Heterogeneous Dose Calculation Research Articles

Heterogeneous Versus Homogeneous Radiation Dose Calculations of Twice-Daily Fractionation in Small Cell Lung Carcinoma.

PurposeThe standard radiotherapy regimen for small cell lung cancer (SCLC) was determined using dose calculations without corrections for tissue heterogeneity, while modern treatments are planned using algorithms accounting for tissue heterogeneity. We assessed differences in dose delivered using heterogeneous and homogeneous dose calculations in a cohort of patients treated for limited-stage small cell lung cancer (LS-SCLC).MethodsThis is a retrospective analysis of 35 patients (three-dimensional conformal radiation therapy (3D-CRT), n = 22; intensity-modulated radiation therapy (IMRT), n = 13) with LS-SCLC treated with chemoradiotherapy from 2011 to 2017. Treatment plans were developed in the Eclipse Treatment Planning System (TPS) version 13.6 using the Analytical Anisotropic Algorithm (AAA). Two plans were generated for each patient with one using the unit relative electron density and the other maintaining the same monitor units (MUs) with tissue density corrections. The prescription was 45 Gy in 30 fractions of 1.5 Gy delivered twice daily. Individuals who underwent replanning within the same treatment course were evaluated using a separate corrected and uncorrected plan sum. Variations greater than 5% in dose to the tumor or organs at risk were considered clinically relevant. A two-sided paired t-test was used to evaluate the statistical significance of the dosimetric differences.ResultsThe percent dose difference between plans without tissue heterogeneity corrections to those with corrections resulted in an overall median difference of -3% (range: -15.1% to 9.6%; p < 0.01) for the dose covering 95% of the planning target volume (PTV D95) and was -5.6% (range: -17.3% to 5.4%; p < 0.01) for lung volume receiving ≥20 Gy (lung V20). For 3D-CRT, the median difference for the PTV D95 was -0.1% (range: -4.7% to 9.6%; p = 0.62) and the lung V20 was -4.2% (range: -9.4 to 5.4; p < 0.01). For IMRT, the median difference for the PTV D95 was -10.0% (range: -15.1% to -5.3%; p < 0.01) and the lung V20 was -8.9% (range: -17.3 to -3.5; p < 0.01).ConclusionTraditional planning without tissue heterogeneity corrections results in an overall decrease in the dose delivered to the target compared with those that incorporate tissue heterogeneity corrections. These differences are modest for 3D treatment plans but may result in clinically relevant differences for the IMRT cohort (>5% deviation).

Monte Carlo Simulation on Heterogeneous Phantom and Physical Dose Calculation on Heterogeneous Organ

Monte Carlo Simulation on Heterogeneous Phantom and Physical Dose Calculation on Heterogeneous Organ

P1.09 Heterogeneous versus Homogeneous Radiation Dose Calculations of Twice Daily Fractionation in Small Cell Lung Carcinoma

P1.09 Heterogeneous versus Homogeneous Radiation Dose Calculations of Twice Daily Fractionation in Small Cell Lung Carcinoma

Radiation Dose to Heart and Great Vessels During Esophageal HDR Brachytherapy

Esophageal cancer patients’ initial treatment is high dose rate (HDR) intraluminal brachytherapy. The treatments are often given as 1800cGy over 3 fractions. Currently, post-treatment CT scans are not required for brachytherapy. During a given treatment fraction, the cranial and caudal extent of the disease is identified under fluoroscopic guidance and a predetermined treatment plan is selected based on the clinical observations. A fraction dose of 600cGy is prescribed at 1cm from the centre axis of the treatment catheter. This will inevitably result in the heart being irradiated. The aim of this study is to assess radiation dose received by the heart and great vessels during HDR intraluminal brachytherapy of esophageal cancers. This study evaluated the treatment plans of 11 patients who underwent a CT scan with the catheter in place following intraluminal HDR brachytherapy of the esophagus. A marker wire was placed into the catheter for better visualization. Post-treatment CT data were imported into the Eclipse BrachyVision planning system. Contours of heart and great vessels (aorta, pulmonary artery, superior vena cava) were drawn. The catheter was digitized and all treatment parameters were reproduced in the planning system resulting in the delivered 3-dimensional dose distribution. Homogeneous and heterogeneous dose calculations were performed and the resulting DVHs analyzed for maximum point doses and volume doses. Comparisons were made with Timmerman’s 3-fraction hypofractionated dose constraint recommendations for normal tissues. Volume doses of organs at risk (OARs) were well below the Timmerman dose constraints; however, maximum point doses calculated in the heart greatly exceeded the respective constraint. The dose to the heart was related to the proximity of the heart to the tumour and hence treatment catheter. Differences in calculated OAR doses using homogeneous and heterogeneous dose calculation engines were minimal. The study’s assessment of dose received to the heart and great vessels validated the accuracy of standardized planning of esophageal HDR brachytherapy. Therefore, current protocols without post-treatment CT are satisfactory and heterogeneous dose calculations are not necessary.

Verification of Acuros XB dose algorithm using 3D printed low-density phantoms for clinical photon beams.

The transport‐based dose calculation algorithm Acuros XB (AXB) has been shown to accurately account for heterogeneities primarily through comparisons with Monte Carlo simulations. This study aims to provide additional experimental verification of AXB for clinically relevant flattened and unflattened beam energies in low density phantoms of the same material. Polystyrene slabs were created using a bench‐top 3D printer. Six slabs were printed at varying densities from 0.23 to 0.68 g/cm3, corresponding to different density humanoid tissues. The slabs were used to form different single and multilayer geometries. Dose was calculated with Eclipse™ AXB 11.0.31 for 6MV, 15MV flattened and 6FFF (flattening filter free) energies for field sizes of 2 × 2 and 5 × 5 cm2. EBT3 film was inserted into the phantoms, which were irradiated. Absolute dose profiles and 2D Gamma analyses were performed for 96 dose planes. For all single slab configurations and energies, absolute dose differences between the AXB calculation and film measurements remained <3% for both fields in the high‐dose region, however, larger disagreement was seen within the penumbra. For the multilayered phantom, percentage depth dose with AXB was within 5% of discrete film measurements. The Gamma index at 2%/2 mm averaged 98% in all combinations of fields, phantoms and photon energies. The transport‐based dose algorithm AXB is in good agreement with the experimental measurements for small field sizes using 6MV, 6FFF and 15MV beams adjacent to various low‐density heterogeneous media. This work provides preliminary experimental grounds to support the use of AXB for heterogeneous dose calculation purposes.

Coupling I-125 permanent implant prostate brachytherapy Monte Carlo dose calculations with radiobiological models.

To investigate the coupling of radiobiological models with patient-specific Monte Carlo (MC) dose calculations for permanent implant prostate brachytherapy (PIPB). To compare radiobiological indices evaluated with different radiobiological models using MC and simulated AAPM TG-43 dose calculations. Three-dimensional dose distributions previously computed using MC techniques with two types of patient models, TG43sim (AAPM TG-43 water-based conditions) and MCDmm (realistic tissues and interseed effects), for 613 PIPB patients are coupled with biological dose and tumour control probability (TCP) models. Two approaches and their extensions are considered to evaluate biological doses, biologically effective dose (BED) and isoeffective dose (IED), as well as two methods to evaluate TCP. Three novel extensions of equivalent uniform biologically effective dose (EUBED) are suggested which consider the spatial distribution of doses within the target volume. Adopted radiobiological model parameter values (α, β, etc) are those suggested by AAPM TG-137, and sensitivity to parameter choice is discussed. MCDmm dose calculations can reveal low doses in the prostate target volume, due to tissue heterogeneities or inter-seed effects; considering these low doses in EUBED calculations can lower TCP estimates by up to 70%, with largest differences in patients with calcifications. There are large variations in biological doses and TCPs evaluated over the 613 patient cohort for each radiobiological model considered, reflecting the spectrum of physical doses calculated for these patients with either MCDmm or TG43sim. Depending on the model details, BED, IED and EUBED are, on average, 6.0-9.8%, 7.4-9.2% and 1.8-15% higher, respectively, with TG43sim than MCDmm. TCP estimates computed using MCDmm dose distributions are much lower than expected based on past treatment outcome studies, suggesting a need to re-assess model parameters when evaluating radiobiological indices coupled with heterogeneous tissue model-based dose calculations. Cohort average differences in biological dose and TCP estimates between radiobiological models are generally larger than differences for any one radiobiological model evaluated with TG43sim or MCDmm dose calculations. However, heterogeneous tissue dose calculations, like MCDmm, can identify clinically-relevant low dose volumes, e.g., in patients with calcifications, which would otherwise be missed with TG-43. In addition to affecting physical dose distributions, these low dose volumes can largely impact radiobiological dose and TCP estimates, which further motivates the clinical implementation of model-based dose calculations for PIPB.

Sci‐Sat AM: Radiation Dosimetry and Practical Therapy Solutions ‐ 04: On 3D Fabrication of Phantoms and Experimental Verification of Patient Dose Computation Algorithms

Purpose:Transport based dose calculation algorithm Acuros XB (AXB) has been shown to accurately account for heterogeneities mostly through comparisons with Monte Carlo simulations. This study aims at providing additional experimental verification for AXB for flattened and unflattened clinical energies in low density phantoms of the same material.Materials and Methods:Polystyrene slabs were created using a bench‐top 3D printer. Six slabs were printed at varying densities from 0.23 g/cm3 to 0.68 g/cm3, corresponding to different density humanoid tissues. The slabs were used to form different single and multilayer geometries. Dose was calculated with AXB 11.0.31 for 6MV, 15MV flattened and 6FFF (flattening filter free) energies for field sizes of 2×2 cm2 and 5×5 cm2. The phantoms containing radiochromic EBT3 films were irradiated. Absolute dose profiles and 2D gamma analyses were performed for 96 dose planes.Results:For all single slab, multislab configurations and energies, absolute dose differences between the AXB calculation and film measurements remained &lt;3% for both fields, with slightly poor disagreement in penumbra. The gamma index at 2% / 2mm averaged 98% in all combinations of fields, phantoms and photon energies.Conclusions:The transport based dose algorithm AXB is in good agreement with the experimental measurements for small field sizes using 6MV, 6FFF and 15MV beams adjacent to low density heterogeneous media. This work provides sufficient experimental ground to support the use of AXB for heterogeneous dose calculation purposes.

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.

Dose Specification for NRG Radiation Therapy Trials

Ambiguity exists in dose reporting in the medical physics community ( 1 Andreo P. Dose to “water-like” media or dose to tissue in MV photons radiotherapy treatment planning: Still a matter of debate. Phys Med Biol. 2015; 60: 309-337 Crossref PubMed Scopus (54) Google Scholar ). While heterogeneous dose calculations are standard, algorithms remain that conduct homogeneous calculations. Even among algorithms that conduct heterogeneous dose calculations, there are many different implementations that may compute dose to water (Dw) or dose to medium (Dm). A consistent method for dose reporting for NRG clinical trials is important to ensure that outcomes can ultimately be correlated to doses specified uniformly. Many NRG clinical trials require specification of Dw. However, it is often unclear if an algorithm is actually computing Dw or Dm. There are also conceptual and practical concerns over some existing methods to convert from Dm to Dw ( 1 Andreo P. Dose to “water-like” media or dose to tissue in MV photons radiotherapy treatment planning: Still a matter of debate. Phys Med Biol. 2015; 60: 309-337 Crossref PubMed Scopus (54) Google Scholar , 2 Ma C.M. Li J. Dose specification for radiation therapy: Dose to water or dose to medium?. Phys Med Biol. 2011; 56: 3073-3089 Crossref PubMed Scopus (100) Google Scholar ). This has sparked debate about dose reporting in some NRG protocols. In this communication, we wish to clarify the Dw and Dm concepts for modern dose algorithms and provide recommendations for dose specification for all NRG clinical trials.

Comparison of Dose Distributions With TG-43 and Collapsed Cone Convolution Algorithms Applied to Accelerated Partial Breast Irradiation Patient Plans

Tocompare the treatment plans for accelerated partial breast irradiation calculated by the new commercially available collapsed cone convolution (CCC) and current standard TG-43-based algorithms for 50 patients treated at our institution with either a Strut-Adjusted Volume Implant (SAVI) or Contura device. We recalculated target coverage, volume of highly dosed normal tissue, and dose to organs at risk (ribs, skin, and lung) with each algorithm. For 1 case an artificial air pocket was added to simulate 10% nonconformance. We performed a Wilcoxon signed rank test to determine the median differences in the clinical indices V90, V95, V100, V150, V200, and highest-dosed 0.1cm(3) and 1.0cm(3) of rib, skin, and lung between the two algorithms. The CCC algorithm calculated lower values on average for all dose-volume histogram parameters. Across the entire patient cohort, the median difference in the clinical indices calculated by the 2 algorithms was <10% for dose to organs at risk, <5% for target volume coverage (V90, V95, and V100), and <4cm(3) for dose to normal breast tissue (V150 and V200). No discernable difference was seen in the nonconformance case. We found that on average over our patient population CCC calculated (<10%) lower doses than TG-43. These results should inform clinicians as they prepare for the transition to heterogeneous dose calculation algorithms and determine whether clinical tolerance limits warrant modification.

Mean Organ Doses Resulting From Non-Human Primate Whole Thorax Lung Irradiation Prescribed to Mid-Line Tissue.

Multi-organ dose evaluations and the effects of heterogeneous tissue dose calculations have been retrospectively evaluated following irradiation to the whole thorax and lung in non-human primates (NHP). A clinical-based approach was established to evaluate actual doses received in the heart and lungs during whole thorax lung irradiation. Anatomical structure and organ densities have been introduced in the calculations to show the effects of dose distribution through heterogeneous tissue. Mean organ doses received by non-human primates undergoing whole thorax lung irradiations were calculated using a treatment planning system that is routinely used in clinical radiation oncology. The doses received by non-human primates irradiated following conventional dose calculations have been retrospectively reconstructed using computerized tomography-based, heterogeneity-corrected dose calculations. The use of dose volume descriptors for irradiation to organs at risk and tissue exposed to radiation is introduced. Mean and partial-volume doses to lung and heart are presented and contrasted. The importance of exact dose definitions is highlighted, and the relevance of precise dosimetry to establish organ-specific dose response relationships in NHP models of acute and delayed effects of acute radiation exposure is emphasized.

SU‐F‐19A‐01: APBI Brachytherapy Treatment Planning: The Impact of Heterogeneous Dose Calculations

Purpose:To quantify the difference in dose to PTV_EVAL and OARs (skin and rib) as calculated by (TG43) and heterogeneous calculations (CCC).Methods:25 patient plans (5 Contura and 20 SAVI) were selected for analysis. Clinical dose distributions were computed with a commercially available treatment planning algorithm (TG43‐D_(w,w)) and then recomputed with a pre‐clinical collapsed cone convolution algorithm (CCCD_(m,m)). PTV_EVAL coverage (V90%, V95%), and rib and skin maximum dose were compared via percent difference. Differences in dose to normal tissue (V150cc, V200cc of PTV_EVAL) were also compared. Changes in coverage and maximum dose to organs at risk are reported in percent change, (100*(TG43 − CCC) / TG43)), and changes in maximum dose to normal tissue are absolute change in cc (TG43 − CCC).Results:Mean differences in V90, V95, V150, and V200 for the SAVI cases were −0.2%, −0.4%, −0.03cc, and −0.14cc, respectively, with maximum differences of −0.78%, −1.7%, 1.28cc, and 1.01cc, respectively. Mean differences in the 0.1cc dose to the rib and skin were −1.4% and −0.22%, respectively, with maximum differences of −4.5% and 16%, respectively. Mean differences in V90, V95, V150, and V200 for the Contura cases were −1.2%, −2.1%, −1.8cc, and −0.59cc, respectively, with maximum differences of −2.0%, −3.16%, −2.9cc, and −0.76cc, respectively. Mean differences in the 0.1cc dose to the rib and skin were −2.6% and −3.9%, respectively, with maximum differences of −3.2% and −5.7%, respectively.Conclusion:The effects of translating clinical knowledge based on D_(w,w) to plans reported in D_(m,m) are minimal (2% or less) on average, but vary based on the type and placement of the device, source, and heterogeneity information.

SU‐E‐CAMPUS‐T‐01: Results of Irradiations Performed On Radiological Physics Center's Anthropomorphic Lung Phantom and Respiratory Simulating Motion Table

Purpose: To present the results of the irradiations performed on Radiological Physics Center's anthropomorphic lung phantom using a respiratory simulating motion table. Methods: The RPC's lung phantom is used to credential institutions to participate in NCI sponsored lung clinical trials. When institutions use breath hold, gating, ITV or tracking techniques to account for patient motion, a motion table was included with the phantom. The institutions designed and delivered 3D or IMRT plans that covered an ovoid shaped target located in the left lung. All irradiations in this analysis included heterogeneity dose calculation algorithms approved by the RTOG. The TLD criteria were ±5% about RPC/Inst=0.97 (about 1.00 for Monte Carlo algorithms). Before March 2012 the film criteria required a DTA &lt;=5 mm in the fall off regions of orthogonal profiles taken through the center of the target. After March 2012 the film criteria changed to require &gt;=80% of the pixels in the ROI in each plane to pass a 5%/5mm gamma calculation. The average of the 3 planes must be &gt;=85%. Results: The lung phantom was irradiated 427 times by 304 institutions. 124 institutions irradiated it 157 times using the motion platform. 79% of the irradiations utilizing the motion platform and 87% of the static irradiations passed the criteria. The overall pass rate for the phantom is 84%. Thirty percent of the moving phantom failures and 67% of the static phantom failures were due to failing solely the film criteria. 4 of 4 (100%) breath hold irradiations, 54 of 68 (79%) gating irradiations, 43 of 61 (70%) ITV irradiations and 23 of the 24 (96%) tracking irradiations passed the criteria. Conclusion: The moving phantom has a pass rate that is statistically significantly different than that for a static phantom. Supported by PHS grant CA10953 and CA081647 from the NCI, DHHS.

SU-D-105-01: Patient-Specific Quality Assurance for Monte Carlo-Calculated Lung SBRT On Cyberknife - Is It Necessary?

Purpose: In transitioning from ray-tracing (RT) to Monte Carlo (MC) dose computation algorithm for Cyberknife lung stereotactic body radiotherapy (SBRT), we realized large differences (10–20%) in PTV coverage to heterogeneous target regions which required dosimetric confirmation. Currently, our practice requires an independent second calculation for all plans. MuCheck software (Oncology Data Systems) is used in lieu of physical measurements for RT plans; however, there exists no commercial software for verifying MC plans. We determined all lung SBRT plans should utilize the MC algorithm and initially be confirmed by direct measurements. Methods: Lung treatment plans were first optimized with RT then recalculated using MC for final optimization and high-resolution dose computation. The MC plan was superimposed on a heterogeneous thorax phantom CT (Standard Imaging 91230) using the phantom overlay tool of the MultiPlan 3.5.2 software. Isodose distributions were manually shifted to place the ion chamber (0.053cc Exradin-A1SL) in a suitably homogeneous region within the CTV. The plan was then delivered to the thorax phantom with the ion chamber placed in the mediastinum insert location. Results: This methodology was used for 33 consecutive lung patients receiving Cyberknife SBRT with PTVs ranging from 10.7–185.9cm3. The mean deviation between measured and MC calculated doses was −2.31%±1.66%. The maximum deviation was −4.69%. Acceptable tolerance for patient-specific QA was considered ±5%. Conclusion: The MC algorithm provides improved accuracy over RT for heterogeneous dose calculation, confirmed by direct point measurements. Patient-specific QA using a heterogeneous lung phantom provided an acceptable anthropomorphic approximation of patient plans calculated with MC. Since the QA results establish satisfactory delivery of MC doses, patient-specific plans calculated on a homogeneous phantom comparing RT and MC algorithms may provide a suitable second check without direct measurement. Conversely, many users may continue direct measurement as a second check until commercial MC verification software becomes available.

Dosimetric impact of Acuros XB deterministic radiation transport algorithm for heterogeneous dose calculation in lung cancer.

The novel deterministic radiation transport algorithm, Acuros XB (AXB), has shown great potential for accurate heterogeneous dose calculation. However, the clinical impact between AXB and other currently used algorithms still needs to be elucidated for translation between these algorithms. The purpose of this study was to investigate the impact of AXB for heterogeneous dose calculation in lung cancer for intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). The thorax phantom from the Radiological Physics Center (RPC) was used for this study. IMRT and VMAT plans were created for the phantom in the Eclipse 11.0 treatment planning system. Each plan was delivered to the phantom three times using a Varian Clinac iX linear accelerator to ensure reproducibility. Thermoluminescent dosimeters (TLDs) and Gafchromic EBT2 film were placed inside the phantom to measure delivered doses. The measurements were compared with dose calculations from AXB 11.0.21 and the anisotropic analytical algorithm (AAA) 11.0.21. Two dose reporting modes of AXB, dose-to-medium in medium (Dm,m) and dose-to-water in medium (Dw,m), were studied. Point doses, dose profiles, and gamma analysis were used to quantify the agreement between measurements and calculations from both AXB and AAA. The computation times for AAA and AXB were also evaluated. For the RPC lung phantom, AAA and AXB dose predictions were found in good agreement to TLD and film measurements for both IMRT and VMAT plans. TLD dose predictions were within 0.4%-4.4% to AXB doses (both Dm,m and Dw,m); and within 2.5%-6.4% to AAA doses, respectively. For the film comparisons, the gamma indexes (± 3%∕3 mm criteria) were 94%, 97%, and 98% for AAA, AXB_Dm,m, and AXB_Dw,m, respectively. The differences between AXB and AAA in dose-volume histogram mean doses were within 2% in the planning target volume, lung, heart, and within 5% in the spinal cord. However, differences up to 8% between AXB and AAA were found at lung∕soft tissue interface regions for individual IMRT fields. AAA was found to be 5-6 times faster than AXB for IMRT, while AXB was 4-5 times faster than AAA for VMAT plan. AXB is satisfactorily accurate for the dose calculation in lung cancer for both IMRT and VMAT plans. The differences between AXB and AAA are generally small except in heterogeneous interface regions. AXB Dw,m and Dm,m calculations are similar inside the soft tissue and lung regions. AXB can benefit lung VMAT plans by both improving accuracy and reducing computation time.

Evaluation of material heterogeneity dosimetric effects using radiochromic film for COMS eye plaques loaded with 125I seeds (model I25.S16)

(1) To measure absolute dose distributions in eye phantom for COMS eye plaques with (125)I seeds (model I25.S16) using radiochromic EBT film dosimetry. (2) To determine the dose correction function for calculations involving the TG-43 formalism to account for the presence of the COMS eye plaque using Monte Carlo (MC) method specific to this seed model. (3) To test the heterogeneous dose calculation accuracy of the new version of Plaque Simulator (v5.3.9) against the EBT film data for this seed model. Using EBT film, absolute doses were measured for (125)I seeds (model I25.S16) in COMS eye plaques (1) along the plaque's central axis for (a) uniformly loaded plaques (14-20 mm in diameter) and (b) a 20 mm plaque with single seed, and (2) in off-axis direction at depths of 5 and 12 mm for all four plaque sizes. The EBT film calibration was performed at (125)I photon energy. MC calculations using MCNP5 code for a single seed at the center of a 20 mm plaque in homogeneous water and polystyrene medium were performed. The heterogeneity dose correction function was determined from the MC calculations. These function values at various depths were entered into PS software (v5.3.9) to calculate the heterogeneous dose distributions for the uniformly loaded plaques (of all four sizes). The dose distributions with homogeneous water assumptions were also calculated using PS for comparison. The EBT film measured absolute dose rate values (film) were compared with those calculated using PS with homogeneous assumption (PS Homo) and heterogeneity correction (PS Hetero). The values of dose ratio (film∕PS Homo) and (film∕PS Hetero) were obtained. The central axis depth dose rate values for a single seed in 20 mm plaque measured using EBT film and calculated with MCNP5 code (both in ploystyrene phantom) were compared, and agreement within 9% was found. The dose ratio (film∕PS Homo) values were substantially lower than unity (mostly between 0.8 and 0.9) for all four plaque sizes, indicating dose reduction by COMS plaque compared with homogeneous assumption. The dose ratio (film∕PS Hetero) values were close to unity, indicating the PS Hetero calculations agree with those from the film study. Substantial heterogeneity effect on the (125)I dose distributions in an eye phantom for COMS plaques was verified using radiochromic EBT film dosimetry. The calculated doses for uniformly loaded plaques using PS with heterogeneity correction option enabled were corroborated by the EBT film measurement data. Radiochromic EBT film dosimetry is feasible in measuring absolute dose distributions in eye phantom for COMS eye plaques loaded with single or multiple (125)I seeds. Plaque Simulator is a viable tool for the calculation of dose distributions if one understands its limitations and uses the proper heterogeneity correction feature.

Algorithms Used in Heterogeneous Dose Calculations Show Systematic Differences as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for Radiation Therapy Oncology Group clinical trials; the irradiations were all done using 6-MV beams. Treatment plans included those for intensity-modulated radiation therapy (IMRT) as well as 3-dimensional conformal therapy (3D-CRT). Heterogeneous plans were developed using Monte Carlo (MC), convolution/superposition (CS), and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target was compared to the calculated dose, as was the planar dose in 3 orthogonal planes. The difference between measured and calculated dose was examined as a function of planning algorithm as well as use of IMRT. PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no difference observed between IMRT and 3D CRT calculation accuracy. Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This issue requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.

Algorithms Used in Heterogeneous Dose Calculations Show Systematic Error as Measured With the Radiological Physics Center's Anthropomorphic Thorax Phantom Used for RTOG Credentialing

To determine the impact of treatment planning algorithm on the accuracy of heterogeneous dose calculations in the Radiological Physics Center (RPC) thorax phantom. We retrospectively analyzed the results of 304 irradiations of the RPC thorax phantom at 221 different institutions as part of credentialing for RTOG clinical trials; the irradiations were all done using 6-MV beams. Heterogeneous treatment plans were developed using Monte Carlo algorithms (MC), convolution/superposition (CS) algorithms and the anisotropic analytic algorithm (AAA), as well as pencil beam (PB) algorithms. For each plan and delivery, the absolute dose measured in the center of a lung target, as well as planar dose measurements throughout the target, were compared to the calculated dose. The difference between measured and calculated dose was examined as a function of planning algorithm, as well as use of IMRT versus 3D conformal therapy, use of a motion platform versus static irradiation, and date of irradiation. Only minor differences were observed in the planar agreement between different algorithms. Absolute dose agreement showed sizeable and significant variation between algorithms. PB algorithms overestimated the dose delivered to the center of the target by 4.9% on average. Surprisingly, CS algorithms and AAA also showed a systematic overestimation of the dose to the center of the target, by 3.7% on average. In contrast, the MC algorithm dose calculations agreed with measurement within 0.6% on average. There was no significant difference observed between IMRT and 3D CRT calculation accuracy, between moving or static irradiations, or as a function of time (i.e., agreement has not improved in recent years). Unexpectedly, advanced treatment planning systems (those using CS and AAA algorithms) overestimated the dose that was delivered to the lung target. This large dosimetric difference requires attention in terms of heterogeneity calculations and potentially in terms of clinical practice.Oral Scientific Abstract 319; TableMean ratio and standard deviation of measured to calculated doseAlgorithmMonte CarloAAAPinnacleXioTomotherapyPBMean0.9940.9590.9680.9550.9710.951Standard Deviation0.0300.0270.0240.0250.0240.027 Open table in a new tab

Dosimetry of 125I and 103Pd COMS eye plaques for intraocular tumors: Report of Task Group 129 by the AAPM and ABS

Dosimetry of eye plaques for ocular tumors presents unique challenges in brachytherapy. The challenges in accurate dosimetry are in part related to the steep dose gradient in the tumor and critical structures that are within millimeters of radioactive sources. In most clinical applications, calculations of dose distributions around eye plaques assume a homogenous water medium and full scatter conditions. Recent Monte Carlo (MC)-based eye-plaque dosimetry simulations have demonstrated that the perturbation effects of heterogeneous materials in eye plaques, including the gold-alloy backing and Silastic insert, can be calculated with reasonable accuracy. Even additional levels of complexity introduced through the use of gold foil "seed-guides" and custom-designed plaques can be calculated accurately using modern MC techniques. Simulations accounting for the aforementioned complexities indicate dose discrepancies exceeding a factor of ten to selected critical structures compared to conventional dose calculations. Task Group 129 was formed to review the literature; re-examine the current dosimetry calculation formalism; and make recommendations for eye-plaque dosimetry, including evaluation of brachytherapy source dosimetry parameters and heterogeneity correction factors. A literature review identified modern assessments of dose calculations for Collaborative Ocular Melanoma Study (COMS) design plaques, including MC analyses and an intercomparison of treatment planning systems (TPS) detailing differences between homogeneous and heterogeneous plaque calculations using the American Association of Physicists in Medicine (AAPM) TG-43U1 brachytherapy dosimetry formalism and MC techniques. This review identified that a commonly used prescription dose of 85 Gy at 5 mm depth in homogeneous medium delivers about 75 Gy and 69 Gy at the same 5 mm depth for specific (125)I and (103)Pd sources, respectively, when accounting for COMS plaque heterogeneities. Thus, the adoption of heterogeneous dose calculation methods in clinical practice would result in dose differences >10% and warrant a careful evaluation of the corresponding changes in prescription doses. Doses to normal ocular structures vary with choice of radionuclide, plaque location, and prescription depth, such that further dosimetric evaluations of the adoption of MC-based dosimetry methods are needed. The AAPM and American Brachytherapy Society (ABS) recommend that clinical medical physicists should make concurrent estimates of heterogeneity-corrected delivered dose using the information in this report's tables to prepare for brachytherapy TPS that can account for material heterogeneities and for a transition to heterogeneity-corrected prescriptive goals. It is recommended that brachytherapy TPS vendors include material heterogeneity corrections in their systems and take steps to integrate planned plaque localization and image guidance. In the interim, before the availability of commercial MC-based brachytherapy TPS, it is recommended that clinical medical physicists use the line-source approximation in homogeneous water medium and the 2D AAPM TG-43U1 dosimetry formalism and brachytherapy source dosimetry parameter datasets for treatment planning calculations. Furthermore, this report includes quality management program recommendations for eye-plaque brachytherapy.

Sci-Thur AM: Planning - 05: Lung SBRT: Dosimetric accuracy of the Analytical Anisotropic Algorithm (AAA) for 6MV FFF RapidArc planning.

Stereotactic Body Radiation Therapy (SBRT) requires the delivery of a high biologically effective dose in only a few fractions. These large doses per fraction can necessitate long treatment times. The Varian Truebeam is capable of RapidArc delivery and also has the optional Flattening Filter Free (FFF) modes which greatly increase the dose rate. We have commissioned the 6MV FFF beam (1400 MU/min) for RapidArc lung SBRT, and verified heterogeneous dose calculations with Monte Carlo (MC). The standard commissioning data was acquired for Varian's Analytical Anisotropic Algorithm (AAA) beam model. Measurements were acquired with the IBA Blue Phantom, using the CC13 and CC01 ion chambers and PTW diode. MLC-defined fields were also acquired for model verification. The Dosimetric Leaf Gap (DLG) was measured and then optimized using RapidArc lung SBRT plans, matching Eclipse with ion chamber measurements. Heterogeneous dose calculations were independently verified using MC. There were some discrepancies regarding leaf transmission and penumbra, but the AAA model was generally well within 2% and 2 mm. A nominal DLG value of 1.6 mm was chosen. A representative lung SBRT case utilizing FFF RapidArc was calculated with MC. For the high dose region, 99% matched Eclipse within 3% and 3 mm. The mean dose difference of the PTV was 0.7%. Although we have observed some minor infield discrepancies between the AAA and Monte Carlo calculations in heterogeneous media, the Eclipse AAA is reasonably accurate for complex FFF, RapidArc, SBRT lung planning.