of the anthropomorphic measurements for OAR doses, underestimation greater than 7 % is possible by the AAA. The results from the AXB are much better than the AAA algorithm. However, underestimations of 4.8 % were found in some of the points even for AXB. For FIF plan, similar trend was seen for PTV and OARs doses in both algorithm.Conclusion:When using the Eclipse TPS for breast cancer, AXB the should be used instead of the AAA algorithm, bearing in mind that the AXB may still underestimate all OAR doses.

A patient immobilization device for prone breast radiotherapy: Dosimetric effects and inclusion in the treatment planning system

PurposeTo assess the dosimetric impact of a patient positioning device for prone breast radiotherapy and assess the accuracy of a treatment planning system (TPS) in predicting this impact. MethodsBeam attenuation and build-up dose perturbations, quantified by ionization chamber and radiochromic film dosimetry, were evaluated for 3 components of the patient positioning device: the carbon fiber baseplate, the support cushions and the support wedge for the contralateral breast. Dose calculations were performed using the XVMC dose engine implemented in the Monaco TPS. All components were included during planning CT acquisition. ResultsBeam attenuation amounted to 7.57% (6MV) and 5.33% (15MV) for beams obliquely intersecting the couchtop–baseplate combination. Beams traversing large sections of the support wedge were attenuated by 12.28% (6MV) and 9.37% (15MV). For the support cushion foam, beam attenuation remained limited to 0.11% (6MV) and 0.08% (15MV) per centimeter thickness. A substantial loss of dose build-up was detected when irradiating through any of the investigated components. TPS dose calculations accurately predicted beam attenuation by the baseplate and support wedge. A manual density overwrite was needed to model attenuation by the support cushion foam. TPS dose calculations in build-up regions differed considerably from measurements for both open beams and beams traversing the device components. ConclusionsIrradiating through the components of the positioning device resulted in a considerable degradation of skin sparing. Inclusion of the device components in the treatment planning CT allowed to accurately model the most important attenuation effect, but failed to accurately predict build-up doses.

A Monte Carlo model for independent dose verification in IMRT and VMAT for the Varian Novalis TX with high definition MLC

Purpose: With intensity modulated radiation therapy (IMRT), the physician can prescribe, design and deliver optimized treatment plans that target the tumor and spare adjacent critical structures. The increased conformity of such plans often comes at the expenses of adding significant complexity to the delivery of the treatment. With volumetrically modulated arc therapy (VMAT), in addition to the modulation of the intensity of the radiation beam, other mechanical parameters such as gantry speed and dose rate are varied during treatment delivery. It is therefore imperative that we develop comprehensive and accurate methods to validate such complex delivery techniques prior to the commencement of the patient’s treatment. Methods : In this study, a Monte Carlo simulation was performed for the high definition multileaf collimator (HD-MLC) of a Varian Novalis TX linac. Our simulation is based on the MCSIM code and provides a comprehensive model of the linac head. After validating the model in reference geometries, treatment plans for different anatomical sites were simulated and compared against the treatment planning system (TPS) dose calculations. All simulations were performed in a cylindrical water phantom as opposed to the patient anatomy, to remove any complexities associated with density effects. Finally, a comparison through gamma analysis of dose plane between the simulation, the TPS and the measurements from the Matrixx array (IBA) was conducted to verify the accuracy of our model against both the measurements and the TPS. Results : Gamma analysis of ten IMRT and ten VMAT cases for different anatomical sites was performed, using a 3%/3 mm passing criterion. The average passing rates were 97.5% and 94.3% for the IMRT and the VMAT plans respectively when comparing the MCSIM and TPS dose calculations. Conclusion : In the present work a Monte Carlo model of a Novalis TX linac which has been tested and benchmarked to produce phase-space files for the treatment head of the linac was used to produce a input phase-space to calculated dose deposition phenomena in different geometries for IMRT and VMAT treatment modalities. The control points defined for the MLC were replaced by blocks with the same characteristics and materials of the linac MLC to speed up the simulation time. With this technique a simulation of a typical IMRT case can be performed with a 10 computer cluster in about 1.02 hours in average. If the number of computer used is increased the computing time can be reduced even more which make our model suitable for clinical use as a second check method to compare the TPS dose calculated. Our results showed that for IMRT and VMAT deliveries with a HD-MLC, there is an average of 95.9% of the points have a gamma index less than 1 with our chosen criterion between our Monte Carlo simulations and the corresponding measurements and TPS calculations in a cylindrical water equivalent phantom. This Monte Carlo code can be used as pre-treatment, independent dose calculation verification for IMRT and VMAT deliveries.

Verification of the pure alanine in PMMA tube dosimeter applicability for dosimetry of radiotherapy photon beams: a feasibility study.

Alanine dosimeters in the form of pure alanine powder in PMMA plastic tubes were investigated for dosimetry in a clinical application. Electron paramagnetic resonance (EPR) spectroscopy was used to measure absorbed radiation doses by detection of signals from radicals generated in irradiated alanine. The measurements were performed for low-dose ranges typical for single-fraction doses often used in external photon beam radiotherapy. First, the dosimeters were irradiated in a solid water phantom to establish calibration curves in the dose range from 0.3 to 3 Gy for 6 and 18 MV X-ray beams from a clinical linear accelerator. Next, the dosimeters were placed at various locations in an anthropomorphic pelvic phantom to measure the dose delivery of a conventional four-field box technique treatment plan to the pelvis. Finally, the doses measured with alanine dosimeters were compared against the doses calculated with a commercial treatment planning system (TPS). The results showed that the alanine dosimeters have a highly sensitive dose response with good linearity and no energy dependence in the dose range and photon beams used in this work. Also, a fairly good agreement was found between the in-phantom dose measurements with alanine dosimeters and the TPS dose calculations. The mean value of the ratios of measured to calculated dose values was found to be near unity. The measured points in the in-field region passed dose-difference acceptance criterion of 3% and those in the penumbral region passed distance-to-agreement acceptance criterion of 3 mm. These findings suggest that the pure alanine powder in PMMA tube dosimeter is a suitable option for dosimetry of radiotherapy photon beams.

Effects of the difference in tube voltage of the CT scanner on dose calculation

Computed tomography (CT) measures the attenuation coefficient of an object and converts the value assigned to each voxel into a CT number. In radiation therapy, the CT number, which is directly proportional to the linear attenuation coefficient, must be converted to an electron density for radiation dose calculations for cancer treatment. However, if various tube voltages are applied to take the patient’s CT image without applying the specific CT number to the electron density conversion curve, the accuracy of the dose calculation is not assured. In this study, changes in CT numbers for different materials due to changes in the tube voltage were demonstrated, and the dose calculation errors in the percentage depth dose (PDD), along with a clinical case were analyzed. The maximum dose difference in the PDD from the treatment planning system (TPS) dose calculation and from the Monte Carlo simulation were 1.3% and 1.1%, respectively, when applying the same CT number to the electron density conversion curve for the 80-kVp and 140-kVp images. In the clinical case, different CT number to electron density conversion curves at tube voltage of 80 kVp and 140 kVp were applied to the same image and the maximum differences in the mean, maximum, and minimum doses were 1.1%, 1.2%, and 1.0%, respectively, at the central region of the phantom and 0.6%, 0.9%, and 0.8%, respectively, at the peripheral region of the phantom.

SU-E-T-276: Dose Calculation Accuracy with a Standard Beam Model for Extended SSD Treatments

Purpose: While most photon treatments are delivered near 100cm SSD or less, a subset of patients may benefit from treatment at SSDs greater than 100cm. A proposed rotating chair for upright treatments would enable isocentric treatments at extended SSDs. The purpose of this study was to assess the accuracy of the Pinnacle3 treatment planning system dose calculation for standard beam geometries delivered at extended SSDs with a beam model commissioned at 100cm SSD. Methods: Dose to a water phantom at 100, 110, and 120cm SSD was calculated with the Pinnacle 3 CC convolve algorithm for 6x beams for 5×5, 10×10, 20×20, and 30×30cm2 field sizes (defined at the water surface for each SSD). PDDs and profiles (depths of 1.5, 12.5, and 22cm) were compared to measurements in water with an ionization chamber. Point-by-point agreement was analyzed, as well as agreement in field size defined by the 50% isodose. Results: The deviations of the calculated PDDs from measurement, analyzed from depth of maximum dose to 23cm, were all within 1.3% for all beam geometries. In particular, the calculated PDDs at 10cm depth were all within 0.7% of measurement. For profiles, the deviations within the central 80% of the field were within 2.2% for all geometries. The field sizes all agreed within 2mm. Conclusion: The agreement of the PDDs and profiles calculated by Pinnacle3 for extended SSD geometries were within the acceptability criteria defined by Van Dyk (±2% for PDDs and ±3% for profiles). The accuracy of the calculation of more complex beam geometries at extended SSDs will be investigated to further assess the feasibility of using a standard beam model commissioned at 100cm SSD in Pinnacle3 for extended SSD treatments.

Independent calculation of monitor units for VMAT and SPORT.

Dose and monitor units (MUs) represent two important facets of a radiation therapy treatment. In current practice, verification of a treatment plan is commonly done in dose domain, in which a phantom measurement or forward dose calculation is performed to examine the dosimetric accuracy and the MU settings of a given treatment plan. While it is desirable to verify directly the MU settings, a computational framework for obtaining the MU values from a known dose distribution has yet to be developed. This work presents a strategy to calculate independently the MUs from a given dose distribution of volumetric modulated arc therapy (VMAT) and station parameter optimized radiation therapy (SPORT). The dose at a point can be expressed as a sum of contributions from all the station points (or control points). This relationship forms the basis of the proposed MU verification technique. To proceed, the authors first obtain the matrix elements which characterize the dosimetric contribution of the involved station points by computing the doses at a series of voxels, typically on the prescription surface of the VMAT/SPORT treatment plan, with unit MU setting for all the station points. An in-house Monte Carlo (MC) software is used for the dose matrix calculation. The MUs of the station points are then derived by minimizing the least-squares difference between doses computed by the treatment planning system (TPS) and that of the MC for the selected set of voxels on the prescription surface. The technique is applied to 16 clinical cases with a variety of energies, disease sites, and TPS dose calculation algorithms. For all plans except the lung cases with large tissue density inhomogeneity, the independently computed MUs agree with that of TPS to within 2.7% for all the station points. In the dose domain, no significant difference between the MC and Eclipse Anisotropic Analytical Algorithm (AAA) dose distribution is found in terms of isodose contours, dose profiles, gamma index, and dose volume histogram (DVH) for these cases. For the lung cases, the MC-calculated MUs differ significantly from that of the treatment plan computed using AAA. However, the discrepancies are reduced to within 3% when the TPS dose calculation algorithm is switched to a transport equation-based technique (Acuros™). Comparison in the dose domain between the MC and Eclipse AAA/Acuros calculation yields conclusion consistent with the MU calculation. A computational framework relating the MU and dose domains has been established. The framework does not only enable them to verify the MU values of the involved station points of a VMAT plan directly in the MU domain but also provide a much needed mechanism to adaptively modify the MU values of the station points in accordance to a specific change in the dose domain.