Dose Grid Research Articles

Clinical impact of DVH uncertainties

Dose-volume histograms (DVH), along with dose and volume metrics, are central to radiotherapy planning. As such, errors have the potential to significantly impact the selection of appropriate treatment plans. Dose distributions that pass tests in one TPS may fail the same tests when transferred to another, even if using identical structures and dose grid information. This work shows the design and implementation of methods for assessing the accuracy of dose and volume computations performed by treatment planning systems (TPS), and other analytical tools. We demonstrate examples where differences in calculations between systems can change the assessment of a plan’s clinical acceptability. Our work also provides a more detailed DVH analysis of single targets than earlier published studies. This is relevant for SRS plans and small structure dose assessments. Very small structures are a particular problem because of their coarse digital representation, and the impact of this is thoroughly examined.Reference DVH curves were derived mathematically, based on Gaussian dose distributions centered on spherical structures. The structures and dose distributions were generated synthetically, and imported into RayStation, MasterPlan, and ProKnow. Corresponding DVHs were analytically derived and taken as ground truth references, for comparison with the commercial DVH calculations. Two commonly used dose metrics PCI and MGI were used to determine the limit of calculation accuracy for small structures. In addition, to measure the DVH differences between a larger range of commercial DVH calculators, the D95 metric from a set of real clinical plans was compared across both the 3 DVH calculators under test, and across a further six TPSs from other hospitals.We show that even slight deviations between the results of DVH calculators can lead to plan check failures, and we illustrate this with the commonly used D95 planning metric. We present clinical data across eight planning systems that highlight instances where plan checks would pass in one software and fail in another due to DVH calculation differences. For the smallest volumes tested, errors of up to 20% were observed in the DVHs. RayStation was tested down to a 3 mm radius sphere (≈0.1 cc) and this showed close to 10% error, reducing to 1% for 10 mm radius (≈4.0 cc) and 0.1% for 20 mm radius (≈33 cc). In clinical plans, the variation in D95 was up to 9% for the smallest volumes, and typically around 2% in the range 0.5 cc-20 cc, and 1% in 20 cc-70 cc, falling to <0.1% for large volumes. Paddick Conformity Index (PCI) and Modified Gradient Index (MGI) are commonly used plan quality indicators for very small volumes. For volumes ≈0.1 cc we observed errors of up to 40% in PCI, and up to 75% in MGI.Our study extends the range of tested DVH calculators in published work, and shows their performance over a wider range of volume sizes. We provide quantitative evidence of the critical need to test the accuracy of DVH calculators in the TPS before clinical use. This work is particularly relevant for both stereotactic plan evaluation and for assessment of small volume doses in published dose constraint recommendations. We demonstrate that significant errors can occur in DVHs for volumes less than 1 cc, even if the volumes themselves are calculated accurately. Even for large structures, deviations between the outputs of DVH calculators can lead to indicated or reported plan check failures if they do not include appropriate tolerances. We urge caution in the use of DVH metrics for these very small volumes and recommend that appropriate DVH uncertainty tolerances are set in organ dose constraints when using them to evaluate clinical plans.

Dosimetric effects of small field size, dose grid size, and variable split-arc methods on gamma pass rates in radiation therapy.

This study investigates the influence of calculation accuracy in peripheral low-dose regions on the gamma pass rate (GPR), utilizing the Acuros XB (AXB) algorithm and ArcCHECK™ measurement. The effects of varying small field sizes, dose grid sizes, and split-arc techniques on GPR were analyzed. Various small field sizes were employed. Thirty-two single-arc plans with dose grid sizes of 2mm and 1mm and prescribed doses of 2, 5, 10, and 20Gy were calculated using the AXB algorithm. In total, 128 GPR plans were examined. These plans were categorized into three sub-fields (3SF), four sub-fields (4SF), and six sub-fields (6SF). The GPR results deteriorated with smaller target sizes and a 2mm dose grid size in a single arc. A similar degradation in GPR was observed with smaller target sizes and a 1mm dose grid size. However, the 1mm dose grid size generally resulted in better GPR compared with the 2mm dose grid size for the same target sizes. The GPR improved with finer split angles and a 2mm dose grid size in the split-arc method. However, no statistically significant improvement was observed with finer split angles and a 1mm dose grid size. This study demonstrates that coarser dose grid sizes result in lower GPRs in peripheral low-dose regions as calculated by AXB with ArcCHECK™ measurement. To enhance GPR, employing split-arc methods and finer dose grid sizes could be beneficial.

Practical experience with initial quality assurance of a high dose rate brachytherapy grid-based Boltzmann solver algorithm.

The purpose of this study was to validate the use of a model-based dose calculation algorithm (MBDCA), Acuros BV, for high dose rate brachytherapy treatment planning for a community-based hospital with a Bravos afterloader. Based on published AAPM recommendations, this work details a practical approach for community-based clinics to complete initial validation of Acuros BV, in order to add a MBDCA to a TG-43 based brachytherapy treatment planning program. Source dimensions and materials used in Acuros BV and TG-43 source models were compared to the physical source. TG-186 testing was completed with standardized test cases externally calculated with Monte Carlo compared to locally calculated with Acuros BV. Point doses calculated using TG-43 were compared to those calculated with Acuros BV in water at various dose grid settings. Secondary dose check software was used to evaluate dose distributions resembling clinical patient plans, both in water and on CT datasets representative of patient anatomy. The major source of discrepancy of source models was the length of modeled steel cable. TG-186 testing showed that the largest differences between Monte Carlo and Acuros BV dose distributions were located along the source axis for cases calculated in water, as well as located in regions of high dose gradients and within the applicator for the case calculated with a generic shielded applicator. An audit of point doses calculated with both TG-43 and Acuros BV in water found that dose grid settings significantly affected agreement. Secondary dose check software indicated that Acuros BV functioned satisfactorily, and a 5% threshold was adopted for secondary dose checks on gynecologic plans. This validation process indicated that Acuros BV met expected standards and affirmed its suitability for integration into this clinical practice's brachytherapy treatment planning.

Dummy run for planning of isotoxic dose-escalated radiation therapy for glioblastoma used in the PRIDE trial (NOA-28; ARO-2024-01; AG-NRO-06)

BackgroundThe PRIDE trial (NOA-28; ARO-2024-01; AG-NRO-06; NCT05871021) is designed to determine whether a dose escalation with 75.0 Gy in 30 fractions can enhance the median overall survival (OS) in patients with methylguanine methyltransferase (MGMT) promotor unmethylated glioblastoma compared to historical median OS rates, while being isotoxic to historical cohorts through the addition of concurrent bevacizumab (BEV). To ensure protocol-compliant irradiation planning with all study centers, a dummy run was planned and the plan quality was evaluated. MethodsA suitable patient case was selected and the computed tomography (CT), magnetic resonance imaging (MRI) and O-(2-[18F]fluoroethyl)-L-tyrosine (FET) positron emission tomography (PET) contours were made available. Participants at the various intended study sites performed radiation planning according to the PRIDE clinical trial protocol. The treatment plans and dose grids were uploaded as Digital Imaging and Communications in Medicine (DICOM) files to a cloud-based platform. Plan quality and protocol adherence were analyzed using a standardized checklist, scorecards and indices such as Dice Score (DSC) and Hausdorff Distance (HD). ResultsMedian DSC was 0.89, 0.90, 0.88 for PTV60, PTV60ex (planning target volume receiving 60.0 Gy for the standard and the experimental plan, respectively) and PTV75 (PTV receiving 75.0 Gy in the experimental plan), respectively. Median HD values were 17.0 mm, 13.9 mm and 12.1 mm, respectively. These differences were also evident in the volumes: The PTV60 had a volume range of 219.1–391.3 cc (median: 261.9 cc) for the standard plans, while the PTV75 volumes for the experimental plans ranged from 71.5–142.7 cc (median: 92.3 cc). The structures with the largest deviations in Dice score were the pituitary gland (median 0.37, range 0.00–0.69) and the right lacrimal gland (median 0.59, range 0.42–0.78). ConclusionsThe deviations revealed the necessity of systematic trainings with appropriate feedback before the start of clinical trials in radiation oncology and the constant monitoring of protocol compliance throw-out the study. Trial registrationNCT05871021

Deep learning for high-resolution dose prediction in high dose rate brachytherapy for breast cancer treatment

Objective. Monte Carlo (MC) simulations are the benchmark for accurate radiotherapy dose calculations, notably in patient-specific high dose rate brachytherapy (HDR BT), in cases where considering tissue heterogeneities is critical. However, the lengthy computational time limits the practical application of MC simulations. Prior research used deep learning (DL) for dose prediction as an alternative to MC simulations. While accurate dose predictions akin to MC were attained, graphics processing unit limitations constrained these predictions to large voxels of 3 mm × 3 mm × 3 mm. This study aimed to enable dose predictions as accurate as MC simulations in 1 mm × 1 mm × 1 mm voxels within a clinically acceptable timeframe. Approach. Computed tomography scans of 98 breast cancer patients treated with Iridium-192-based HDR BT were used: 70 for training, 14 for validation, and 14 for testing. A new cropping strategy based on the distance to the seed was devised to reduce the volume size, enabling efficient training of 3D DL models using 1 mm × 1 mm × 1 mm dose grids. Additionally, novel DL architecture with layer-level fusion were proposed to predict MC simulated dose to medium-in-medium (D m,m ). These architectures fuse information from TG-43 dose to water-in-water (D w,w ) with patient tissue composition at the layer-level. Different inputs describing patient body composition were investigated. Main results. The proposed approach demonstrated state-of-the-art performance, on par with the MC D m,m maps, but 300 times faster. The mean absolute percent error for dosimetric indices between the MC and DL-predicted complete treatment plans was 0.17% ± 0.15% for the planning target volume V 100, 0.30% ± 0.32% for the skin D 2cc , 0.82% ± 0.79% for the lung D 2cc , 0.34% ± 0.29% for the chest wall D 2cc and 1.08% ± 0.98% for the heart D 2cc . Significance. Unlike the time-consuming MC simulations, the proposed novel strategy efficiently converts TG-43 D w,w maps into precise D m,m maps at high resolution, enabling clinical integration.

Electron streaming dose measurements and calculations on a 1.5 T MR-Linac.

To evaluate the accuracy of different dosimeters and the treatment planning system (TPS) for assessing the skin dose due to the electron streaming effect (ESE) on a 1.5 T magnetic resonance (MR)-linac. Skin dose due to the ESE on an MR-linac (Unity, Elekta) was investigated using a solid water phantom rotated 45° in the x-y plane (IEC61217) and centered at the isocenter. The phantom was irradiated with 1 × 1, 3 × 3, 5 × 5, 10 × 10, and 22 × 22cm2 fields, gantry at 90°. Out-of-field doses (OFDs) deposited by electron streams generated at the entry and exit surface of the angled phantom were measured on the surface of solid water slabs placed±20.0cm from the isocenter along the x-direction. A high-resolution MOSkin™ detector served as a benchmark due to its shallower depth of measurement that matches the International Commission on Radiological Protection (ICRP) recommended depth for skin dose assessment (0.07mm). MOSkin™ doses were compared to EBT3 film, OSLDs, a diamond detector, and the TPS where the experimental setup was modeled using two separate calculation parameters settings: a 0.1cm dose grid with 0.2% statistical uncertainty (0.1cm, 0.2%) and a 0.2cm dose grid with 3.0% statistical uncertainty (0.2cm, 3.0%). OSLD, film, the 0.1cm, 0.2%, and 0.2cm, 3.0% TPS ESE doses, underestimated skin doses measured by the MOSkin™ by as much as -75.3%, -7.0%, -24.7%, and -41.9%, respectively. Film results were most similar to MOSkin™ skin dose measurements. These results show that electron streams can deposit significant doses outside the primary field and that dosimeter choice and TPS calculation settings greatly influence the reported readings. Due to the steep dose gradient of the ESE, EBT3 film remains the choice for accurate skin dose assessment in this challenging environment.

A physically constrained Monte Carlo-Neural Network coupling algorithm for BNCT dose calculation.

In boron neutron capture therapy (BNCT)-a form of binary radiotherapy-the primary challenge in treatment planning systems for dose calculations arises from the time-consuming nature of the Monte Carlo (MC) method. Recent progress, including the use of neural networks (NN), has been made to accelerate BNCT dose calculations. However, this approach may result in significant dose errors in both the tumor and the skin, with the latter being a critical organ in BNCT. Furthermore, owing to the lack of physical processes in purely NN-based approaches, their reliability for clinical dose calculations in BNCT is questionable. In this study, a physically constrained MC-NN (PCMC-NN) coupling algorithm is proposed to achieve fast and accurate computation of the BNCT three-dimensional (3D) therapeutic dose distribution. This approach synergizes the high precision of the MC method with the speed of the NN and utilizes physical conservation laws to constrain the coupling process. It addresses the time-consuming issue of the traditional MC method while reducing dose errors. Clinical data were collected from 113 glioblastoma patients. For each patient, the 3D dose distributions for both the coarse and detailed dose grids were calculated using the MC code PHITS. Among these patients, the data from 14 patients were allocated to the test set, 9 to the validation set, and the remaining to the training set. A neural network, 3D-Unet, was built based on the coarse grid dose and patient CT information to enable fast and accurate computation of the 3D detailed grid dose distribution of BNCT. Statistical evaluations, including relative deviation, dose deviation, mean absolute error (MAE), and mean absolute percentage error (MAPE) were conducted. Our findings suggested that the PCMC-NN algorithm substantially outperformed the traditional NN and interpolation methods. Furthermore, the proposed algorithm significantly reduced errors, particularly in the skin and GTV, and improved computational accuracy (hereinafter referred to simply as 'accuracy') with a MAPE range of 1.6%-4.0% and a maximum MAE of 0.3Gy (IsoE) for different organs. The dose-volume histograms generated by the PCMC-NN aligned well with those obtained from the MC method, further validating its accuracy. The PCMC-NN algorithm enhanced the speed and accuracy of BNCT dose calculations by combining the MC method with the NN algorithm. This indicates the significant potential of the proposed algorithm for clinical applications in optimizing treatment planning.

Evaluation of Patient-Specific Quality Assurance in Single Isocenter Multitarget Stereotactic Radiosurgery of Brain Metastases Using Octavius 4D for Flattening Filter Free Beams.

To evaluate the patient-specific quality assurance (PSQA) in Single Isocenter Multitarget (SIMT) Stereotactic Radiosurgery (SRS) of brain metastases using Octavius 4D for flattening filter free (FFF) X-ray beams. Octavius 4D with 1000 SRS detector array was used for doing PSQA in fifteen patients planned using volumetric modulated arc therapy (VMAT) based stereotactic radiosurgery (SRS) of SIMT technique for treating multiple brain metastases. Measurements were made both in standard setup as well as in special setup through which the region of measurement is increased. The obtained results are compared with TPS reference doses at different dose grid sizes and minimum dose cutoff thresholds for both 6X-FFF and 10X-FFF energies. The percentages of measurable targets were 83.34% and 93.54% in standard and special set respectively. In standard set, the average gamma passing rate was 97.63±1.26% and 97.78±1.09% for 6X-FFF and 10X-FFF respectively. The mean gamma passing rates was 97.97±0.99% and 97.85±1.13% for 6X-FFF and 10X-FFF in special set respectively. PSQA passing rate decreases with increasing TPS grid size in both the energies. The mean gamma passing rate for 6X-FFF was 97.97±0.99% and 96.94±1.11% at 10% and 25% cutoff dose respectively. The mean gamma passing rate for 10X-FFF at 10% and 25% dose cutoff was 97.85±1.13% and 97.12±1.29% respectively. Low-dose masking effect was more in 6X-FFF than observed with 10X-FFF energy. The performance of Octavius 1000 SRS for PSQA of VMAT based SRS were evaluated and found satisfactory at 6X-FFF and 10X-FFF energies in SIMT of brain metastases.

Multicenter Multivendor Evaluation of Dose Volume Histogram Creation Consistencies for 8 Commercial Radiation Therapy Dosimetric Systems

PurposeTo evaluate dose volume histogram (DVH) construction differences across eight major commercial treatment planning systems (TPSs) and dose reporting systems (DRSs) for clinically treated plans of various anatomical sites and target sizes. Materials and MethodsDose files from ten selected clinically treated plans with a hypo-fractionation, stereotactic radiotherapy prescription or sharp dose gradients such as head and neck plans ranging from prescription doses of 18 Gy in 1 fraction to 70 Gy in 35 fractions, each calculated at 0.25 cm and 0.125 cm grid size, were created and anonymized in Eclipse TPS, and exported to seven other major TPS (Pinnacle, RayStation, Elements) and DRS (MIM, Mobius, ProKnow, and Velocity) systems for comparison. Dose-volume constraint points of clinical importance for each plan were collected from each evaluated system (D0.03cc[Gy], volume, and the mean dose were used for structures without specified constraints). Each reported constraint type and structure volume was normalized to the value from Eclipse for a pairwise comparison. A Wilcoxon Rank Sum test was used for statistical significance and a multivariable regression model was evaluated adjusting for plan, grid size, and distance to target center. ResultsFor all DVH points relative to Eclipse, all systems reported median values within 1.0% difference of each other, however they were all different from Eclipse. Considering mean values, Pinnacle, RayStation, and Elements averaged at 1.038, 1.046, 1.024 respectively, while MIM, Mobius, ProKnow, and Velocity reported 1.026, 1.050, 1.033 and 1.022 respectively relative to Eclipse. Smaller dose grid size improved agreement between the systems marginally without statistical significance. For structure volumes relative to Eclipse, larger differences are seen across all systems with a range in median values up to 3.0% difference and mean up to 10.1% difference. ConclusionLarge variations were observed between all systems. Eclipse generally reported, at statistically significant levels, lower values than all other evaluated systems. The non-significant change resulting from lowering the dose grid resolution indicates that this resolution may be less important than other aspects of calculating DVH curves, such as the 3D modeling of the structure.

Assessment of Dynamic Contrast Enhanced (DCE) MRI for Detection of Radiotherapy Induced Alteration in Mandibular Vasculature

We aim to determine the kinetics of DCE-MRI changes in various mandibular risk volumes based on radiation (RT) dose received. Eighty-eight head and neck cancer (HNC) patients (Pts) who underwent definitive RT were enrolled in this prospective study after IRB approval and informed consent. Images were acquired at pre-RT (Baseline), 3 weeks after RT start date (Mid-RT), 3 mos post-RT (PostRT1), and 6 mos post-RT (PostRT2). Manually segmented mandibular volumes on T2-weighted images were propagated to co-registered DCE-MRIs. Planning CTs and dose grids were also co-registered to corresponding baseline T2 images to create 3-D dose subvolumes. These were used to create 3 risk subvolumes; <30 Gy, 30-50 Gy, and >50 Gy ROIs. DCE images of different timepoints (TPs) were deformably co-registered and the dose subvolumes were propagated to each TP. We used the extended-Tofts model to generate the vascular quantitative maps (Ktrans and Ve). Each subvolume histogram parameters were extracted at each TP. Wilcoxon Signed Rank test was used to compare the changes at different TPs compared to baseline. We classified Pts' delta parameters at different TPs -based on our prior extensive QA assessment- into Pts with stable vascular profile (±25% change), Pts with significant increase (>25% change) and Pts with significant decrease (<-25%). Chi-square test was used to assess the change at different TPs. For <30 Gy subvolumes, there were no significant changes (p > 0.05) in the studied DCE parameters at all TPs except a significant decrease (p < 0.001) in median Ktrans at PostRT2. For 30-50 Gy subvolumes, there was a significant increase in median Ktrans that started at MidRT (p = 0.006) and continued at PostRT1 (p = 0.04) but recovered to baseline values at PostRT2. Median Ve on the other hand only showed significant increase at PostRT1 (p = 0.001), but other TPs were not significantly different compared to baseline. Similarly, subvolumes >50 Gy showed same kinetics as in 30-50 Gy with significant increase of Ktrans at MidRT and PostRT1 and significant increase in Ve in only PostRT1 (P <0.05). For <30 Gy, there was significant increase in the number of Pts with stable or decrease in Ktrans at PostRT2 compared to earlier TPs (70% vs. 60% at PostRT1 and 54% at MidRT p = 0.003). 30-50 Gy subvolumes showed similar profile like <30 Gy with significant increase in the percentage of Pts with recovery at PostRT2. However, for >50 Gy, there was no significant increase in the number of Pts who recovered at PostRT2 (p = 0.3). Ve showed no significant increase in the percentage of Pts with recovery at different TPs (p > 0.05). Results showed that for all dose mandibular subvolumes, there is an acute vascular insult that tends to recover at +6 months post-RT except for a selective group of patients who continue to have persistence of the vascular insult at high dose subvolumes. These findings are of importance for future selection of high risk population for prophylactic intervention against osteoradionecrosis.

Accuracy of Electron Density and Planning Dosimetry in a Novel High-Quality CBCT Imaging System

A high-quality Cone-Beam Computed Tomography (CBCT) imaging system has been FDA approved for imaging guidance and dose calculation in radiotherapy. This study aims to evaluate the accuracy of the relative electron density in CBCT images acquired in this CBCT imaging system in a phantom study and its dosimetric impact on treatment planning in a patient study. Astoichiometric CT calibration was performed with a CIRS phantom (SunNuclear, Model 062M) to generate the HU-electron density curve for two tube voltages, 125kVp and 140 kVp, respectively. The phantom has a longitudinal length of 26.5 cm and is equipped with interchangeable inserts of various compositions, supplied by the vendor. Measurements were taken with solid water plates added to both ends of the phantom to allow adequate scattering and repeated for various clinical protocols with different combinations of tube voltages and exposures. The accuracy of the relative electron density of the CBCT imaging system was verified by comparing the calculated electron density from the Hounsfield Units (HU) measurements obtained from a Gammex phantom to the relative electron densities provided in vendor's specifications. To benchmark the relative electron density of the CBCT imaging system against a standard helical CT simulator, ten clinical plans that were created on CT simulation images were copied and recalculated on the CBCT images acquired immediately after the CT simulation, the latter of which was a standard procedure in current radiotherapy care for all patients who had given their consent to participate in the IRB-approved imaging study. The dose grids used in these calculations were 2.5mm x 2.5mm x 3mm. The Gamma passing rate was calculated using a standard 3mm/3% criterion with a 10% threshold. Ourresults showed the difference between the averaged CBCT calibration curves acquired at tube voltages of 125 kVP and 140 kVp was less than 2%. The mean discrepancy of the relative electron densities from vendor's specification was 0.0045 with a range between -0.02 and 0.04. Relative electron densities in all inserts were within 2% from the vendor's specifications except the cortical bone insert. Gamma passing rate was between 96.02% and 98.49% with mean value of 97.4% and a standard deviation of 0.95%. We consider this reflects the fact that the CT simulation and CBCT imaging were performed in separated rooms, which resulted in slight anatomical deformation that could negatively impact the Gamma passing rate. The CBCT imaging system provides sufficient accuracy of electron density for dose calculation, and the dose distribution calculated on the CBCT images is clinically equivalent to those calculated on helical CT images. The enhanced imaging quality of CBCT could further extend the role of imaging guidance to planning for adaptive radiotherapy, potentially reducing the need for re-simulation and interruptions in the radiotherapy course.

End-to-End Verification and Dosimetric Comparative Study of Brainlab Monte Carlo and Eclipse Acuros XB Photon Dose Calculation Algorithm for VMAT SRS Planning

We commissioned the Brainlab Monte Carlo Dose Calculation Algorithm for VMAT SRS planning and verified it by measurements and comparisons with two other algorithms: Pencil beam and Eclipse Acuros. The end-to-end verification and dosimetric comparative study results are reported in this abstract. Brainlab Elements Cranial SRS (v3.0) provides single lesion planning using VMAT optimization, thus allowing dose modulation with MLC leaf positioning, dose rate, and gantry speed. We commissioned and performed quality assurance for the available dose calculation algorithms, including Pencil Beam (PB) algorithm with only reference beam data (RBM PB) and PB with full beam data (Elements PB), and Monte Carlo (MC) algorithm XVMC. Calculation between the two were compared to each other and to Acuros XB dose calculation algorithm by Varian. Open field MLC, static cone, dynamic MLC, and conical cone plans were calculated with each algorithm. IBA CC01 ionization chamber was used for each measurement, and small field correction factors as determined in TRT-483 were applied. End-to-end tests were performed using an anthropomorphic stereotactic end-to-end verification (STEEV) phantom from CIRS and a phantom/diode array, StereoPHAN/SRSMapCHECK from SunNuclear. Each phantom was CT scanned with 0.5 mm slice thickness. Four clinical plans were optimized in Brainlab Elements Treatment Planning System (TPS). Each plan was calculated with the following algorithms: Elements PB, RBM PB, MC, and Acuros XB with 1 mm dose grid size. The delivered dose was measured with a SunNuclear SRSMapCHECK diode array detector. The calculated dose was compared with the measured dose by Gamma analysis with DTA 1%/1mm, 2%/1mm, and 3%/1mm. The results of point dose measurements for field size larger than 1 × 1 cm2 among measured dose, dose calculated by pencil beam algorithms, and dose calculated by Monte Carlo algorithm were in excellent agreement (1.45% ± 0.7%). For the small rectangular field sizes (0.4 × 1, 0.2 × 1 cm2), Monte Carlo calculation showed superior accuracy and robustness to pencil beam algorithm when compared with measured dose, with percent dose difference 13% and -2.2%, respectively. Monte Carlo calculation have higher gamma results compared with pencil beam algorithm due more accurate calculations with heterogenous materials. Acuros XB showed comparable accuracy with MC in terms of gamma passing rate. The SRS MapCHECK measurement agrees dosimetrically with the plans calculated by Pencil Beam algorithm and the Monte Carlo algorithm XVMC by Brainlab, as well as the Acuros XB dose calculation algorithm by Varian. Point dose calculation for small field dosimetry and gamma analysis using SRS MapCHECK both showed that Monte Carlo algorithm is more robust and is superior in regions of heterogenous materials. Therefore, we recommend that Monte Carlo algorithm for dose calculation be used in clinical SRS and SRT cases.

Requirements for dose calculation on an active scanned proton beamline for small, shallow fields

Introduction:A growing interest in using proton pencil beam scanning in combination with collimators for the treatment of small, shallow targets, such as ocular melanoma or pre-clinical research emerged recently. This study aims at demonstrating that the dose of a synchrotron-based PBS system with a dedicated small, shallow field nozzle can be accurately predicted by a commercial treatment planning system (TPS) following appropriate tuning of both, nozzle and TPS. Materials:A removable extension to the clinical nozzle was developed to modify the beam shape passively. Five circular apertures with diameters between 5 to 34mm, mounted 72cm downstream of a range shifter were used. For each collimator treatment plans with spread-out Bragg peaks (SOBP) with a modulation of 3 to 30mm were measured and calculated with GATE/Geant4 and the research TPS RayStation (RS11B-R). The dose grid, multiple coulomb scattering and block discretization resolution were varied to find the optimal balance between accuracy and performance. Results:For SOBPs deeper than 10mm, the dose in the target agreed within 1% between RS11B-R, GATE/Geant4 and measurements for aperture diameters between 8 to 34mm, but deviated up to 5% for smaller apertures. A plastic taper was introduced reducing scatter contributions to the patient (from the pipe) and improving the dose calculation accuracy of the TPS to a 5% level in the entrance region for large apertures. Conclusion:The commercial TPS and GATE/Geant4 can accurately calculate the dose for shallow, small proton fields using a collimator and pencil beam scanning.

Fully automated segmentally boosted VMAT.

Treatment planning for volumetric modulated arc therapy (VMAT) typically involves the use of multiple arcs to achieve sufficient intensity modulation. Alternatively, we can perform segment boosting to achieve similar intensity modulation while also reducing the number of control points used. Here, we propose the MetaPlanner Boosted VMAT (MPBV) approach, which generates boosted VMAT plans through a fully automated framework. The proposed MPBV approach is an open-source framework that consists of three main stages: meta-optimization of treatment plan hyperparameters, fast beam angle optimization on a coarse dose grid to select desirable segments for boosting, and final plan generation (i.e., constructing the boosted VMAT arc and performing optimization). Performance for the MPBV approach is evaluated on 21 prostate cases and 6 head and neck cases using clinically relevant plan quality metrics (i.e., target coverage, dose conformity, dose homogeneity, and OAR sparing). As compared to two baseline methods with multiple arcs, MPBV maintains or improves dosimetric performance for the evaluated metrics while substantially reducing average estimated delivery times (from 2.6 to 2.1 min). Our proposed MPBV approach provides an automated framework for producing high-quality VMAT plans that uses fewer control points and reduces delivery time as compared to traditional approaches with multiple arcs. MPBV applies automated treatment planning to segmentally boosted VMAT to address the beam utilization inefficiencies of traditional VMAT approaches that use multiple full arcs.

Stability of dosiomic features against variations in dose calculation: An analysis based on a cohort of prostate external beam radiotherapy patients.

Interest in using higher order features of the planned 3D dose distributions (i.e., dosiomics) to predict radiotherapy outcomes is growing. This is driving many retrospective studies where historical data are mined to train machine learning models; however, recent decades have seen considerable advances in dose calculation that could have a direct impact on the dosiomic features such studies seek to extract. Is it necessary to recalculate planned dose distributions using a common algorithm if retrospective datasets from different institutions are included? Does a change in dose calculation grid size part way through a retrospective cohort, introduce bias in the extracted dosiomic features? The purpose of this study is to assess the stability of dosiomic features against variations in three factors: the dose calculation algorithm type, version, and dose grid size. Dose distributions for 27 prostate patients who received EBRT were recalculated in the Eclipse Treatment Planning System (Varian Medical Systems, Palo Alto, California, USA) using two algorithms (AAA and Acuros XB), two versions (version 13.6 and 15.6), and three dose grids (2, 2.5s, and 3mm) - 12 dose distributions for each patient. Ninety-three dosiomic features were extracted from each dose distribution and each of the following regions-of-interest: high dose PTV (PTV_High), 1cm rind around PTV_High (PTV_Ring), low dose PTV (PTV_Low), rectum, and bladder using PyRadiomics. The coefficient of variation (CV) was calculated for each dosiomic feature. Hierarchical clustering was used to group features with high and low variability. Three-way repeated measures ANOVA was performed to investigate the effect of the three different factors on dosiomic features that were classified with high variation. Additionally, CVs were calculated for cumulative dose volume histograms (DVHs) to test their ability to detect the variations in dose distributions. For PTV_Ring, PTV_Low, and rectum, all the dosiomic features had low CV (average CV ≤ 0.26) across the varying dose calculation conditions. For PTV_High, six dosiomic features showed CV>0.26, and dose calculation algorithm type and grid size were the major sources of within-patient variation. For bladder, one dosiomic feature had average CV>0.26, but none of the three dose calculation-related factors led to a statistically significant variation. The CVs for all the DVHs were very small (CV<0.05). For all the regions-of-interest examined in this study, the majority of the dosiomic features were stable against variations in dose calculation; however, some of the dosiomic features for PTV_High and bladder had significant variations due to differences in dose calculation details. DVHs were detecting less variation than dosiomic features.

Initial clinical applications treating pediatric and adolescent patients using MR-guided radiotherapy.

To demonstrate the clinical applications and feasibility of online adaptive magnetic resonance image guided radiotherapy (MRgRT) in the pediatric, adolescent and young adult (AYA) population. This is a retrospective case series of patients enrolled onto a prospective study. All pediatric (age < 18) and AYA patients (age< 30), treated on the Elekta Unity MR linear accelerator (MRL) from 2019 to 2021 were enrolled onto a prospective registry. Rationale for MRgRT included improved visualization of and alignment to the primary tumor, re-irradiation in a critical area, ability to use smaller margins, and need for daily adaptive replanning to minimize dose to adjacent critical structures. Step-and-shoot intensity-modulated radiation treatment (IMRT) plans were generated for all Unity patients with a dose grid of 3 mm and a statistical uncertainty of < 1% per plan. A total of 15 pediatric and AYA patients have been treated with median age of 13 years (range: 6 mos - 27 yrs). Seven patients were <10 yo. The clinical applications of MRgRT included Wilms tumor with unresectable IVC thrombus (n=1), Ewing sarcoma (primary and metastatic, n=3), recurrent diffuse intrinsic pontine glioma (DIPG, n=2), nasopharyngeal carcinoma (n=1), clival chordoma (n=1), primitive neuroectodermal tumor of the pancreas (n=1), recurrent gluteo-sacral germ cell tumor (n=1), C-spine ependymoma (n=1), and posterior fossa ependymoma (n=1). Two children required general anesthesia. One AYA patient could not complete the MRgRT course due to tumor-related pain exacerbated by longer treatment times. Two AYA patients experienced anxiety related to treatment on the MRL, one of which required daily Ativan. No patient experienced treatment interruptions or unexpected toxicity. MRgRT was well-tolerated by pediatric and AYA patients. There was no increased use of anesthesia outside of our usual practice. Dosimetric advantages were seen for patients with tumors in critical locations such as adjacent to or involving optic structures, stomach, kidney, bowel, and heart.

Dosimetric Impact of Simulated Daily Adaptive Radiotherapy with Significantly Reduced Setup Margins in the Definitive Treatment of Head and Neck Cancer

<h3>Purpose/Objective(s)</h3> Even when treating head and neck cancer (HNC) with daily image-guided radiation therapy, setup margins of 3-5 mm are required to ensure adequate coverage of disease due to setup error. These margins lead to additional normal tissue dose that may compromise acute and long-term quality-of-life. The use of daily adaptive radiotherapy (ART) using an online adaptive system significantly reduces the need for a PTV margin, since the targets and organs-at-risk (OAR) are recontoured each fraction. In this study, we hypothesized using daily ART (simulated) with 1 mm margins based on our involved nodal radiotherapy (INRT) paradigm would improve coverage and reduce OAR dose. <h3>Materials/Methods</h3> Ten patients with HNC treated on our institutional phase II INRT trial were selected for this study. Cone-beam CT scans were uploaded to the ETHOS emulator for daily adaptive replanning which was simulated using fractions 5, 10, 15, 20, 25, 30, and 35. Two plans were generated with a 1 mm (reduced) and a 5 mm (conventional) PTV expansion. The dose grid from the delivered, conventional margin plan was transposed onto the CBCT and adapted contours. Separately, a reduced target margin plan optimized on the online adaptive system was created. PTV coverage (70 Gy-gross/primary disease, 66.5 Gy-suspicious nodes, 63 Gy-extended volume surrounding the primary disease) and D1 were calculated for target structures along with the dose to OARs. Comparisons performed via paired two tailed t-test. <h3>Results</h3> Six patients with oropharynx cancer and four patients with larynx cancer were included. Median V100% coverage for 70 Gy (96% vs 95.6%), 66.5 Gy (98.5% vs 76.5%), and 63 Gy (98.9% vs 74.9%) were improved with the adaptive planning (all p-values<0.03). Plan homogeneity was improved in the ART plan (e.g., PTV70 median D1% 72.8 Gy vs 74.8 Gy, all p-values<0.02). The total median dose reduction was 10.8 Gy (interquartile range, IQR 8.0-12.6 Gy) for the ipsilateral submandibular gland (SMG), 12.2 Gy (IQR 7.3-23.5 Gy) for the contralateral SMG, 8.3 Gy (IQR 3.8-12.6 Gy) for the ipsilateral parotid, 8.1 Gy (IQR 4.4-11.0 Gy) for the contralateral parotid, 7.0 Gy (IQR 4.5-9.1 Gy) for larynx, 6.6 Gy (IQR 2.7-8.4 Gy) for oral cavity, and 8.6 Gy (IQR 5.9-12.1 Gy) for constrictors. <h3>Conclusion</h3> Simulated daily ART using a 1 mm PTV margin provided significant improvement over conventional PTV margins in coverage and critical OAR dosimetry. This small proof-of-concept study suggests this approach may lead to improved quality-of-life while preserving or improving tumor coverage. Further evaluation is needed to assess the resources needed to implement daily adaptations. A phase II randomized trial of daily ART is underway to determine if dosimetric improvements lead to similar gains in patient-reported outcomes.

Evaluation of a High-Resolution Large-Area Two-Dimensional Detector Array for Pre-Treatment Verification of Multiple-Target SRS Plans Using a Single Isocenter

<h3>Purpose/Objective(s)</h3> Pre-treatment verification of multiple-target SRS plans using a single isocenter is challenging because of small target sizes and multiple off-isocenter targets. Targets located further away from the isocenter are more sensitive to rotational setup variations, increasing the risk of geometrical miss. It demands high-resolution detector for small target sizes and large detector area for measuring multiple targets at once. The purpose of this study is to evaluate a high-resolution large-area two-dimensional (2D) detector array for pre-treatment verification of multiple-target SRS plans using a single isocenter. <h3>Materials/Methods</h3> A 2D detector array was evaluated for this purpose. It is based on a silicon complementary metal-oxide-semiconductor platform, which enables a compact design, fast read-out, and high pixel density with each pixel comprising a radiation-sensitive photodiode, a capacitor and three transistors. Its spatial resolution is 0.4 mm, with 105,000 pixels across a large active area of 120 × 140 mm². Ten multiple-target SRS plans were selected such that each plan has at least 2 targets appear on a longitudinal plane cross the isocenter. All plans were created in a treatment planning system using 6 MV flattening filter free photon beam. The typical plan consisted of one or two coplanar arcs, and two to three non-coplanar arcs. All plans were delivered in patient geometry with the detector center aligned to the plan isocenter. The detector plane was rotated to the corresponding desired longitudinal plane to measure the composite dose in a single measurement. The smallest target measured was 0.12 cc. The largest center-to-center target separation measured was 12 cm. Six of the plans in which the target separations were relatively short were able to be cross verified with a patient-specific quality assurance (PSQA) tool, whose detector area is of 77 × 77 mm². The measured 2D dose distributions in true composite were compared with the calculated dose distributions. Gamma index analysis was performed using 3%/1 mm criteria with a 10% dose threshold. <h3>Results</h3> With the 2D detector array, the gamma passing rates are all greater than 95% except one, which is 93%. The average gamma-passing rate is 97.7%. For the 6 plans cross verified, the average gamma-passing rates are 98.2% and 96.1% with the 2D detector array and PSQA tool, respectively. <h3>Conclusion</h3> The 2D detector array was demonstrated to be suitable and offers an efficient way for pre-treatment verification of multiple-target SRS plans using a single isocenter. The advantages of this 2D array include: 1) High-resolution of 0.4 mm avoids dose interpolations for plans calculated with 0.5 -1.0 mm dose grid and potentially avoids "false positive" results due to more measured points; 2) Large active area of 120 × 140 mm² allows simultaneous verification of multiple targets in true composite in patient geometry.

Estimation of Cumulative Organ Maximum Dose and Confidence Intervals for MRI-Guided Adaptive Radiotherapy of Abdominal Cancer

<h3>Purpose/Objective(s)</h3> The maximum dose (Dmax), e.g., D0.03cc (dose to 0.03cc volume), of adjacent organs at risk (OARs) is a primary dose-limiting constraint in MRI-guided adaptive radiotherapy (MRgART) of abdominal cancer. We utilized inverse consistency error (ICE) maps to estimate voxel-wise uncertainties in deformable image registration (DIR) based dose accumulation (DDA) and confidence intervals in cumulative OARs Dmax during MRgART for pancreatic cancer. <h3>Materials/Methods</h3> Motion-average reference (first fraction) and subsequent daily MRIs, collected in MRgART for five pancreatic cancer patients treated with 33 Gy in 5 fractions on a 1.5T MR-Linac, were registered using a free-form DIR algorithm based on feature similarity scoring metrics and were used to accumulate daily doses. Forward and reverse DIRs were performed, and ICE maps were calculated in the reference image. The correlation between ICE and actual registration error (RE) was assessed by identifying anatomical landmarks (63) in a reference-daily image pair and measuring the landmark REs (after forward-DIR) and ICEs (after forward and reverse DIRs). To account for the deformation vector field spatial uncertainty affecting the DDA, for each voxel in the reference image we associated a cloud of dose grid points centered at the voxel landing coordinates in the daily image with a radius proportional to the ICE length. We considered that the cloud would encompass the "true" registration point with a high likelihood. The uncertainty in the dose mapped to each voxel from each fraction was determined as the standard deviation of the grid point doses within the cloud. The maxima and minima of the dose distributions were computed from the cumulative uncertainties, and confidence intervals for Dmax were determined. <h3>Results</h3> The measured REs and ICEs for landmarks indicated a positive correlation between these errors (Pearson's correlation coefficient r=0.795, p-value=7.04 × 10⁻¹⁵), RE being about twice ICE. The ICEs averaged 1.5 mm and 2.5 mm in the duodenum and stomach. For the duodenum, cumulative D0.03cc and D1cc at the end of treatment ranged from 85.7% to 100.4% of the plan constraint value (confidence intervals included). The uncertainty intervals were between 1.1% and 2.2% in four cases and approximately 7% for the fifth case. Cumulative D0.03cc in the stomach varied from 89.4% to 102.8%, with uncertainty intervals that extended from 2.5% to 6.4%. All Dmax lower limits were below 100%. <h3>Conclusion</h3> Voxel-wise uncertainties in DIR-based dose accumulation during MRgART of abdominal cancer can be estimated using an ICE-based technique. The derived confidence intervals for cumulative Dmax (D1cc or D0.03cc) of OARs (the duodenum and stomach) can be utilized to guide daily dose adaptation throughout MRgART.

Dosimetry Validation Study for Automated Head and Neck Cancer Organs at Risk Segmentation Using Stratified Learning and Neural Architecture Search

<h3>Purpose/Objective(s)</h3> Organs at risk (OARs) segmentation is an essential process in head and neck (H&N) cancer radiotherapy. We have reported high automated segmentation geometric accuracy of Stratified Learning and Neural Architecture Search method in terms of Dice Score (DSC) from our previous study. In this study, we would evaluate the dosimetric influence of our automated approach before integrating this method into the clinical workflow. <h3>Materials/Methods</h3> To measure the dosimetric effects brought by the OARs' variance, the intensity-modulated radiotherapy (IMRT) dose plans of 10 head and neck cancer patients were replanned using the original tumor target volumes and three substitute OAR contours permutations (deep learning generated stratified organs at risk segmentation (SOARS), SOARS revised by physician (SOARS-revised), and OAR delineated from scratch by physician (human reader)). We further examined the clinical dosimetric accuracy and the clinical reference OAR contours were overlaid on top of each replanned dose grid to evaluate the dosimetric differences. <h3>Results</h3> After replanning, SOARS and SOARS-revised contours have slightly smaller Diff (max dose) as compared to human reader contours (3.4%, 3.5% vs. 4.1%). For the Diff (mean dose), human reader, SOARS, SOARS-revised achieves similar results, i.e., 5.3%, 5.0%, and 5.0%, respectively. However, more OARs from the human reader have dose variations larger than 10% or 20% as compared to SOARS and SOARS-revised. Overall, our results indicate that using OAR contours from human reader, SOARS, and SOARS-revised lead to generally comparable dose accuracy in clinical practice. SOAR-related OAR contours have fewer OARs with dose error larger than 10% or 20%. <h3>Conclusion</h3> This study further validates the clinical applicability of a deep learning based automated H&N OAR segmentation method by comparing dosimetry of plans using OAR contours generated automatically and by a human reader to the gold standard contours. The dose variations calculated after planning on automated segmentation contours are less than 5%. Our proposed automated H&N OAR segmentation method not only achieves high geometric accuracy but also helps deliver treatment beams with little variances.