Monte Carlo Treatment Planning System Research Articles

Total body irradiation delivered using a dedicated Co-60 TBI unit: Evaluation of dosimetric uniformity and dose verification.

This work presents the dosimetric characteristics of Total Body Irradiation (TBI) delivered using a dedicated Co-60 TBI unit. We demonstrate the ability to deliver a uniform dose to the entire patient without the need for a beam spoiler or patient-specific compensation. Full dose distributions are calculated using an in-house Monte Carlo treatment planning system, and cumulative dose distributions are created by deforming the dose distributions within two different patient orientations. Sample dose distributions and profiles are provided to illustrate the plan characteristics, and dose and DVH statistics are provided for a heterogeneous cohort of patients. The patient cohort includes adult and pediatric patients with a range of 132-198cm in length and 16.5-37.5cm in anterior-posterior thickness. With the exception of the lungs, a uniform dose of 12Gy is delivered to the patient with nearly the entire volume receiving a dose within 10% of the prescription dose. Mean lung doses (MLDs) are maintained below the estimated threshold for radiation pneumonitis, with MLDs ranging from 7.3 to 9.3Gy (estimated equivalent dose in 2Gy fractions (EQD2 ) of 6.2-8.5Gy). Dose uniformity is demonstrated across five anatomical locations within the patient for which mean doses are all within 3.1% of the prescription dose. In-vivo dosimetry demonstrates excellent agreement between measured and calculated doses, with 78% of measurements within ±5% of the calculated dose and 99% within ±10%. These results demonstrate a state-of-the-art TBI planning and delivery system using a dedicated TBI unit and hybrid in-house and commercial planning techniques which provide comprehensive dosimetric data for TBI treatment plans that are accurately verified using in-vivo dosimetry.

PO-1816 Dosimetric verification of a Monte Carlo treatment planning system for MR-guided proton therapy

PO-1816 Dosimetric verification of a Monte Carlo treatment planning system for MR-guided proton therapy

A novel Monte Carlo (MC) dose model for small MLC fields of the cyberknife® M6TM radiosurgery system using the EGSnrc.

The multi-leaf collimator (MLC)-equipped CyberKnife® M6 radiosurgery system (CKM6) (Accuray Inc., Sunnyvale, CA) has been increasingly employed for stereotactic radiosurgery (SRS) to treat relatively small lesions. However, achieving an accurate dose distribution in such cases is usually challenging due to the combination of numerous small fields≤(30×30)mm2 . In this study, we developed a new Monte Carlo (MC) dose model for the CKM6 system using the EGSnrc to investigate dose variations in the small fields. The dose model was verified for the static MLC fields ranging from (53.8×53.9) to (7.6×7.7) mm2 at 800mm source to axis distance in a water phantom, based on the computed doses of Accuray Precision® (Accuray Inc.) treatment planning system (TPS). We achieved a statistical uncertainty of ≤4% by simulating 30-50 million incident particles/histories. Then, the treatment plans were created for the same fields in the TPS, and the corresponding measurements were performed with MapCHECK2 (Sun Nuclear Corporation), a standard device for patient-specific quality assurance (PSQA). Results of the MC simulations, TPS, and MapCHECK2 measurements were inter-compared. An overall difference in dosimetric parameters such as profiles, tissue maximum ratio (TMR), and output factors (OF) between the MC simulations and the TPS results was found ≤3% for (53.8×53.9-15.4×15.4) mm2 MLC fields, and it rose to 4.5% for the smallest (7.6mm×7.7mm) MLC field. The MapCHECK2 results showed a deviation ranging from -1.5% to + 4.5% compared to the TPS results, whereas the deviation was within±2.5% compared with the MC results. Overall, our MC dose model for the CKM6 system showed better agreement with measurements and it could serve as a secondary dose verification tool for the patient-specific QA in small fields.

Commissioning of a synchrotron-based proton beam therapy system for use with a Monte Carlo treatment planning system

This work tackles the commissioning and validation of a novel combination of a synchrotron-based proton beam therapy system (Hitachi, Ltd.) for use with a Monte Carlo treatment planning system (TPS). Four crucial aspects in this configuration have been investigated: (1) Monte Carlo-based correction performed by the TPS to the measured integrated depth-dose curves (IDD), (2) circular spot modelling with a single Gaussian function to characterize the synchrotron physical spot, which is elliptical, (3) the modelling of the range shifter that enables using only one set of measurements in open beams, and (4) the Monte Carlo dose calculation model in small fields.Integrated depth-dose curves were measured with a PTW Bragg peak chamber and corrected, with a Monte Carlo model, to account for energy absorbed outside the detector. The elliptical spot was measured by IBA Lynx scintillator, EBT3 films and PTW microDiamond. The accuracy of the TPS (RayStation, RaySearch Laboratories) at spot modelling with a circular Gaussian function was assessed.The beam model was validated using spread-out Bragg peak (SOBP) fields. We took single-point doses at several depths through the central axis using a PTW Farmer chamber, for fields between 2 × 2cm and 30 × 30cm. We checked the range-shifter modelling from open-beam data. We tested clinical cases with film and an ionization chamber array (IBA Matrix).Sigma differences for spots fitted using 2D images and 1D profiles to elliptical and circular Gaussian models were below 0.22 mm. Differences between SOBP measurements at single points and TPS calculations for all fields between 5 × 5 and 30 × 30cm were below 2.3%. Smaller fields had larger differences: up to 3.8% in the 2 × 2cm field. Mean differences at several depths along the central axis were generally below 1%. Differences in range-shifter doses were below 2.4%. Gamma test (3%, 3 mm) results for clinical cases were generally above 95% for Matrix and film.Approaches for modelling synchrotron proton beams have been validated. Dose values for open and range-shifter fields demonstrate accurate Monte Carlo correction for IDDs. Elliptical spots can be successfully modelled using a circular Gaussian, which is accurate for patient calculations and can be used for small fields. A double-Gaussian spot can improve small-field calculations. The range-shifter modelling approach, which reduces clinical commissioning time, is adequate.

Monte Carlo modeling of the Elekta Versa HD and patient dose calculation with EGSnrc/BEAMnrc.

IntroductionNumerous studies have proven the Monte Carlo method to be an accurate means of dose calculation. Although there are several commercial Monte Carlo treatment planning systems (TPSs), some clinics may not have access to these resources. We present a method for routine, independent patient dose calculations from treatment plans generated in a commercial TPS with our own Monte Carlo model using free, open‐source software.Materials and methodsA model of the Elekta Versa HD linear accelerator was developed using the EGSnrc codes. A MATLAB script was created to take clinical patient plans and convert the DICOM RTP files into a format usable by EGSnrc. Ten patients’ treatment plans were exported from the Monaco TPS to be recalculated using EGSnrc. Treatment simulations were done in BEAMnrc, and doses were calculated using Source 21 in DOSXYZnrc. Results were compared to patient plans calculated in the Monaco TPS and evaluated in Verisoft with a gamma criterion of 3%/2 mm.ResultsOur Monte Carlo model was validated within 1%/1‐mm accuracy of measured percent depth doses and profiles. Gamma passing rates ranged from 82.1% to 99.8%, with 7 out of 10 plans having a gamma pass rate over 95%. Lung and prostate patients showed the best agreement with doses calculated in Monaco. All statistical uncertainties in DOSXYZnrc were less than 3.0%.ConclusionA Monte Carlo model for routine patient dose calculation was successfully developed and tested. This model allows users to directly recalculate DICOM RP files containing patients’ plans that have been exported from a commercial TPS.

On the impact of absorbed dose specification, tissue heterogeneities, and applicator heterogeneities on Monte Carlo-based dosimetry of Ir-192, Se-75, and Yb-169 in conventional and intensity-modulated brachytherapy for the treatment of cervical cancer.

The purpose of this study was to evaluate the impact of dose reporting schemes and tissue/applicator heterogeneities for 192 Ir-, 75 Se-, and 169 Yb-based MRI-guided conventional and intensity-modulated brachytherapy. Treatment plans using a variety of dose reporting and tissue/applicator segmentation schemes were generated for a cohort (n=10) of cervical cancer patients treated with 192 Ir-based Venezia brachytherapy. Dose calculations were performed using RapidBrachyMCTPS, a Geant4-based research Monte Carlo treatment planning system. Ultimately, five dose calculation scenarios were evaluated: (a) dose to water in water (Dw,w ); (b) Dw,w taking the applicator material into consideration (Dw,wApp ); (c) dose to water in medium (Dw,m ); (d and e) dose to medium in medium with mass densities assigned either nominally per structure (Dm,m (Nom) ) or voxel-by-voxel (Dm,m ). Ignoring the plastic Venezia applicator (Dw,wApp ) overestimates Dm,m by up to 1% (average) with high energy source (192 Ir and 75 Se) and up to 2% with 169 Yb. Scoring dose to water (Dw,wApp or Dw,m ) generally overestimates dose and this effect increases with decreasing photon energy. Reporting dose other than Dm,m (or Dm,m Nom ) for 169 Yb-based conventional and intensity-modulated brachytherapy leads to a simultaneous overestimation (up to 4%) of CTVHR D90 and underestimation (up to 2%) of bladder D2cc due to a significant dip in the mass-energy absorption ratios at the depths of nearby targets and OARs. Using a nominal mass-density assignment per structure, rather than a CT-derived voxel-by-voxel assignment for MRI-guided brachytherapy, amounts to a dose error up to 1% for all radionuclides considered. The effects of the considered dose reporting schemes trend correspondingly between conventional and intensity-modulated brachytherapy. In the absence of CT-derived mass densities, MRI-only-based dosimetry can adequately approximate Dm,m by assigning nominal mass densities to structures. Tissue and applicator heterogeneities do not significantly impact dosimetry for 192 Ir and 75 Se, but do for 169 Yb; dose reporting must be explicitly defined since Dw,m and Dw,w may overstate the dosimetric benefits.

Back to the Future: Helium Ion Therapy 2020

Helium ions (4He) remained clinically unexploited worldwide since the shutdown of the clinical trials at the Lawrence Berkeley Laboratory (LBL) despite their favorable physical and biophysical characteristics. 4He afford minimal lateral scattering compared to protons, with a significantly reduced fragmentation tail compared to carbon ions and enhancement of bio-effects with relative biological effectiveness (RBE) values ranging between 1.3 and ∼3. Here, we outline milestones and on-going preparations for 4He clinical trials set for this year at our facility. Investigation of physical and biological phenomena of 4He was performed. Dosimetry was conducted in both homogenous and heterogenous (clinical-like) settings, verifying prediction of the first (non-clinical) 4He Monte Carlo (MC) and GPU-accelerated based treatment planning system (gTPS). Towards translation in patients, these systems are supporting development and integration of the first clinical treatment planning system. Additionally, in vitro, in vivo and in silico biological studies were carried out to establish robust prediction of biological effects for 4He. Prescription doses, fractionation schemes, tissue radio-sensitivity (α/βx) assignment, algorithm for effective dose calculation, and RBE model were selected for the upcoming clinical trials using 4He. In water and in anthropomorphic head phantoms, measurements yielded dose values within ∼2% against MC and gTPS predictions. For RBE modeling, measured RBE in vitro and in vivo indicates clinically relevant uncertainty of ±5-10% across different RBE model as a function of the various endpoints (dose, linear energy transfer and tissue type). Based on improved performance in the studied cell lines and tissues, as well as its accessibility in the literature, the modified microdosimetric kinetic model (mMKM) was selected as the clinical model for 4He RBE prediction. Through interdisciplinary collaboration between physicians, physicists and radiobiologists, we have addressed the prospective challenges and clinical decisions necessary for translating active scanning 4He to the clinic. Developed physical/biological models were successfully integrated in both research and commercial systems and validated against in vitro/in vivo experimental data. Together, these efforts mark an important step towards clinical translation 4He ion-beam therapy with state-of-the-art technologies.

A Multimodality Image Guided Precision Radiation Research Platform: Integrating X-ray, Bioluminescence, and Fluorescence Tomography With Radiation Therapy

Small animal irradiation is crucial to the investigation of radiobiological mechanisms. The paradigm of clinical radiation therapy is trending toward high-precision, stereotactic treatment. However, translating this scheme to small animal irradiation is challenging owing to the lack of high-quality image guidance. To overcome this obstacle, we developed a multimodality image guided precision radiation platform. The platform consists of 4 modules: x-ray computed tomography (CT), bioluminescence tomography (BLT), fluorescence molecular tomography (FMT), and radiation therapy. CT provides animal anatomy and material density for radiation dose calculation, as well as body contour for BLT and FMT reconstruction. BLT and FMT provide tumor localization to guide radiation beams and molecular activity to evaluate treatment outcome. Furthermore, we developed a Monte Carlo-based treatment planning system (TPS) for 3-dimensional dose calculation, calibrated it using radiochromic films sandwiched in a water-equivalent phantom, and validated it using invivo dosimeters surgically implanted into euthanized mice (n = 4). Finally, we performed image guided irradiation on mice bearing orthotopic breast and prostate tumors and confirmed radiation delivery using γH2AX histology. The Monte Carlo-based TPS was successfully calibrated by benchmarking simulation dose against film measurement. For invivo dosimetry measured in the euthanized mice, the average difference between the TPS calculated dose and measured dose was 3.86% ± 1.12%. Following the TPS-generated treatment plan, we successfully delivered 20 Gy dose to an animal bearing an orthotopic prostate tumor using 4 BLT-guided radiation beams and 5 Gy dose to an animal bearing an orthotopic breast tumor using a single FMT-guided radiation beam. γH2AX histology presented significantly more DNA damage in irradiated tumors and thus validated the dose delivery accuracy. Combined with Monte Carlo TPS, this multimodality CT/BLT/FMT image guided small animal radiation platformcan specifically localize tumors, accurately calculate dose distribution, precisely guide radiation delivery, and molecularly evaluate treatment response. It provides an advanced toolset for radiobiology and translational cancer research.

Measurement of surface dose in an MR-Linac with optically stimulated luminescence dosimeters for IMRT beam geometries.

This study aims to measure the surface dose on an anthropomorphic phantom for intensity-modulated radiation therapy (IMRT) plans treated in a 1.5T magnetic resonance (MR)-Linac. Previous studies have used Monte Carlo programs to simulate surface dose and have recognized high surface dose as a potential limiting factor for the MR-Linac; however, to our knowledge surface dose measurement for clinical plans has not yet been published. Given the novelty of the MR-Linac, it is important to perform in vivo measurements of a potentially dose-limiting factor such as surface dose when moving forward for clinical use. Optically stimulated luminescence dosimeters (OSLDs) were used on an anthropomorphic phantom. Intensity-modulated radiation therapy plans were generated to treat a near-surface breast tumor in the phantom. The tumor was treated with 2, 3, 5, 7, and 9 beam IMRT plans with a 1.5T MR-Linac using a 7-MV photon beam. The plans were generated in a Monte Carlo treatment planning system (TPS) capable of modeling magnetic field effects. The surface dose was sampled in seven locations on the surface surrounding the planning target volume (PTV), and in two different OSLD configurations with the dosimeters measuring water equivalent depths of 0.16 and 0.64mm. The TPS was used to estimate the doses at the OSLD locations. In addition, MR images were taken of a pork belly with and without an OSLD placed anteriorly placed to determine the effect of an OSLD on image fidelity. For the 3, 5, 7, and 9-beam configurations, surface doses were approximately half that of the prescription dose to the simulated tumor, although the magnitude of the skin dose relative to the prescription is certainly also dependent on individual patient anatomy. The general trend for both TPS and measurements was that the greater the number of beams, the lower the skin doses and dose readings; also, with increasing numbers of beams, doses at shallow depths become lower relative to deeper depths. The MR images showed that the presence of the OSLD did not induce clinically relevant geometric distortions or intensity differences. To our knowledge, this study is the first of its kind to experimentally measure the surface dose in an MR-Linac for IMRT plans. This study has explored the use of OSLDs to measure in vivo surface dose in a clinical setting. OSLDs may be used to measure skin dose clinically when there are concerns of skin radiation burns and near-surface toxicity. Optically stimulated luminescence dosimeters are promising devices for in vivo surface dosimetry in an MR-Linac.

Surface and near-surface dose measurements at beam entry and exit in a 1.5 T MR-Linac using optically stimulated luminescence dosimeters

The objective of this study is to measure surface and near-surface dose at entry and exit surfaces in a 1.5 T MR-Linac (Elekta AB, Stockholm, Sweden) using optically stimulated luminescence dosimeters (OSLDs). OSLDs were expected to be useful for measuring surface dose in a strong magnetic field because they can be taped to undersides to measure exit dose, and their dose response have been shown to be reasonably insensitive to variations in beam angle, beam energy, and magnetic fields.The surface and near-surface dose at the entry and exit of a 20 cm thick solid water phantom was measured with OSLDs for 5 × 5, 10 × 10, and 22 × 22 cm2 field sizes. The solid water phantom was elevated off the couch top to produce an air gap of 3.7 cm so as to observe the electron return effect (ERE) near the beam exit surface. Measurement depths ranged from surface to 15 mm deep from entry and exit surfaces. The phantom dose distribution was also computed in the Monaco (Elekta AB, Stockholm, Sweden) Monte Carlo treatment planning system (TPS).For the 5 × 5, 10 × 10, and 22 × 22 cm2 field sizes the surface dose at depth 0 mm was extrapolated from OSLD measurements to be 10.9%, 12.0%, and 13.5%. The surface entry dose was found to be far less field size-dependent compared to a conventional linac, likely due to a lack of electronic contamination due to the strong magnetic field perpendicular to the beam. The ERE effect was observed in the measurements near the exit surface of the phantom, and was in close agreement with the TPS calculation.

Can intermediate-energy sources lead to elevated bone doses for prostate and head & neck high-dose-rate brachytherapy?

PurposeSeveral radionuclides with high (60Co, 75Se) and intermediate (169Yb, 153Gd) energies have been investigated as alternatives to 192Ir for high-dose-rate brachytherapy. The purpose of this study was to evaluate the impact of tissue heterogeneities for these five high- to intermediate-energy sources in prostate and head & neck brachytherapy. Methods and MaterialsTreatment plans were generated for a cohort of prostate (n = 10) and oral tongue (n = 10) patients. Dose calculations were performed using RapidBrachyMCTPS, an in-house Geant4-based Monte Carlo treatment planning system. Treatment plans were simulated using 60Co, 192Ir, 75Se, 169Yb, and 153Gd as the active core of the microSelectron v2 source. Two dose calculation scenarios were presented: (1) dose to water in water (Dw,w), and (2) dose to medium in medium (Dm,m). ResultsDw,w overestimates planning target volume coverage compared with Dm,m, regardless of photon energy. The average planning target volume D90 reduction was ∼1% for high-energy sources, whereas larger differences were observed for intermediate-energy sources (1%–2% for prostate and 4%–7% for oral tongue). Dose differences were not clinically relevant (<5%) for soft tissues in general. Going from Dw,w to Dm,m, bone doses were increased two- to three-fold for 169Yb and four- to five-fold for 153Gd, whereas the ratio was close to ∼1 for high-energy sources. ConclusionsDw,w underestimates the dose to bones and, to a lesser extent, overestimates the dose to soft tissues for radionuclides with average energies lower than 192Ir. Further studies regarding bone toxicities are needed before intermediate-energy sources can be adopted in cases where bones are in close vicinity to the tumor.

Imaging and Dosimetry Study of Inter-fraction Setup Error in a Murine Xenograft Flank Tumor Radiation Model.

Modern small animal irradiation platforms provide for accurate delivery of radiation under 3D image guidance. However, leveraging these improvements currently comes at the cost of lower-throughput experimentation. Herein, we characterized setup accuracy and dosimetric robustness for mock/sham irradiation of a murine xenograft flank tumor model using the X-RAD SmART+ with the vendor-supplied Monte Carlo (MC) treatment planning system (SmART ATP). The chosen beam arrangement was parallel-opposing using a 20 mm square collimator, aligned off-axis for ipsilateral lung sparing. Using a cohort of five mice imaged with cone beam computed tomography (CBCT) over five consecutive mock-irradiation fractions, we compared inter-fraction setup variability resulting from a vendor-supplied multi-purpose bed with anesthesia nose cone with a more complicated immobilization solution with an integrated bite block with nose cone and Styrofoam platform. A hypothetical "high-throughput" image-guidance scenario was investigated, wherein the day 1 stage coordinates (resulting from CBCT guidance) were applied on days 2-5. Daily inter-fraction setup errors were evaluated per specimen (days 2-5) using CBCT-derived offsets from day 1 stage coordinates. Using the CBCT images and Monte Carlo dose calculation, 3D dosimetric plan robustness was evaluated for the vendor-supplied immobilization solution, for both the high-throughput guidance scenario as well as for use of daily CBCT-based alignment. Inter-fraction setup offset magnitude was 3.6 (±1.5) mm for the vendor-supplied immobilization compared to 3.3 (±1.8) mm for the more complicated solution. For the vendor-supplied immobilization, we found that daily CBCT was needed to adequately cover the flank tumors dosimetrically, given our chosen treatment approach.

Comparative photon and proton dosimetry for patients with mediastinal lymphoma in the era of Monte Carlo treatment planning and variable relative biological effectiveness

BackgroundExisting pencil beam analytical (PBA) algorithms for proton therapy treatment planning are not ideal for sites with heterogeneous tissue density and do not account for the spatial variations in proton relative biological effectiveness (vRBE). Using a commercially available Monte Carlo (MC) treatment planning system, we compared various dosimetric endpoints between proton PBA, proton MC, and photon treatment plans among patients with mediastinal lymphoma.MethodsEight mediastinal lymphoma patients with both free breathing (FB) and deep inspiration breath hold (DIBH) CT simulation scans were analyzed. The original PBA plans were re-calculated with MC. New proton plans that used MC for both optimization and dose calculation with equivalent CTV/ITV coverage were also created. A vRBE model, which uses a published model for DNA double strand break (DSB) induction, was applied on MC plans to study the potential impact of vRBE on cardiac doses. Comparative photon plans were generated on the DIBH scan.ResultsRe-calculation of FB PBA plans with MC demonstrated significant under coverage of the ITV V99 and V95. Target coverage was recovered by re-optimizing the PT plan with MC with minimal change to OAR doses. Compared to photons with DIBH, MC-optimized FB and DIBH proton plans had significantly lower dose to the mean lung, lung V5, breast tissue, and spinal cord for similar target coverage. Even with application of vRBE in the proton plans, the putative increase in RBE at the end of range did not decrease the dosimetric advantages of proton therapy in cardiac substructures.ConclusionsMC should be used for PT treatment planning of mediastinal lymphoma to ensure adequate coverage of target volumes. Our preliminary data suggests that MC-optimized PT plans have better sparing of the lung and breast tissue compared to photons. Also, the potential for end of range RBE effects are unlikely to be large enough to offset the dosimetric advantages of proton therapy in cardiac substructures for mediastinal targets, although these dosimetric findings require validation with late toxicity data.

Feasibility of a GATE Monte Carlo platform in a clinical pretreatment QA system for VMAT treatment plans using TrueBeam with an HD120 multileaf collimator.

PurposeTo evaluate the quality of patient‐specific complicated treatment plans, including commercialized treatment planning systems (TPS) and commissioned beam data, we developed a process of quality assurance (QA) using a Monte Carlo (MC) platform. Specifically, we constructed an interface system that automatically converts treatment plan and dose matrix data in digital imaging and communications in medicine to an MC dose‐calculation engine. The clinical feasibility of the system was evaluated.Materials and MethodsA dose‐calculation engine based on GATE v8.1 was embedded in our QA system and in a parallel computing system to significantly reduce the computation time. The QA system automatically converts parameters in volumetric‐modulated arc therapy (VMAT) plans to files for dose calculation using GATE. The system then calculates dose maps. Energies of 6 MV, 10 MV, 6 MV flattening filter free (FFF), and 10 MV FFF from a TrueBeam with HD120 were modeled and commissioned. To evaluate the beam models, percentage depth dose (PDD) values, MC calculation profiles, and measured beam data were compared at various depths (Dmax, 5 cm, 10 cm, and 20 cm), field sizes, and energies. To evaluate the feasibility of the QA system for clinical use, doses measured for clinical VMAT plans using films were compared to dose maps calculated using our MC‐based QA system.ResultsA LINAC QA system was analyzed by PDD and profile according to the secondary collimator and multileaf collimator (MLC). Values for MC calculations and TPS beam data obtained using CC13 ion chamber (IBA Dosimetry, Germany) were consistent within 1.0%. Clinical validation using a gamma index was performed for VMAT treatment plans using a solid water phantom and arbitrary patient data. The gamma evaluation results (with criteria of 3%/3 mm) were 98.1%, 99.1%, 99.2%, and 97.1% for energies of 6 MV, 10 MV, 6 MV FFF, and 10 MV FFF, respectively.ConclusionsWe constructed an MC‐based QA system for evaluating patient treatment plans and evaluated its feasibility in clinical practice. We observed robust agreement between dose calculations from our QA system and measurements for VMAT plans. Our QA system could be useful in other clinical settings, such as small‐field SRS procedures or analyses of secondary cancer risk, for which dose calculations using TPS are difficult to verify.

P291] Comparison of beam model implementation methods for commissioning of Monte Carlo code in proton beam therapy centre

Purpose In proton beam therapy (PBT), Monte Carlo (MC) methods offer more accurate dose recalculation than analytical treatment planning system (TPS) used in clinical routine. The purpose of this work was to implement a PBT centre beam model in MC code. This is essential when employing the code as a support tool for quality assurance procedures and using data from MC simulation as an additional information for the clinical TPS. Furthermore, different beam implementation methods were compared in order to find the most adequate approach to simulate beam source and obtain the best agreement between MC simulations and TPS. Methods Simulations of proton pencil beams in water phantom were performed in order to find MC specific beam parameters that characterize the beam model in the PBT centre. A GPU-accelerated MC code was used, which significantly reduces the dose distribution calculation time. Two beam model implementation methods were considered: (i) a paraxial beam approach and (ii) an emittance model. The paraxial beam approach is determined from depth-dose-distribution (DDD) measurements and TPS calculations in a water phantom. The emittance model additionally accounts for the measurement of beam profiles in air. The beam shape and dose values for single beam simulations using the two methods were compared with TPS, which was used as a reference. Results A perfect agreement in beam range, within binning uncertainty of 0.1 mm, was obtained for single beam DDDs calculated with MC code and TPS comparing both beam model implementation approaches. The simulated lateral beam size fit to TPS reference values for the paraxial beam approach and emittance model differing up to 10% (delta_sigma 0.5 mm). The minor difference of up to 0.2 mm in the beam size was observed between the emittance model and the paraxial beam approach. Conclusions The preliminary results showed that the beam size in the water phantom for the paraxial beam approach, the emittance model and TPS are in good agreement, even though, the emittance model considers actual variation of the beam size in air. For the clinical application these findings must be further verified by MC simulations and experimentally.

Agility MLC transmission optimization in the Monaco treatment planning system.

The Monaco Monte Carlo treatment planning system uses three‐beam model components to achieve accuracy in dose calculation. These components include a virtual source model (VSM), transmission probability filters (TPFs), and an x‐ray voxel Monte Carlo (XVMC) engine to calculate the dose in the patient. The aim of this study was to assess the TPF component of the Monaco TPS and optimize the TPF parameters using measurements from an Elekta linear accelerator with an Agility™ multileaf collimator (MLC). The optimization began with all TPF parameters set to their default value. The function of each TPF parameter was characterized and a value was selected that best replicated measurements with the Agility™ MLC. Both vendor provided fields and a set of additional test fields were used to create a rigorous systematic process, which can be used to optimize the TPF parameters. It was found that adjustment of the TPF parameters based on this process resulted in improved point dose measurements and improved 3D gamma analysis pass rates with Octavius 4D. All plans calculated with the optimized beam model had a gamma pass rate of > 95% using criteria of 2% global dose/2 mm distance‐to‐agreement, while some plans calculated with the default beam model had pass rates as low as 88.4%. For measured point doses, the improvement was particularly noticeable in the low‐dose regions of the clinical plans. In these regions, the average difference from the planned dose reduced from 4.4 ± 4.5% to 0.9 ± 2.7% with a coverage factor (k = 2) using the optimized beam model. A step‐by‐step optimization guide is provided at the end of this study to assist in the optimization of the TPF parameters in the Monaco TPS. Although it is possible to achieve good clinical results by randomly selecting TPF parameter values, it is recommended that the optimization process outlined in this study is followed so that the transmission through the TPF is characterized appropriately.

Validation of a Software Upgrade in a Monte Carlo Treatment Planning System by Comparison of Plans in Different Versions.

Purpose:Validation of a new software version of a Monte Carlo treatment planning system through comparing plans generated by two software versions in volumetric-modulated arc therapy (VMAT) for lung cancer.Materials and Methods:Three patients who were treated with 60 Gy/30 fractions in Elekta Synergy™ linear accelerator by VMAT technique with 2% statistical uncertainty (SU) were chosen for the study. Multiple VMAT plans were generated using two different software versions of Monaco treatment planning system TPS (V5.10.02 and V5.11). By keeping all other parameters constant, originally accepted plans were recalculated for the SUs of 0.5%, 1%, 2%, 3%, 4%, and 5%. For plan evaluation, the metrics compared were conformity Index (CI), homogeneity Index (HI), dose coverage to planning target volume (PTV), organ at risk (OAR) doses to spinal cord, pericardium, bilateral lungs-PTV, esophagus, liver, normal tissue integral dose (NTID), volumes receiving dose >5 and >10 Gy, calculation time (tCT), and gamma pass rates.Results:In both versions, CI and HI improved as the SU increased from 0.5% to 5%. No significant dose difference was observed in Dmean to PTV, bilateral lungs-PTV, pericardium, esophagus, liver, normal tissue volume receiving >5, and >10 Gy and NTID. It was observed that while the tCT and gamma pass rates decreased, the maximum dose to PTV increased as the SU increased. No other significant dose differences were observed between the two MC versions compared.Conclusion:For lung VMAT plans, in both versions, SU could be accepted up to 3% per plan with reduced tCT without compromising plan quality and deliverability by accepting variations in point dose and an inhomogeneous dose within the target. The plan quality of Monaco™V5.10.02 was similar to Monaco™TPS-V5.11 except for tCT.

Quantification and comparison the dosimetric impact of two treatment couch model in VMAT.

The use of Monte Carlo treatment planning systems (TPS) in radiation therapy has increased the dosimetric accuracy of VMAT treatment sequences. However, this accuracy is compromised by not including the treatment couch into the treatment planning process. Therefore, the impact of the treatment couch on radiation delivery output was determined, and two different couch models (uniform couch model A vs two components model B) were included and tested in the Monaco TPS to investigate which model can better quantify the couch influence on radiation dose. Relative attenuation measurements were performed following procedures outlined by TG‐176 with three phantom positions for A–B direction: on the left half (L), in the center (C) and on the right half (R) of the couch. As well as absolute dose comparison of static fields of 10 × 10 cm2 that were delivered through the couch tops with that calculated in the TPS with the couch model at 2 mm and 5 mm computing grid size respectively. The most severe percentage deviation was 4.60% for the phantom positioned at the left half of the couch with 5 mm grid size at gantry angle 120°. The couch model was included in the TPS with a uniform ED of 0.26 g/cm3 or a two component model with a fiber 0.52 g/cm3 and foam core 0.1 g/cm3. After including the treatment couch, the maximum mean dose attenuation was reduced from 3.68% without couch included to (0.60, 0.83, 0.72, and 1.02) % for model A and model B at 2 and 5 mm voxel grid size. The results obtained showed that Model A performed better than the model B, demonstrating lower deviations from measurements and better robustness against dose grid resolution changes. Considering the results of this study, we propose the systematic introduction of the couch Model A in clinical routine. All the reported findings are valid for the Elekta iBEAM ® evo Extension 415 couch and these methods can also be used for other couch model.

Comparative Evaluation of Treatment Planning Systems Used in Lung SBRT

The effectiveness of lung SBRT depends on the choice of treatment planning system (TPS) used for dose calculations. Whereas Monte-Carlo (MC) based TPS calculate the most accurate dose, conventional planning systems cannot adequately account for dose variation caused by heterogeneous lung tissue. We aim to evaluate the performance of several commercial TPS used in lung SBRT. We hypothesize that the use of MC-based TPS will result in more accurate and consistent delivered doses in lung SBRT. SBRT plans for 30 early-stage lung cancer patients (31 targets) were retrospectively evaluated in this multi-institutional study. Patient data with structure sets were anonymized and sent to four participating institutions to plan on five different TPS: one pencil-beam (PB), two convolution-superposition (CS1 & CS2), and two Monte Carlo (MC1 & MC2) systems. A test case with homogenous slab phantom was also included to allow benchmarking of each TPS. Patients were grouped according to PTV size: small (<25cc), medium (25 – 60cc), large (>60cc); and location: island, adjacent to chest-wall/mediastinum, and mixed. All patients were initially planned with MC1TPS to deliver 50Gy PTV dose in 5 fractions using 7-12 non-coplanar, conformal, 6 MV fields. Treatment plans were optimized for > 95% PTV dose coverage as per the Radiation Therapy Oncology Group guidelines. Using the same beam parameters, calculation geometry, and MUs, all patient plans were re-computed on other TPS. Dose-volume-histograms (DVH) for PTV and organs-at-risk (OARs) including lung, esophagus, trachea, heart, and spinal cord were evaluated. Two-tailed student t-test was used for statistical analysis. Average PTV was 53.5cc (range: 8.6 – 149.4cc). The largest dose difference was found between MC1 and PB for small island targets. Compared to MC1, average values of PTV Dmean for all patients were higher by 12.5%, 3%, 0.5% and 2% respectively in PB, CS1, CS2 and MC2 TPS (p < 0.01). Larger dose differences were seen for PTV Dmin: 21.8%, 4.6%, 10.6% and 12.8% (p < 0.01), indicating greater sensitivity of dose calculation algorithms in the target periphery due to density changes. Whereas the prescription dose conformality (CI) was similar in CS and MC TPS, dose spillage (D2cm) was 6.2%, 4.8%, 1.5% and 0.9% higher respectively for PB, CS1, CS2 and MC2 (p< 0.01). Compared to MC1, average lung V20 showed an 11.4% increase in PB, but was 2.4%, 2.4% and 4.4% lower in CS1, CS2 and MC2 TPS respectively (p < 0.01). Other OARs showed similar trend with MC1 having closest agreement with MC2, followed by CS2, CS1 and PB. Conventional TPS for lung SBRT result in significant dose variation depending on the calculation algorithm. MC based TPS provide the greatest accuracy and consistency in delivered doses, thereby allowing easier plan comparison among different institutions. In light of observed dose differences, current RTOG dose guidelines may require further evaluation or revision.

A Monte Carlo model and its commissioning for the Leksell Gamma Knife Perfexion radiosurgery system.

To develop and commission a Monte Carlo (MC) simulation model for the Leksell Gamma Knife (LGK) Perfexion (PFX) radiosurgery system. We previously established a source model for MC simulations of the LGK PFX for the purpose of the treatment planning system (TPS) dose verification and plan evaluation. To make practical and effective use of the model in clinic, several issues need to be addressed. First, thorough commissioning procedures are needed to ensure the validity of the model parameters, such as the source-to-focus (STF) distance, the source solid angle. Second, an efficient source particle sampling method is required to facilitate dose calculations for multitarget and multishot configurations in patient treatment plans. Third, inseparably, it is interesting to know the dose difference between the two GK TPS algorithms (TMR and convolution) and the MC method in extreme heterogeneous cases resulting from the inhomogeneous effect. We report our recent development in addressing these issues. Phantoms with the frame fiducials were manually created in the format of DICOM CT image to eliminate the uncertainties associated with scanner artifacts and image registration. The created homogeneous phantom was used to calibrate the model parameters to match the output factors with the manufacturer provided data, and the heterogeneous phantom with multilayer materials was used to study the inhomogeneous effect. The agreement between the MC calculation and TPS was very good for the homogeneous spherical phantom. The difference of the full width at half maximum (FWHM) of the profiles was less than 1 mm except for the profile for 16 mm collimator along z-axis (less than 2 mm). For the extreme heterogeneous test case, it was shown that the TMR algorithm can overestimate the target dose by up to 22% using the measure of dose volume parameter D95. The agreement between the MC method and the TPS convolution method was better (within 3.6%) for the target near the center of phantom, however, discrepancy (up to 10.7%) existed for the target close to the skull. The difference between the two TPS dose algorithms was about 11%. Considerable dose difference may result from the effect of heterogeneity, such as in the regions of the air cavities and bones. As the MC method has been extensively used in conventional external beams, it is worthwhile for further investigation in applying the MC method to accurate dose planning in the new GK PFX radiosurgery platform.