Validation of an in vivo transit dosimetry algorithm using Monte Carlo simulations and ionization chamber measurements.

PURPOSE: Transit dosimetry is a safety tool based on the transit images acquired during treatment. Forward-projection transit dosimetry software, as PerFRACTION, compares the transit images acquired with an expected image calculated from the DICOM plan, the CT, and the structure set. This work aims to validate PerFRACTION expected transit dose using PRIMO Monte Carlo simulations and ionization chamber measurements, and propose a methodology based on MPPG5a report. METHODS: The validation process was divided into three groups of tests according to MPPG5a: basic dose validation, IMRT dose validation, and heterogeneity correction validation. For the basic dose validation, the fields used were the nine fields needed to calibrate PerFRACTION and three jaws-defined. For the IMRT dose validation, seven sweeping gaps fields, the MLC transmission and 29 IMRT fields from 10 breast treatment plans were measured. For the heterogeneity validation, the transit dose of these fields was studied using three phantoms: 10 , 30 , and a 3 cm cork slab placed between 10 cm of solid water. The PerFRACTION expected doses were compared with PRIMO Monte Carlo simulation results and ionization chamber measurements. RESULTS: Using the 10 cm solid water phantom, for the basic validation fields, the root mean square (RMS) of the difference between PerFRACTION and PRIMO simulations was 0.6%. In the IMRT fields, the RMS of the difference was 1.2%. When comparing respect ionization chamber measurements, the RMS of the difference was 1.0% both for the basic and the IMRT validation. The average passing rate with a γ(2%/2 mm, TH = 20%) criterion between PRIMO dose distribution and PerFRACTION expected dose was 96.0% ± 5.8%. CONCLUSION: We validated PerFRACTION calculated transit dose with PRIMO Monte Carlo and ionization chamber measurements adapting the methodology of the MMPG5a report. The methodology presented can be applied to validate other forward-projection transit dosimetry software.

PRIMO Monte Carlo software as a tool for commissioning of an external beam radiotherapy treatment planning system.

Background: The purpose was to validate the PRIMO Monte Carlo software to be used during the commissioning of a treatment planning system (TPS). Materials and methods: The Acuros XB v. 16.1 algorithm of the Eclipse was configured for 6 MV and 6 MV flattening-filter-free (FFF) photon beams, from a TrueBeam linac equipped with a high-definition 120-leaf multileaf collimator (MLC). PRIMO v. 0.3.64.1814 software was used with the phase space files provided by Varian and benchmarked against the reference dosimetry dataset published by the Imaging and Radiation Oncology Core-Houston (IROC-H). Thirty Eclipse clinical intensity-modulated radiation therapy (IMRT)/volumetric modulated arc therapy (VMAT) plans were verified in three ways: 1) using the PTW Octavius 4D (O4D) system; 2) the Varian Portal Dosimetry system and 3) the PRIMO software. Clinical validation of PRIMO was completed by comparing the simulated dose distributions on the O4D phantom against dose measurements for these 30 clinical plans. Agreement evaluations were performed using a 3% global/2 mm gamma index analysis. Results: PRIMO simulations agreed with the benchmark IROC-H data within 2.0% for both energies. Gamma passing rates (GPRs) from the 30 clinical plan verifications were (6 MV/6MV FFF): 99.4% ± 0.5%/99.9% ± 0.1%, 99.8% ± 0.4%/98.9% ± 1.4%, 99.7% ± 0.4%/99.7% ± 0.4%, for the 1), 2) and 3) verification methods, respectively. Agreement between PRIMO simulations on the O4D phantom and 3D dose measurements resulted in GPRs of 97.9% ± 2.4%/99.7% ± 0.4%. Conclusion: The PRIMO software is a valuable tool for dosimetric verification of clinical plans during the commissioning of the primary TPS.

Gamma passing rates of daily EPID transit images correlate to PTV coverage for breast cancer IMRT treatment plans.

PURPOSE: The use of the transit image obtained with the electronic portal-imaging device (EPID) is becoming an extended method to perform in-vivo dosimetry. The transit images acquired during each fraction can be compared with a predicted image, if available, or with a baseline image, usually the obtained in the first fraction. This work aims to study the dosimetric impact of the failing fractions and to evaluate the appropriateness of using a baseline image in breast plans. MATERIAL AND METHODS: Twenty breast patients treated in a Halcyon were retrospectively selected. For each patient and fraction, the treatment plan was calculated over the daily CBCT image. For each fraction, the differences respect to the treatment plan values of OARs and PTV dosimetric parameters were analyzed: ΔDmean , ΔD95%, ΔD98%, ΔD2%, ΔV36Gy, ΔV38.5Gy, and ΔV43.5Gy. Daily fractions were ranked according to the differences found in the dosimetric parameters between the treatment plan and the daily CBCT to establish the best fraction. The daily transit images acquired in every fraction were compared to the first fraction using the global gamma index with the Portal Dosimetry tool. The comparison was repeated using the best fraction image as a baseline. We assessed the correlation of the dosimetric differences obtained from the CBCT images-based treatment plans with the gamma index passing rates obtained using first fraction and best fraction as baseline. RESULTS: Average values of -11.6% [-21.4%, -3.3%] and -3.2% [-1.0%, -10.3%] for the ∆PTVD98% and ∆PTVD95% per every 10% decrease in the passing rate were found, respectively. When using the best fraction as baseline patients were detected with failing fractions that were not detected with the first fraction as baseline. CONCLUSION: The gamma passing rates of daily transit images correlate with the coverage loss parameters in breast IMRT plans. Using first fraction image as baseline can lead to the non-detectability of failing fractions.

Treatment time of image-guided radiotherapy with a Halcyon 2.0 system.

INTRODUCTION: The treatment fraction time is a key indicator of the external beam radiotherapy process. The Halcyon system was designed to improve the clinical workflow, according to the manufacturer (Varian Medical Systems). Few works studied the actual delivery efficiency of the Halcyon system. This work analyzed the treatment time on a Halcyon 2.0 unit for a variety of sites along a period of 9 months. MATERIALS AND METHODS: Treatment time included patient setup, image acquisition, image-guided online couch correction, and radiation delivery time. Data were extracted from the ARIA oncology information system and were studied as a function of the treatment site, the delivery modality, and the time from the first day of treatments with the Halcyon 2.0 system in our institution. RESULTS: A total of 8599 fractions were delivered during the analyzed period (69.5% from VMAT plans, and 30.5% from IMRT plans). The number of fractions by site ranged from 30 for anal canal to 1933 for prostate. Five sites (prostate, lung, pelvis with prostate, breast, and gynecological sites) accounted for the 84% of the fractions. After a 2-week adaptation period of the staff, the daily mean treatment time was reduced to less than 12 min. The mean treatment time of all the fractions amounted to 10.5 ± 3.8 min. CONCLUSIONS: The Halcyon 2.0 allowed delivering online image-guided radiation therapy in all fractions with total treatment time consistently below the 12-min standard time slot, for most of the analyzed treatment sites.

Monte Carlo-based independent dose verification of radiosurgery HyperArc plans.

PURPOSE: To investigate the feasibility of using the free PRIMO Monte Carlo software for independent dose check of cranial SRS plans designed with the Varian HyperArc (HA) technique. MATERIALS AND METHODS: In this study, the PRIMO Monte Carlo software v. 0.3.64.1800 was used with the phase-space files (v. 2, Feb. 27, 2013) provided by Varian for 6 MV flattening-filter-free (FFF) photon beams from a Varian TrueBeam linear accelerator (linac), equipped with a Millennium 120 multileaf collimator (MLC). This configuration was validated by comparing the percentage depth doses (PDDs), lateral profiles and relative output factors (OFs) simulated in a water phantom against measurements for field sizes from 1 × 1 to 40 × 40 cm2. The agreement between simulated and experimental relative dose curves was evaluated using a global (G) gamma index analysis. In addition, the accuracy of PRIMO to model the MLC was investigated (dosimetric leaf gap, tongue and groove, leaf transmission and interleaf leakage). Thirty-five HA SRS plans computed in the Eclipse treatment planning system (TPS) were simulated in PRIMO. The Acuros XB algorithm v. 16.10 (dose to medium) was used in Eclipse. Sixty targets with diameters ranging from 6 to 33 mm were included. Agreement between the dose distributions given by Eclipse and PRIMO was evaluated in terms of 3D global gamma passing rates (GPRs) for the 2 %/2 mm criteria. RESULTS: Average GPR greater than 95 % with the 2 %(G)/1 mm criteria were obtained over the PDD and profiles of each field size. Differences between PRIMO calculated and measured OFs were within 0.5 % in all fields, except for the 1 × 1 cm2 with a discrepancy of 1.5 %. Regarding the MLC modeling in PRIMO, an agreement within 3 % was achieved between calculated and experimental doses. Excellent agreement between PRIMO and Eclipse was found for the 35 HA plans. The 3D global GPRs (2 %/2 mm) for the targets and external patient contour were 99.6 % ± 1.1 % and 99.8 % ± 0.5 %, respectively. CONCLUSIONS: According to the results described in this study, the PRIMO Monte Carlo software, in conjunction with the 6X FFF Varian phase-space files, can be used as secondary dose calculation software to check stereotactic radiosurgery plans from Eclipse using the HyperArc technique.

Clinical Experience in Prostate Ultrahypofractionated Radiation Therapy With an Online Adaptive Method.

PURPOSE: This study aimed to describe the feasibility of the online adaptive radiation therapy (oART) method developed at the Hospital Quirónsalud Barcelona for prostate cancer, using a standard C-arm linear accelerator (linac) and without the support of artificial intelligence. METHODS AND MATERIALS: The first 18 patients treated at the Hospital Quirónsalud Barcelona with the developed oART method were included. An ultrahypofractionated radiation therapy scheme consisting of 7 × 6.1 Gy was used. Patients were treated on 2 conventional Varian C-arm linacs. For each patient, a reference plan based on a planning computed tomography (pCT) scan was generated using the Eclipse system. On each treatment session, the pCT scan was rigidly registered with the daily cone beam computed tomography (CT) scan. The pCT-based target (prostate) and organs at risk were mapped onto the cone beam CT images and manually adapted to take into account the anatomy of the day. The reference plan was then copied to the cone beam CT scan, and a full reoptimization was done for the current anatomy (adapted plan). For each treatment session, the unaltered reference plan was recomputed on the daily cone beam CT scan by mimicking the soft-tissue alignment performed per our standard procedure (nonadapted plan). Over the 126 adapted sessions from the 18 patients, a dosimetric comparison of adapted against nonadapted plans was done. RESULTS: A significant difference in the target coverage was found between the adapted and nonadapted plans (97.1 vs 90.4; P < .001) in favor of adapting. Without online adaptation, the optimal coverage of the prostate was not attained in 35% of fractions. Adapting allows for the improvement of the target coverage with compliance of all organ-at-risk dose constraints in all treatment fractions. CONCLUSIONS: The oART technique described in this study is technically feasible with a C-arm linac. To our knowledge, this is the first clinical experience with oART for prostate cancer including full replanning and delivered with a C-arm linac without artificial intelligence capability.

Validation of virtual water phantom software for pre-treatment verification of single-isocenter multiple-target stereotactic radiosurgery.

The aim of this study was to benchmark the accuracy of the VIrtual Phantom Epid dose Reconstruction (VIPER) software for pre-treatment dosimetric verification of multiple-target stereotactic radiosurgery (SRS). VIPER is an EPID-based method to reconstruct a 3D dose distribution in a virtual phantom from in-air portal images. Validation of the VIPER dose calculation was assessed using several MLC-defined fields for a 6 MV photon beam. Central axis percent depth doses (PDDs) and output factors were measured with an ionization chamber in a water tank, while dose planes at a depth of 10 cm in a solid flat phantom were acquired with radiochromic films. The accuracy of VIPER for multiple-target SRS plan verification was benchmarked against Monte Carlo simulations. Eighteen multiple-target SRS plans designed with the Eclipse treatment planning system were mapped to a cylindrical water phantom. For each plan, the 3D dose distribution reconstructed by VIPER within the phantom was compared with the Monte Carlo simulation, using a 3D gamma analysis. Dose differences (VIPER vs. measurements) generally within 2% were found for the MLC-defined fields, while film dosimetry revealed gamma passing rates (GPRs) ≥95% for a 3%/1 mm criteria. For the 18 multiple-target SRS plans, average 3D GPRs greater than 93% and 98% for the 3%/2 mm and 5%/2 mm criteria, respectively. Our results validate the use of VIPER as a dosimetric verification tool for pre-treatment QA of single-isocenter multiple-target SRS plans. The method requires no setup time on the linac and results in an accurate 3D characterization of the delivered dose.

Monte Carlo simulation of conical collimators for stereotactic radiosurgery with a 6 MV flattening-filter-free photon beam.

PURPOSE: Conical collimators, or cones, are tertiary collimators that attach to a radiotherapy linac and are suited for the stereotactic radiosurgery treatment of small brain lesions. The small diameter of the most used cones makes difficult the acquisition of the dosimetry data needed for the commissioning of treatment planning systems. Although many publications report dosimetric data of conical collimators for stereotactic radiosurgery, most of the works use different setups, which complicates comparisons. In other cases, the cone output factors reported do not take into account the effect of the small cone diameter on the detector response. Finally, few data exist on the dosimetry of cones with flattening-filter-free (FFF) beams from modern linac models. This work aims at obtaining a dosimetric characterization of the conical collimators manufactured by Brainlab AG (Munich, Germany) in a 6 MV FFF beam from a TrueBeam STx linac (Varian Medical Systems). METHODS: Percentage depth dose curves, lateral dose profiles and cone output factors were obtained using Monte Carlo simulations for the cones with diameters of 4, 5, 6, 7.5, 8, 10, 12.5, 15, 17.5, 20, 25, and 30 mm. The simulation of the linac head was carried out with the PRIMO Monte Carlo software, and the simulations of the cones and the water phantom were run with the general-purpose Monte Carlo code PENELOPE. The Monte Carlo model was validated by comparing the simulation results with measurements performed for the cones of 4, 5, and 7.5 mm of diameter using a stereotactic field diode, a microDiamond detector and EBT3 radiochromic film. In addition, for those cones, simulations and measurements were done for comparison purposes, by reproducing the experimental setups from the available publications. RESULTS: The experimental data acquired for the cones of 4, 5, and 7.5 mm validated the developed Monte Carlo model. The simulations accurately reproduced the experimental depths of maximum dose and the dose ratio at 20- and 10-cm depth (PDD20/10 ). A good agreement was obtained between simulated and experimental lateral dose profiles: The differences in the full-width at half-maximum were smaller than 0.2 mm, and the differences in the penumbra 80%-20% were smaller than 0.25 mm. The difference between the simulated and the average of the experimental output factors for the cones of 4, 5, and 7.5 mm of diameter was 0.0%, 0.0%, and 3.0%, respectively, well within the statistical uncertainty of the simulations (4.4% with coverage factor k = 2). It was also found that the simulated cone output factors agreed within 2% with the average of output factors reported in the literature for a variety of setup conditions, detectors, beam qualities, and cone manufacturers. CONCLUSION: A Monte Carlo model of cones for stereotactic radiosurgery has been developed and validated. The cone dosimetry dataset obtained in this work, consisting of percentage depth doses, lateral dose profiles and output factors, is useful to benchmark data acquired for the commissioning of cone-based radiosurgery treatment planning systems.

PRIMO Monte Carlo software benchmarked against a reference dosimetry dataset for 6 MV photon beams from Varian linacs.

BACKGROUND: The software PRIMO for the Monte Carlo simulation of radiotherapy linacs could potentially act as a independent calculation system to verify the calculations of treatment planning systems. We investigated the suitability of the PRIMO default beam parameters to produce accurate dosimetric results for 6 MV photon beams from Varian Clinac 2100 linacs and 6 MV flattening-filter-free photon beams from Varian TrueBeam linacs. METHODS: Simulation results with the DPM algorithm were benchmarked against a published reference dosimetry dataset based on point measurements of 25 dosimetric parameters on a large series of linacs. Studied parameters (for several field sizes and depths) were: PDD, off-axis ratios, and output factors for open fields and IMRT/SBRT-style fields. For the latter, the output factors were also determined with radiochromic film and with a small-sized ionization chamber. Benchmark data, PRIMO simulation results and our experimental results were compared. RESULTS: PDD, off-axis ratios, and open-field output factors obtained from the simulations with the PRIMO default beam parameters agreed with the benchmark data within 2.4% for Clinac 2100, and within 1.3% for TrueBeam. Higher differences were found for IMRT/SBRT-style output factors: up to 2.8% for Clinac 2100, and up to 3.3% for TrueBeam. Experimental output factors agreed with benchmark data within 1.0% (ionization chamber) and within 1.9% (radiochromic film). CONCLUSIONS: PRIMO default initial beam parameters for 6 MV photon beams from Varian Clinac 2100 linacs and 6 MV FFF photon beams from Varian TrueBeam linacs allowed agreement within 3.3% with a dosimetry database based on measurements of a high number of linacs. This finding represents a first step in the validation of PRIMO for the independent verification of radiotherapy plans.

Assessment of the Monitor Unit Objective tool for VMAT in the Eclipse treatment planning system.

AIM: This work aims to achieve the highest possible monitor units (MU) reduction using the MU Objective tool included in the Eclipse treatment planning system, while preserving the plan quality. BACKGROUND: The treatment planning system Eclipse (Varian Medical Systems, Palo Alto, CA) includes a control mechanism for the number of monitor units of volumetric modulated arc therapy (VMAT) plans, named the MU Objective tool. MATERIAL AND METHODS: Forty prostate plans, 20 gynecological plans and 20 head and neck plans designed with VMAT were retrospectively studied. Each plan (base plan) was optimized without using the MU Objective tool, and it was re-optimized with different values of the Maximum MU (MaxMU) parameter of the MU Objective tool. MU differences were analyzed with a paired samples t-test and changes in plan quality were assessed with a set of parameters for OARs and PTVs. RESULTS: The average relative MU difference [Formula: see text] considering all treatment sites, was the highest when MaxMU = 400 (-4.2%, p < 0.001). For prostate plans, the lowest [Formula: see text] was obtained (-3.7%, p < 0.001). For head and neck plans [Formula: see text] was -7.3% (p < 0.001) and for gynecological plans [Formula: see text] was 7.0% (p = 0.002). Although similar MU reductions were observed for both sites, for some gynecological plans maximum differences were greater than 10%. All the assessed parameters for PTVs and OARs sparing showed average differences below 2%. CONCLUSION: For the three studied clinical sites, establishing MaxMU = 400 led to the optimum MU reduction, maintaining the original dose distribution and dosimetric parameters practically unaltered.

Absorbed dose distributions from ophthalmic 106 Ru/106 Rh plaques measured in water with radiochromic film.

PURPOSE: Brachytherapy with 106 Ru/106 Rh plaques offers good outcomes for small-to-medium choroidal melanomas and retinoblastomas. The dose measurement of the plaques is challenging, due to the small range of the emitted beta particles and steep dose gradients involved. The scarce publications on film dosimetry of 106 Ru/106 Rh plaques used solid phantoms. This work aims to develop a practical method for measuring the absorbed dose distribution in water produced by 106 Ru/106 Rh plaques using EBT3 radiochromic film. METHODS: Experimental setups were developed to determine the dose distribution at a plane perpendicular to the symmetry axis of the plaque and at a plane containing the symmetry axis. One CCA and two CCX plaques were studied. The dose maps were obtained with the FilmQA Pro 2015 software, using the triple-channel dosimetry method. The measured dose distributions were compared to published Monte Carlo simulation and experimental data. RESULTS: A good agreement was found between measurements and simulations, improving upon published data. Measured reference dose rates agreed within the experimental uncertainty with data obtained by the manufacturer using a scintillation detector, with typical differences below 5%. The attained experimental uncertainty was 4.1% (k = 1) for the perpendicular setup, and 7.9% (k = 1) for the parallel setup. These values are similar or smaller than those obtained by the manufacturer and other authors, without the need of solid phantoms that are not available to most users. CONCLUSIONS: The proposed method may be useful to the users to perform quality assurance preclinical tests of 106 Ru/106 Rh plaques.

Technical Note: Monte Carlo study of 106 Ru/106 Rh ophthalmic plaques including the 106 Rh gamma spectrum.

PURPOSE: To assess the influence of the 106 Rh gamma spectrum on the Monte Carlo simulation of 106 Ru/106 Rh ophthalmic plaques, which has been neglected without a quantitative estimation in all previous publications. METHODS: Simulations were run with the penelope 2014 Monte Carlo code for radiation transport. Depth-dose distributions in water were simulated for the plaque models CCA, CCC, CCX and CIA. In addition to the 106 Rh beta spectrum, all gamma components from the 106 Rh gamma spectrum were included in the simulations. Depth-dose curves were compared with those obtained without considering the 106 Rh gamma spectrum. Moreover, half-value (HVL) and tenth-value layers (TVL) were estimated for the 106 Rh gamma spectrum in water, PMMA, stainless steel and lead. Some practical radiation protection applications were discussed. Parallel computing was implemented to reduce computing time. RESULTS: The contribution of the 106 Rh gamma spectrum on the depth-dose curves is negligible at depths of clinical interest. The HVL and TVL of the 106 Rh gamma spectrum were found to be similar to those of 137 Cs. The air-kerma rate at 1 m for a CCA plaque in typical clinical conditions was about 0.4µGym2h-1, resulting in equivalent doses at that point elow 0.05 mSv during a treatment. The air-kerma rate would be underestimated by a factor of 5 if the 106 Rh gamma spectrum were not considered. Also, a freely available software tool was developed to ease parallelization of penelope 2014 simulations that use penmain as steering main program. CONCLUSIONS: The influence of the 106 Rh gamma spectrum is not relevant for clinical purposes, thus validating the common assumption from the literature. However, for simulations at large distances from the plaques, such as for radiation shielding assessment and estimation of dose to personnel, the gamma spectrum from 106 Rh must be taken into account to obtain accurate results.

Investigating the spatial accuracy of CBCT-guided cranial radiosurgery: A phantom end-to-end test study.

PURPOSE: To evaluate the spatial accuracy of a frameless cone-beam computed tomography (CBCT)-guided cranial radiosurgery (SRS) using an end-to-end (E2E) phantom test methodology. METHODS AND MATERIALS: Five clinical SRS plans were mapped to an acrylic phantom containing a radiochromic film. The resulting phantom-based plans (E2E plans) were delivered four times. The phantom was setup on the treatment table with intentional misalignments, and CBCT-imaging was used to align it prior to E2E plan delivery. Comparisons (global gamma analysis) of the planned and delivered dose to the film were performed using a commercial triple-channel film dosimetry software. The necessary distance-to-agreement to achieve a 95% (DTA95) gamma passing rate for a fixed 3% dose difference provided an estimate of the spatial accuracy of CBCT-guided SRS. Systematic (∑) and random (σ) error components, as well as 95% confidence levels were derived for the DTA95 metric. RESULTS: The overall systematic spatial accuracy averaged over all tests was 1.4mm (SD: 0.2mm), with a corresponding 95% confidence level of 1.8mm. The systematic (Σ) and random (σ) spatial components of the accuracy derived from the E2E tests were 0.2mm and 0.8mm, respectively. CONCLUSIONS: The E2E methodology used in this study allowed an estimation of the spatial accuracy of our CBCT-guided SRS procedure. Subsequently, a PTV margin of 2.0mm is currently used in our department.

Experimental HPGe coaxial detector response and efficiency compared to Monte Carlo simulations.

The peak efficiency for photons hitting the frontal surface of a medium volume n-type HPGe coaxial detector is mapped using acutely collimated beams of energies between 31 and 383 keV from a (133)Ba radioactive source. Simulated values obtained with the Monte Carlo radiation transport code penelope, using a model that respected actual detector dimensions and physical constants while varying dead-layer thicknesses, allowed us to fit the experimental results in the detector bulk but not near its rim. The spectra of a (137)Cs source were measured using the detector shielded from the natural background radiation, with and without a broad angle collimator. The corresponding simulated spectra, using the fitted dead-layer thicknesses, underestimate the continuum component of the spectra and overestimate the peak efficiency, by less than ten percent in the broad angle collimator arrangement. The simulated results are sensitive to the photon attenuation coefficients.