Monte Carlo Based Dose Calculation Research Articles

Digital phantom versus patient-specific radiation dosimetry in adult routine thorax CT examinations.

The aim of this study was to compare the organ doses assessed through a digital phantom-based and a patient specific-based dosimetric tool in adult routine thorax computed tomography (CT) examinations with reference to physical dose measurements performed in anthropomorphic phantoms. Two Monte Carlo based dose calculation tools were used to assess organ doses in routine adult thorax CT examinations. These were a digital phantom-based dosimetry tool (NCICT, National Cancer Institute, USA) and a patient-specific individualized dosimetry tool (ImpactMC, CT Imaging GmbH, Germany). Digital phantoms and patients were classified in four groups according to their water equivalent diameter (Dw). Normalized to volume computed tomography dose index (CTDIvol), organ dose was assessed for lungs, esophagus, heart, breast, active bone marrow, and skin. Organ doses were compared to measurements performed using thermoluminescent detectors (TLDs) in two physical anthropomorphic phantoms that simulate the average adult individual as a male (Alderson Research Labs, USA) and as a female (ATOM Phantoms, USA). The average percent difference of NCICT to TLD and ImpactMC to TLD dose measurements across all organs in both sexes was 13% and 6%, respectively. The average±1 standard deviation in dose values across all organs with NCICT, ImpactMC, and TLDs was±0.06 (mGy/mGy),±0.19 (mGy/mGy), and±0.13 (mGy/mGy), respectively. Organ doses decreased with increasing Dw in both NCICT and ImpactMC. Organ doses estimated with ImpactMC were in closer agreement to TLDs compared to NCICT. This may be attributed to the inherent property of ImpactMC methodology to generate phantoms that resemble the realistic anatomy of the examined patient as opposed to NCICT methodology that incorporates an anatomical discrepancy between phantoms and patients.

Validation of Monte Carlo-based calculations for megavolt electron beams for IORT and FLASH-IORT

In Intra-Operative Radiation Therapy (IORT) the tumour site is surgically exposed and normal tissue located around the tumour may be avoided. Electron applicators would require large surgical incisions; therefore, the preferred mechanism for beam collimation is the IORT cone system. FLASH radiotherapy (FLASH-RT) involves the treatment of tumours at ultra-high dose rates and the IORT cone system can also be used. This study validates the Monte Carlo-based calculations for these small electron beams to accurately determine the dose characteristics of each possible cone-energy combination as well as custom-built alloy cutouts attached to the end of the IORT cone. This will contribute to accurate dose distribution and output factor calculations that are essential to all radiation therapy treatments. A Monte Carlo (MC) model was modelled for electron beams produced by a Siemens Primus LINAC and the IORT cones. The accelerator was built with the component modules available in the BEAMnrc code. The phase-space file generated by the BEAM simulation was used as the source input for the subsequent DOSXYZnrc simulations. Percentage Depth Dose (PDD) data and profiles were extracted from the dose distributions obtained with the DOSXYZnrc simulations. These beam characteristics were compared with measured data for 6, 12, and 18 MeV electron beams for the IORT open cones of diameters 19, 45, and 64 mm and irregularly shaped cutouts. The MC simulations could replicate electron beams within a criterion of 3%/3 mm. Applicator factors were within 0.7%, and cone factors showed good agreement, except for the 9 mm cone size.Based on the successful comparisons between measurement and MC-calculated dose distributions, output factors for the open cones and for small irregularly shaped IORT beams, it may be concluded that the Monte Carlo based dose calculation could replicate electron beams used for IORT and FLASH-IORT.

Hip Imaging in Children With Cerebral Palsy: Estimation and Intrapatient Comparison of Patient-Specific Radiation Doses of Low-Dose CT and Radiography.

Hip displacement is the second most common orthopedic problem affecting children with cerebral palsy (CP). Routine radiographic hip surveillance typically involves an anteroposterior (AP) pelvis radiograph. Unfortunately, this imaging protocol is limited by its projectional technique and the positioning challenges in children with CP. Alternatively, hip low-dose computed tomography (LDCT) has been advocated as a more accurate strategy for imaging surveillance as it provides biofidelic details of the hip that is independent of patient positioning. However, the tradeoff is the (presumed) higher radiation dose to the patient. The goal of this study is to estimate patient-specific radiation doses of hip LDCTs and AP pelvis radiographs in CP patients, and perform an intrapatient dose comparison. A search of our imaging database was performed to identify children with CP who underwent hip LDCT and AP pelvis radiograph within 6 months of each other. The LDCTs were performed using weight-adjusted kVp and tube current modulation, whereas the radiographs were obtained with age-/size-adjusted kVp/mAs. The patient-specific organ and effective doses for LDCT were estimated by matching the patients to a nonreference pediatric phantom library from the National Cancer Institute Dosimetry System for Computed Tomography database with Monte Carlo-based dosimetry. The patient-specific organ and effective doses for radiograph were estimated using the National Cancer Institute Dosimetry System for Radiography and Fluoroscopy with Monte Carlo-based dose calculation. Dose conversion k-factors of dose area product for radiography and dose length product for LDCT were adapted, and the estimation results were compared with patient-specific dosimetry. Our study cohort consisted of 70 paired imaging studies from 67 children (age, 9.1 ± 3.3 years). The patient-specific and dose length product-based effective doses for LDCT were 0.42 ± 0.21 mSv and 0.59 ± 0.28 mSv, respectively. The patient-specific and dose area product-based effective doses for radiography were 0.14 ± 0.09 mSv and 0.08 ± 0.06 mSv, respectively. The radiation dose for a hip LDCT is ~4 times higher than pelvis radiograph, but it is still very low and poses minimal risk to the patient.

Comparison of organ and effective dose estimations from different Monte Carlo simulation-based software methods in infant CT and comparison with direct phantom measurements.

PurposeComputational dosimetry software is routinely used to evaluate the organ and effective doses from computed tomography (CT) examinations. Studies have shown a significant variation in dose estimates between software in adult cohorts, and few studies have evaluated software for pediatric dose estimates. This study aims to compare the primary organ and effective doses estimated by four commercially available CT dosimetry software to thermoluminescent dosimeter (TLD) measurements in a 1‐year‐old phantom.MethodsOne hundred fifteen calibrated LiF (Mg, Cu, P)‐TLD 100‐H chips were embedded within an anthropomorphic phantom representing a 1‐year‐old child at positions that matched the approximate location of organs within an infant. The phantom was scanned under three protocols, each with whole‐body coverage. The mean absorbed doses from 25 radiosensitive organs and skeletal tissues were determined from the TLD readings. Effective doses for each of the protocols were subsequently calculated using ICRP 103 formalism. Dose estimates by the four Monte Carlo–based dose calculation systems were determined and compared to the directly measured doses.ResultsMost organ doses determined by computation dosimetry software aligned to phantom measurements within 20%. Additionally, comparisons between effective doses are calculated using computational and direct measurement methods aligned within 20% across the three protocols. Significant variances were found in bone surface dose estimations among dosimetry methods, likely caused by differences in bone tissue modeling.ConclusionAll four‐dosimetry software evaluated in this study provide adequate primary organ and effective dose estimations. Users should be aware, however, of the possible estimated uncertainty associated with each of the programs.

Abstract 3060: Temporal analysis of type 1 interferon activation in tumor cells following external beam radiotherapy or targeted radionuclide therapy

Abstract Background: Radiation (RT) activates a type 1 interferon (IFN1) response and this is critical to the effect of RT in priming a response to immune checkpoint blockade (ie anti-CTLA4). However, little is known about the time course of this effect. Clinical interest in utilizing systemically administered targeted radionuclide therapy agents (TRT) is growing as these agents can be used to target multiple sites of disease including micro-metastases. It is unclear how IFN1 activation induced by continuous delivery of RT during exponential decay of a TRT source will compare to that induced following instantaneous external beam RT (EBRT). Here we report the time course of IFN1 response following RT in vitro and in vivo. Methods: For in vitro studies, we utilized murine models of melanoma (B16, B16 STING knockout, B78), and head and neck squamous cell carcinoma (MOC2). EBRT was prescribed to 2.5 Gy, 12 Gy, or 3 fractions of 8 Gy. For in vivo studies, syngeneic C57BL/6 mice were engrafted with either B78 or MOC2 cells on the flank and RT was delivered when mean tumor size was 100-150 mm3. EBRT was prescribed to 2.5 Gy or 12 Gy. For TRT, we used 90Y conjugated to NM600, an alkylphosphocholine analog that exhibits selective uptake and retention in tumor cells of nearly any type, including B78 and MOC2. Tumor-specific dosimetry for 90Y-NM600 was determined using sequential 86Y-NM600 PET/CT imaging (3h, 24h, 48h, 72h) and a Monte Carlo based dose calculation platform. TRT was prescribed to a cumulative absorbed dose of 2.5 Gy or 12 Gy. Following delivery of RT, cells or tumors were harvested at 1d, 7d, and 14d post RT and RNA was isolated. Gene expression of Ifn-β and IFN response elements was quantified by qPCR and normalized to untreated controls. Results: We observed significant IFN1 activation in all cell lines, with peak activation in B78, B16, and MOC2 cell lines occurring 7, 7, and 1 days, respectively, following RT for all doses. This effect was STING-dependent. Select IFN response genes remained upregulated at 14 days following RT. IFN1 activation following STING agonist treatment in vitro was identical to RT suggesting time course differences between cell lines were mediated by STING pathway kinetics and not DNA damage susceptibility. In vivo delivery of EBRT and TRT to B78 and MOC2 tumors resulted in a comparable time course and magnitude of IFN1 activation. In the MOC2 model, the combination of 90Y-NM600 and anti-CTLA-4 therapy reduced tumor growth and prolonged survival compared to single agent therapy. Conclusions: We report the time course of the STING-dependent IFN1 response following RT in multiple murine tumor models. We show the potential of TRT to stimulate IFN1 activation that is comparable to that observed with EBRT of equivalent cumulative dose. Further evaluation of the timing and magnitude of IFN1 response following EBRT and TRT may be critical to the optimal integration with immunotherapies. Citation Format: Justin C. Jagodinsky, Amber M. Bates, Reinier Hernandez, Joseph J. Grudzinski, Ian R. Marsh, Ishan Chakravarty, Ian S. Arthur, Luke M. Zangl, Ryan J. Brown, Erin J. Nystuen, Sarah E. Emma, Caroline Kerr, Won Jong Jin, Peter M. Carlson, Jonathan W. Engle, Eduardo Aluicio-Sarduy, Todd E. Barnhart, Trang Le, KyungMann Kim, Bryan P. Bednarz, Jamey P. Weichert, Ravi B. Patel, Zachary S. Morris. Temporal analysis of type 1 interferon activation in tumor cells following external beam radiotherapy or targeted radionuclide therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3060.

Multimodal Monte Carlo treatment system capable of microdosimetry with PHITS

Boron neutron capture therapy (BNCT) is a radiation therapy that combines neutrons and boron drugs. An industry-academia-government collaboration team is currently developing the linac-based treatment device and several peripheral devices to establish this therapy. In the project, a demonstration device, “iBNCT001”, for a linac-based neutron source applicable to BNCT treatment is being developed. We are developing Tsukuba Plan, a multimodal Monte Carlo-based treatment planning system, in addition to the linac-based neutron source device. Tsukuba-Plan has adopted a Particle and Heavy Ion Transport code System (PHITS) as the Monte Carlo-based dose calculation engine. PHITS has a microdosimetric function that can compute a lineal energy distribution and determine relative biological effectiveness through combination with the stochastic microdosimetric kinetic model parameters. By utilising these functions, Tsukuba-Plan is expected to estimate the weighted doses administered to the tumour region and normal tissues in the irradiation field more accurately.

Development of CT Effective Dose Conversion Factors from Clinical CT Examinations in the Republic of Korea

The aim of this study was to determine the conversion factors for the effective dose (ED) per dose length product (DLP) for various computed tomography (CT) protocols based on the 2007 recommendations of the International Commission on Radiological Protection (ICRP). CT dose data from 369 CT scanners and 13,625 patients were collected through a nationwide survey. Data from 3793 patients with a difference in height within 5% of computational human phantoms were selected to calculate ED and DLP. The anatomical CT scan ranges for 11 scan protocols (adult-10, pediatric-1) were determined by experts, and scan lengths were obtained by matching scan ranges to computational phantoms. ED and DLP were calculated using the NCICT program. For each CT protocol, ED/DLP conversion factors were calculated from ED and DLP. Estimated ED conversion factors were 0.00172, 0.00751, 0.00858, 0.01843, 0.01103, 0.02532, 0.01794, 0.02811, 0.02815, 0.02175, 0.00626, 0.00458, 0.00308, and 0.00233 mSv∙mGy−1∙cm−1 for the adult brain, intra-cranial angiography, C-spine, L-spine, neck, chest, abdomen and pelvis, coronary angiography, calcium scoring, aortography, and CT examinations of pediatric brain of <2 years, 4–6 years, 9–11 years, and 13–15 years, respectively. We determined ED conversion factors for 11 CT protocols using CT data obtained from a nationwide survey in Korea and Monte Carlo-based dose calculations.

Abstract 477: Comparing type 1 interferon activation in tumor cells following external beam radiotherapy versus targeted radionuclide therapy

Abstract Background: Radiation (RT) activates a type 1 interferon (IFN-1) response and, in preclinical studies, this is critical to the effect of RT in priming a response to immune checkpoint blockade. Recent studies report the impact of external beam radiotherapy (EBRT) dose and fractionation on IFN-1 response, however little is known about the time course of this effect. Clinical interest in utilizing systemically administered targeted radionuclide therapy agents (TRT) is growing. It is unclear how IFN-1 activation induced by continuous delivery of RT during exponential decay of a TRT source will compare to that induced following instantaneous EBRT. Here we report the time course of IFN-1 response following RT in vitro and in vivo. Methods: For in vitro studies, we utilized murine models of melanoma (B16, B16 STING knockout, B78), and head and neck cancer (MOC2). EBRT was prescribed to 12 Gy, 20 Gy, or 3 fractions of 8 Gy. For in vivo studies, syngeneic C57BL/6 mice were engrafted with either B78 or MOC2 cells on the flank and RT was delivered when mean tumor size was ~ 150 mm3. EBRT was prescribed to 2 Gy or 12 Gy. For TRT, we used 90Y conjugated to NM600, an alkylphosphocholine analog that exhibits selective uptake and retention in tumor cells of nearly any type, including B78 and MOC2. Tumor-specific dosimetry for 90Y-NM600 was determined using sequential 86Y-NM600PET/CT imaging (3h, 24h, 72h) and a Monte Carlo based dose calculation platform. TRT was prescribed to a dose of 2 Gy or 12 Gy. Following delivery of RT in vitro or in vivo, cells or tumors were harvested at 24h, 7d, and 14 d post RT and RNA was isolated. Gene expression of Ifn-β and IFN response elements (Oas2, Oas3, and Mx1) was quantified by qPCR and normalized to untreated controls. Results: In agreement with prior studies, we observed significant IFN-1 activation 24 hours following 8 Gy x 3 in our B16, B78, and MOC2 tumor cell lines. At 12 Gy and 20 Gy, we continued to detect significant IFN-1 activation. Peak activation was 7d following EBRT for all doses, raising the potential for a lead-time bias when comparing fractionated to single dose treatments. For all doses, select IFN response genes remained upregulated at 14d. In vivo delivery of EBRT and TRT to B78 and MOC2 tumors resulted in a comparable time course of IFN-1 activation peaking 7d after RT and persisting to 14d. Using a STING knockout variant of B16 melanoma, we confirmed that activation of IFN-1 response by RT was STING dependent at all time points evaluated. Conclusions: We report the time course for activation of a STING-dependent IFN-1 response following RT in multiple murine tumor models. We observe maximal activation 7d following both EBRT and TRT. We show the potential of TRT to generate EBRT comparable IFN-1 activation. Further studies evaluating the time course and magnitude of IFN-1 response following TRT may be critical in metastatic settings to integrating this modality with immunotherapy. Citation Format: Justin C. Jagodinsky, Ian S. Arthur, Juliana S. Castillo, Ishan Chakravarty, Luke M. Zangl, Ryan J. Brown, Ravi B. Patel, Wonjon J. Jin, Peter M. Carlson, Reinier Hernandez, Joseph J. Grudzinski, Ian R. Marsh, Jamey P. Weichert, Bryan P. Bednarz, Zachary S. Morris. Comparing type 1 interferon activation in tumor cells following external beam radiotherapy versus targeted radionuclide therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 477.

Experimental benchmarking of RayStation proton dose calculation algorithms inside and outside the target region in heterogeneous phantom geometries

PurposeThe aim of the presented study was to complement existing literature on benchmarking proton dose by comparing dose calculations with experimental measurements in heterogeneous phantom. Points of interest inside and outside the target were considered to quantify the magnitude of calculation uncertainties in current and previous proton therapy practice that might especially have an impact on the dose in organs at risk (OARs). MethodsThe RayStation treatment planning system (RaySearch Laboratories), offering two dose calculation algorithms for pencil beam scanning in proton therapy, i.e., Pencil Beam (PB) and Monte Carlo (MC), was utilized. Treatment plans for a target located behind the interface of the heterogeneous tissues were generated. Dose measurements within and behind the target were performed in a water phantom with embedded slabs of various tissue equivalent materials and 24 PinPoint ionization chambers (PTW). In total 12 test configurations encompassing two different target depths, oblique beam incidence of 30 degrees and range shifter, were considered. ResultsPB and MC calculated doses agreed equally well with the measurements for all test geometries within the target, including the range shifter (mean dose differences ± 3%). Outside the target, the maximum dose difference of 9% (19%) was observed for MC (PB) for the oblique beam incidence and inserted range shifter. ConclusionThe accuracy of MC dose algorithm was superior compared to the PB algorithm, especially outside the target volumes. MC based dose calculation should therefore be preferred in treatment scenarios with heterogeneities, especially to reduce clinically relevant uncertainties for OARs.

Verification of a Monte Carlo dose calculation engine in proton minibeam radiotherapy in a passive scattering beamline for preclinical trials.

Proton minibeam radiation therapy (pMBRT) is a novel therapeutic strategy that combines the benefits of proton therapy with the remarkable normal tissue preservation observed with the use of submillimetric spatially fractionated beams. This promising technique has been implemented at the Institut Curie-Proton therapy centre (ICPO) using a first prototype of a multislit collimator. The purpose of this work was to develop a Monte Carlo-based dose calculation engine to reliably guide preclinical studies at ICPO. The whole "Y1"-passive beamline at the ICPO, including pMBRT implementation, was modelled using the Monte Carlo GATE v. 7.0 code. A clinically relevant proton energy (100 MeV) was used as starting point. Minibeam generation by means of the brass collimator used in the first experiments was modelled. A virtual source was modelled at the exit of the beamline nozzle and outcomes were compared with dosimetric measurements performed with EBT3 gafchromic films and a diamond detector in water. Dose distributions were recorded in a water phantom and in rat CT images (7-week-old male Fischer rats). The dose calculation engine was benchmarked against experimental data and was then used to assess dose distributions in CT images of a rat, resulting from different irradiation configurations used in several experiments. It reduced computational time by an order of magnitude. This allows us to speed up simulations for in vivo trials, where we obtained peak-to-valley dose ratios of 1.20 ± 0.05 and 6.1 ± 0.2 for proton minibeam irradiations targeting the tumour and crossing the rat head. Tumour eradication was observed in the 67 and 22% of the animals treated respectively. A Monte Carlo dose calculation engine for pMBRT implementation with mechanical collimation has been developed. This tool can be used to guide and interpret the results of in vivo trials. This is the first Monte Carlo dose engine for pMBRT that is being used to guide preclinical trials in a clinical proton therapy centre.

Is Monte Carlo-Based Dose Calculation Algorithm Required for All Lung Tumors Treated with Stereotactic Body Radiotherapy?

The primary objective of this study is to compare the accuracy of a Boltzmann equation solvers algorithm against a Monte Carlo based dose calculation algorithm for island and peripheral lung tumors treated with Stereotactic Body Radiotherapy (SBRT). Ten lung SBRT patients considered for this retrospective study, five patients with an island lung tumor and five patients with a chest wall or lateral tumor . For each patient, a simulation CT scan and a 10-phase 4DCT scans were acquired. The initial delivered plans were planned with dynamic conformal arc technique using a planning system that utilizes the x-ray voxel Monte Carlo (XVMC) algorithm. The patients’ treatment plans were transferred to a treatment planning system utilizes the Acuros AXB algorithm. These plans were retrospectively re-calculated keeping the initial arc angles and multi leaf collimator (MLC) segments. Validation plans were generated by projecting the plans into a 4D detector/cylindrical diode detector array quality assurance (QA) phantom. Quality assurance plans were delivered using a 6X SRS beam. Measured dose distributions were compared with calculated dose distributions using 3% 3mm gamma analysis. Dose-volumetric data including the maximum dose, minimum dose, D95% for internal tumor volume (ITV) and planning tumor volume (PTV) were compared for the two algorithms using the Wilcoxon Rank Sum test. All analyses were performed using data and decision management software with two-tailed p-values reported and a p-value of <0.05 considered significant. The table below summarizes the results for dose-volumetric data for all 10 patients. For the island tumors the ITV minimum dose and the PTV minimum dose showed a trend for a difference between the AXB and XVMC algorithms (p=0.06, p=0.06, respectively). There was no difference observed between the ITV max, or PTV max . For the peripheral tumors there was no difference observed for any of the dose volumetric data. Passing rate for both algorithms is above 90% with (3mm, 3%) gamma analysis. The XVMC requires 16% more monitor unit than the AXB algorithm. The dosimetric performance of the AXB is comparable to the XVMC algorithm. This indicates that a Monte Carlo based dose calculation algorithm is not the only algorithm that can be used for lung tumors treated with SBRT. The AXB algorithm can be used clinically even for island type lung tumor where high heterogeneity is present. Slight differences in minimum doses were noted for island tumors; however, these differences are usually clinically acceptable.Abstract 3828; Table 1CriteriaXVMCAXBITV Max dose (%)131.2±5.6130.5±7.1ITV Min dose (%)106.7 ±6.2108.2 ±9.9PTV Max dose (%)131.2 ±6.2130.6 ±6.7PTV Min dose (%)83.1±5.386.0±3.9D95%101.4±1.9100.6±1.7Gamma (3%, 3mm)95.0 ± 3.093.7 ± 2.0 Open table in a new tab

Dosimetric Considerations for Ytterbium-169, Selenium-75, and Iridium-192 Radioisotopes in High-Dose-Rate Endorectal Brachytherapy

To investigate differences between prescribed and postimplant calculated dose in 192Ir high-dose-rate endorectal brachytherapy (HDR-EBT) by evaluating dose to clinical target volume (CTV) and organs at risk (OARs) calculated with a Monte Carlo-based dose calculation software, RapidBrachyMC. In addition, dose coverage, conformity, and homogeneity were compared among the radionuclides 192Ir, 75Se, and 169Yb for use in HDR-EBT. Postimplant dosimetry was evaluated using 23 computed tomography (CT) images from patients treated with HDR-EBT using the 192Ir microSelectron v2 (Elekta AB, Stockholm, Sweden) source and the Intracavitary Mold Applicator Set (Elekta AB, Stockholm, Sweden), which is a flexible applicator capable of fitting a tungsten rod for OAR shielding. Four tissue segmentation schemes were evaluated: (1) TG-43 formalism, (2) materials and nominal densities assigned to contours of foreign objects, (3) materials and nominal densities assigned to contoured organs in addition to foreign objects, and (4) materials specified as in (3) but with voxel mass densities derived from CT Hounsfield units. Clinical plans optimized for 192Ir were used, with the results for 75Se and 169Yb normalized to the D90 of the 192Ir clinical plan. In comparison to segmentation scheme 4, TG-43-based dosimetry overestimates CTV D90 by 6% (P = .00003), rectum D50 by 24% (P = .00003), and pelvic bone D50 by 5% (P = .00003) for 192Ir. For 169Yb, CTV D90 is overestimated by 17% (P = .00003) and rectum D50 by 39% (P = .00003), and pelvic bone D50 is significantly underestimated by 27% (P = .007). Postimplant dosimetry calculations also showed that a 169Yb source would give 20% (P = .00003) lower rectum V60 and 17% (P = .00008) lower rectum D50. Ignoring high-Z materials in dose calculation contributes to inaccuracies that may lead to suboptimal dose optimization and disagreement between prescribed and calculated dose. This is especially important for low-energy radionuclides. Our results also show that with future magnetic resonance imaging-based treatment planning, loss of CT density data will only affect calculated dose in nonbone OARs by 2% or less and bone OARs by 13% or less across all sources if material composition and nominal mass densities are correctly assigned.

Monte-Carlo Modelling For Evaluation Of Two Different Calculation Algorithms

Modeling of linac head (VARIAN Trilogy) for 6 MeV photon beam was performed using BEAMnrc code package (BEAMnrc 2017). The DOSXYZnrc code was used to determine the percentage depth dose (PDD profiles) and beam profiles for different symmetric square field sizes, i.e., 5cm X 5cm; and 40cm X 40cm. The DICOM images of Alderson Radiation Therapy (ART) RANDO Phantom was used. Four field 3D-CRT treatment plans were generated using AAA, PBC, and Monte-Carlo (MC). It was found that nominal energy of 5.7 MeV with FWHM of 1.2 mm provides best matching of modelled and working linac. All three 3D-CRT plans calculated with AAA, PBC and MC on a Pelvic Rando phantom, were compared using CERR (Computational Environment for Radiotherapy Research) and MATLAB 2013b. It was found that AAA and PBC have comparable results, although in case of tissue interfaces and inhomogeneous media AAA provides better accuracy in comparison to PBC. It can also be observed that AAA and PBC underestimate doses in comparison to MC in the soft muscle tissue which includes OARs such as bladder, bowel bag, and PTV TOTAL. It was found that both AAA and PBC fails to account for tissue air interface accurately and shows variation of 30-40% whereas for surface dose variation of +/-10% was observed. In homogeneous media (muscle tissue) AAA and PBC underestimate doses in comparison to MC. These commercially available algorithms overestimates and underestimates dose values as compared to MC based dose calculation for low and high dose regions specially.

Retina dose as a predictor for visual acuity loss in 106Ru eye plaque brachytherapy of uveal melanomas

Background and purposeTo evaluate the retina dose as a risk factor associated with loss of visual acuity (VA) in 106Ru plaque brachytherapy. Material/methods45 patients receiving 106Ru plaques brachytherapy (median follow-up 29.5 months) were included in this study. An in-house developed treatment planning system with Monte Carlo based dose calculation was used to perform treatment planning and dose calculation. Risk factors associated with loss of VA were evaluated using the Cox proportional hazards models, Kaplan–Meier estimates and Pearson correlation coefficients. ResultsA significant correlation was found between VA loss and mean (r = 0.49, p = 0.001) and near maximum (r = 0.47, p = 0.001) retina dose D2% and tumor basal diameter (r = 0.50, p < 0.001). The Kaplan–Meier and Cox proportional hazards model yielded a significantly higher risk for VA loss (>0.3Snellen) for patients receiving a maximum dose of >500 Gy (p = 0.002). A Cox multivariate analysis including the macula dose (p = 0.237) and basal diameter (p = 0.791) showed that a high maximum retinal dose is the best risk factor (p = 0.013) for VA loss. ConclusionThe study showed that retina dose (D2% and Dmean) is a suitable predictor for VA loss.

Estimating Second Malignancy Risk in Intensity-Modulated Radiotherapy and Volumetric-Modulated Arc Therapy using a Mechanistic Radiobiological Model in Radiotherapy for Carcinoma of Left Breast.

Objectives:The aim of this study is to estimate second cancer risk (SCR) in intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT) using a mechanistic radiobiological model. The model also takes into account patient age at exposure and the gender-specific correction factors of SCR.Materials and Methods:Fifty IMRT and VMAT plans were selected for the study. Monte Carlo-based dose calculation engine was used for dose calculation. Appropriate model parameters were taken from the literature for the mechanistic model to calculate excess absolute risk (EAR), lifetime attributable risk, integral dose and relative risk (RR) for lungs, contralateral breast, heart, and spinal cord.Results:The mean monitor unit (MU) in IMRT and VMAT plans were 751.1 ± 133.3 and 1004.8 ± 180, respectively, for IMRT and VMAT. The mean EAR values with age correction were 44.6 ± 11.9, 11.2 ± 6.4, 5.4 ± 4.0, 1.4 ± 0.5, and 0.3 ± 0.2 for left lung, right lung, contralateral breast, heart, and spinal cord, respectively, for the IMRT treatments and 54.6 ± 20.6, 30.2 ± 12.0, 13.8 ± 8.6, 1.6 ± 0.6, and 0.9 ± 0.5 for the VMAT treatments in units of 10,000 PY. The RR of 6.7% and 9.1%, respectively, for IMRT and VMAT found in our study using computational models is in close comparison with the value reported in a large epidemiological breast cancer study.Conclusions:VMAT plans had a higher risk of developing second malignancy in lung, contralateral breast, heart, and cord compared to IMRT plans. However, the increase in risk was found to be marginal compared to IMRT. Incorporating the age correction factor decreased the risk of contralateral breast SCR. No strong correlation was found between EAR and MU.

The Correlation of Clinical Outcome and Radiobiological Modeling of Tumor Control Probability (TCP) in Patients With Lung Tumors Treated With SBRT Using the Monte Carlo–Based Dose Calculation Algorithm

The Correlation of Clinical Outcome and Radiobiological Modeling of Tumor Control Probability (TCP) in Patients With Lung Tumors Treated With SBRT Using the Monte Carlo–Based Dose Calculation Algorithm

O22. Monte-Carlo commissioning of small irregularly-shaped intra-operative radiotherapy beams

Introduction The objective of this study is to validate Monte Carlo (MC) based calculations for small irregularly shaped intra-operative radiotherapy (IORT) electron beams. Treatment is delivered using an electron beam passing through an IORT cone with custom built irregularly-shaped alloy cut-outs. Materials and methods All measurements were performed on the Siemens Primus LINAC utilising the PTW MP3 therapy beam analyser and PTW pinpoint chamber. Percentage depth dose and profiles at various depths were obtained for the 6 MeV, 12 MeV and 18 MeV electron beams with the standard 10 × 10 cm 2 and 5 cm circle applicators for MC commissioning purposes. Identical measurements were done with the IORT cones of diameters 19 mm, 45 mm and 64 mm with and without irregularly-shaped alloy cut-outs. Output factors for all energy/applicator, energy/cone and energy/cut-out combinations were measured. DOSXYZnrc was used to calculate 3D dose distributions in a voxel-based phantom using the individual phase space files (obtained from BEAMnrc simulations) as the source. The data-extraction tools BEAMDP, MCSHOW, Grace, Verisoft and STATDOSE were used to extract and analyse the simulation results. Results For all the energies investigated, direct comparison between measurement and simulation of R max , R 90 , R 80 , and R 50 yielded differences of less than 1 mm for the standard 10 × 10 cm 2 and 5 cm circle applicators used for MC commissioning. For the IORT cones and cut-outs, maximum differences of 1.5 mm and 1.0 mm respectively were observed for these points. All points that failed the Gamma Index (GI) criteria of 3 mm/3% for profile comparisons were in the high dose gradient regions and/or at the extreme lateral extent of the beam. A maximum difference of 4.04% and 7.82% in cone factor and cut-out factor respectively was observed for the smallest cone and cut-out for 6 MeV. The statistical variance of all MC calculations was below 1%. Conclusion The MC based dose calculation system evaluated in this study has been successfully validated and is acceptable for clinical implementation.

SU-G-TeP1-14: SRS Dose Calculation Accuracy Comparison Between Pencil Beam and Monte Carlo Algorithms

Purpose: Stereotactic radiosurgery (SRS) outcomes are related to the delivered dose to the target and to surrounding tissue. We have commissioned a Monte Carlo based dose calculation algorithm to recalculated the delivered dose planned using pencil beam calculation dose engine. Methods: Twenty consecutive previously treated patients have been selected for this study. All plans were generated using the iPlan treatment planning system (TPS) and calculated using the pencil beam algorithm. Each patient plan consisted of 1 to 3 targets and treated using dynamically conformal arcs or intensity modulated beams. Multi-target treatments were delivered using multiple isocenters, one for each target. These plans were recalculated for the purpose of this study using a single isocenter. The CT image sets along with the plan, doses and structures were DICOM exported to Monaco TPS and the dose was recalculated using the same voxel resolution and monitor units. Benchmark data was also generated prior to patient calculations to assess the accuracy of the two TPS against measurements using a micro ionization chamber in solid water. Results: Good agreement, within −0.4% for Monaco and +2.2% for iPlan were observed for measurements in water phantom. Doses in patient geometry revealed up to 9.6% differences for single target plans and 9.3% for multiple-target-multiple-isocenter plans. The average dose differences for multi-target-single-isocenter plans were approximately 1.4%. Similar differences were observed for the OARs and integral dose. Conclusion: Accuracy of the beam is crucial for the dose calculation especially in the case of small fields such as those used in SRS treatments. A superior dose calculation algorithm such as Monte Carlo, with properly commissioned beam models, which is unaffected by the lack of electronic equilibrium should be preferred for the calculation of small fields to improve accuracy.

MO-FG-CAMPUS-TeP3-03: Calculation of Proton Pencil Beam Properties with Full Beamline Model in TOPAS

Purpose: Introducing Monte Carlo based dose calculation algorithms into proton therapy planning systems (TPS) leads to improved accuracy. However accurate modelling of the proton pencil beam impinging the patient is necessary. Current approaches rely on measurement-driven reconstruction of phase-space and spectrum properties, typically constrained to analytical model functions. In this study a detailed Monte Carlo model of the complete cyclotron-based delivery system was created with the aim of providing more representative beam properties at treatment position. Methods: A model of the Varian Probeam proton system from the cyclotron exit to isocenter was constructed in the TOPAS Monte Carlo framework. The beam evolution through apertures and magnetic elements was validated using Transport/Turtle calculations and additionally against measurements from the Probeam™ system at Scripps Proton Therapy Center (SPTC) in San Diego, CA. A voxelized water phantom at isocenter allowed for comparison of the dose-depth curve from the Probeam model with that of a corresponding Gaussian beam over the entire energy range (70–240 MeV). Measurements of relative beam fluence cross-profiles and depth-dose curves at and around isocenter were also compared to the MC results. Results: The simulated TOPAS beam envelope was found to agree with both the Transport/Turtle and measurements to within 5% for most of the beamline. The MC predicted energy spectrum at isocenter was found to deviate increasingly from Gaussian at energies below 160 MeV. The corresponding effects on the depth dose curve agreed well with measurements. Conclusion: Given the flexibility of TOPAS and available details of the delivery system, an accurate characterization of a proton pencil beam at isocenter is possible. Incorporation of the MC derived properties of the proton pencil beam can eliminate analytical approximations and ultimately increase treatment plan accuracy and quality. Both authors are employees of Varian Medical Systems

SU‐F‐T‐81: Treating Nose Skin Using Energy and Intensity Modulated Electron Beams with Monte Carlo Based Dose Calculation

Purpose:Treating nose skin with an electron beam is of a substantial challenge due to uneven nose surfaces and tissue heterogeneity, and consequently could have a great uncertainty of dose accuracy on the target. This work explored the method using Monte Carlo (MC)‐based energy and intensity modulated electron radiotherapy (MERT), which would be delivered with a photon MLC in a standard medical linac (Artiste).Methods:The traditional treatment on the nose skin involves the usage of a bolus, often with a single energy electron beam. This work avoided using the bolus, and utilized mixed energies of electron beams. An in‐house developed Monte Carlo (MC)‐based dose calculation/optimization planning system was employed for treatment planning. Phase space data (6, 9, 12 and 15 MeV) were used as an input source for MC dose calculations for the linac. To reduce the scatter‐caused penumbra, a short SSD (61 cm) was used. A clinical case of the nose skin, which was previously treated with a single 9 MeV electron beam, was replanned with the MERT method. The resultant dose distributions were compared with the plan previously clinically used. The dose volume histogram of the MERT plan is calculated to examine the coverage of the planning target volume (PTV) and critical structure doses.Results:The target coverage and conformality in the MERT plan are improved as compared to the conventional plan. The MERT can provide more sufficient target coverage and less normal tissue dose underneath the nose skin.Conclusion:Compared to the conventional treatment technique, using MERT for the nose skin treatment has shown the dosimetric advantages in the PTV coverage and conformality. In addition, this technique eliminates the necessity of the cutout and bolus, which makes the treatment more efficient and accurate.