Purpose: To compare plan quality among photon volumetric modulated arc therapy (VMAT) and intensity-modulated proton therapy (IMPT) with robustness using three different proton beam delivery systems with various spot size (sigma) ranges: cyclotron-generated proton beams (CPBs) (sigma: 2.7-7.0 mm), linear accelerator proton beams (LPBs) (sigma: 2.9-5.5 mm), and linear accelerator proton mini beams (LPMBs) (sigma: 0.8-3.9 mm) for the treatment of head and neck (HN) cancer with bilateral neck irradiation. Methods: Ten patients treated for oropharynx cancer with bilateral neck irradiation were planned using CPBs, LPBs, LPMBs, and VMAT. The homogeneity index (HI), mean body dose, and defined volumetric doses for selected critical organs-at-risk (OARs) were compared. Set-up uncertainties of +/- 3 mm and +/- 3.5% range uncertainties were included in robust evaluation using V-95%Rx > 95% (Volume that covers 95% of the target volume at 95% of the prescription (Rx) dose) to high dose and low dose CTV volumes (CTV_70 Gy and CTV_56 Gy). VMAT and proton plans were compared in terms of OAR doses and mean body dose only. Homogeneity Indices were compared among IMPT plans in addition to OAR doses. The Wilcoxon signed-rank test was used to evaluate statistical differences between evaluation metrics for VMAT plans and all proton plan types. Results: OAR dose metrics were improved by 2% to 30% from CPB plans to LPB or LPMB plans. Compared to photon VMAT plans, all OAR doses except for mandible dose metrics were improved by 2% to 53% for all proton plans. The mean body dose was also improved by 7.5% from CPB to LPB and by 10.8% from CPB to LPMB. In addition, the mean body dose was also improved by 44% from VMAT to CPB, by 48% from VMAT to LPB, and by 50% from VMAT to LPMB plans. Compared to CPB plans, HI was significantly better (p < 0.05) for the LPB and LPMB plans. HI also improved considerably from VMAT to CPB, LPB, and LPMB. For both CTV_70 Gy and CTV_56 Gy, average robust evaluation across all worst-case scenarios was slightly better for CPB plans, with an average of V-95%Rx of the CTV_70 Gy of 97.6% +/- 1.22%, followed by 97.2% +/- 1.31% and 97.2% +/- 1.35% for LPB and LPMB plans, respectively. Robustness for CTV_56 Gy showed comparable robustness across all proton plan types, with an average V-95%Rx of 97.4% +/- 0.87% for CPB, 97.4% +/- 1.21%, and 97.5% +/- 1.08% for CPB, LPB, and LPMB plans, respectively. Conclusion: With decreased spot size, the LPB and LPMB are excellent alternatives to VMAT and CPB therapy and can significantly reduce the dose to normal tissue.

PURPOSE:Volumetric-modulated arc therapy for total body irradiation (VMAT-TBI) is a novel radiotherapy technique that has been implemented at our institution. The purpose of this work is to investigate possible failure modes (FMs) in the treatment process and to develop a quality control (QC) program for VMAT-TBI following TG-100 guidelines.METHODS:We formed a multidisciplinary team to map out the complete treatment process of VMAT-TBI following the AAPM TG-100 guidelines. This process map gives a visual representation of the VMAT-TBI workflow from the CT simulation, image processing, contouring, treatment planning, to treatment delivery. From the process map, potential FMs were identified. The occurrence (O), detectability (D), and severity of impact (S) of each FM were assigned according to scoring criteria (1-10) by the multidisciplinary team. A risk priority number (RPN) was calculated from average O, S, and D of each FM (RPN = O x S x D). High risk FMs were identified as 20% of the FMs having the highest RPN scores. After the FMEA analysis, fault-tree analysis (FTA) was performed for each major step of the treatment process to determine the effects of potential failures to the treatment outcome. Effective QC methods were identified to prevent the high risk failures and to improve the safety of the VMAT-TBI program.RESULTS:We identified a total of 55 sub-processes and 128 FMs from the VMAT-TBI workflow. The top five high-risk FMs were: (1) Prescription and/or OAR constraints changed during planning and not communicated to the planner, (2) Patient moves or breathes too heavily during the upper body CT scan (3) Patient moves during the lower body CT scan, (4) Treatment planning system not calculating total body DVH metrics correctly for TBI, (5) Improper optimization criteria used or not sufficient optimization, resulting in suboptimal dose coverage, OAR sparing or excessive hotspots during treatment planning. Two FMs have average severity scores ≥8: Incorrect PTV subdivision/isocenter placement and Prescription and/or OAR constraints changed during planning and not communicated to the planner. Quality assurance and QC interventions including staff training, standard operating procedures, and quality checklists were implemented based on the FMEA and FTA.CONCLUSION:FM and effect analysis was performed to identify high-risk FMs of our VMAT-TBI program. FMEA and FTA were effective in identifying potential FMs and determining the best quality management (QM) measures to implement in the VMAT-TBI program.

The purpose of this work is to objectively assess variability of intercampus plan quality for head-and-neck (HN) cancer and to test utility of a priori feasibility dose-volume histograms (FDVHs) as planning dose goals. In this study, 109 plans treated from 2017 to 2019 were selected, with 52 from the main campus and 57 from various regional centers. For each patient, the planning computed tomography images and contours were imported into a commercial program to generate FDVHs with a feasibility value (f-value) ranging from 0.0 to 0.5. For 10 selected organs-at-risk (OARs), we used the Dice similarity coefficient (DSC) to quantify the overlaps between FDVH and clinically achieved DVH of each OAR and determined the f-value associated with the maximum DSC (labeled as f-max). Subsequently, 10 HN plans from the regional centers were replanned with planning dose goals guided by FDVHs. The clinical and feasibility-guided auto-planning (FgAP) plans were evaluated using our institutional criteria. Among plans from the main campus and regional centers, the median f-max values were statistically significantly different (p < 0.05) for all OARs except for the left parotid (p = 0.622), oral cavity (p = 0.057), and mandible (p = 0.237). For the 10 FgAP plans, the median values of f-max were 0.21, compared to 0.37 from the clinical plans. With comparable dose coverage to the tumor volumes, the significant differences (p < 0.05) in the median f-max and corresponding dose reduction (shown in parenthesis) for the spinal cord, larynx, supraglottis, trachea, and esophagus were 0.27 (8.5 Gy), 0.3 (7.6 Gy), 0.19 (5.9 Gy), 0.19 (8.9 Gy), and 0.12 (4.0 Gy), respectively. In conclusion, the FDVH prediction is an objective quality assurance tool to evaluate the intercampus plan variability. This tool can also provide guideline in planning dose goals to further improve plan quality.

For patients undergoing stereotactic body radiation therapy for lung cancer, their tumor positions may vary due to anatomical changes. This study is to investigate whether adaptive re-planning is necessary for patients with large tumor position displacements observed from daily kV-cone-beam computed tomography (kV-CBCT). We selected 16 fractions from 16 patients with recorded treatment couch shifts greater than 1.5 cm under kV-CBCT guidance. The treatment positions for these patients were manually restored in kV-CBCTs via bone-to-bone alignments (B2B) and tumor-to-tumor alignments (T2T) with corresponding planning CTs. The tumor volumes, including PTVs, ITVs, and GTVs, were transferred from the planning CTs to these kV-CBCTs. With the planned beam configurations and treatment isocenters, kV-CBCTs were imported into the treatment planning system for dose recalculations. To minimize uncertainties of the Hounsfield Unit (HU) in kV-CBCTs, uniformed HU values were assigned to the externals, ITVs, and lungs. The percentage volumes of GTVs, ITVs, and PTVs receiving the prescription dose (VRx) and the dose to the normal structures were analyzed. Seven out of the 16 patients were identified with >5mm tumor position displacements after subtracting the recorded couch shifts from the shifts of B2B alignment. For T2T alignments, 9 out of 16 (56.3%) patients had VRx of PTV <95% (the planning goal) with 91.4% as the lowest, while VRx of the GTV and ITV remained 100% for all 16 patients. For B2B alignments, 14 out of 16 (87.5%) patients have VRx of PTV <95%; 5 patients (31.3%) had VRx of ITV <95%; and 4 patients (25.0%) had VRx of GTV <99%. T2T alignment with 5 mm PTV margin was found superior to B2B alignment, resulting in adequate dose coverage to the ITVs, even for tumors with large positional changes. Adaptive re-planning may not be necessary under these scenarios.

Purpose:To investigate the impact of tumor position displacements (TPDs) on tumor dose coverage in photon and proton stereotactic body radiation therapy (SBRT) treatments for lung cancer patients. Methods:From our institutional database of 2877 fractions from 770 lung cancer patients treated with photon SBRT in 2017-2021, 163 fractions from 88 patients with recorded iso-center shifts of >1.5 cm in any direction under kV-cone-beam CT guidance were identified. By double registrations with bony and tumor alignments, the difference between the iso-center shifts of these two alignments was categorized as TPDs. One fraction from each of 15 patients who had TPD magnitudes >3 mm were selected for this study. For each patient, one proton plan using intensity modulated proton therapy (IMPT) with robust optimization was generated retrospectively. All photon plans had V100%RX>99% of GTVs and V100%RX>98% of ITVs. Proton plans were evaluated with two worse-case scenario (voxelwise worst and worst scenario) using 5mm and 3.5% uncertainty to achieve the same planning goals as the corresponding photon plans. These two evaluation proton plans were named proton-1st and proton-2nd plans. The dosimetric effect of TPD was simulated by shifting tumor contours with the corresponding shift on patient specific planning CT and by recalculating the dose of the original plan. Results:The range of magnitude of TPDs was 3.58-28.71 mm. In photon plans, TPDs did not impact tumor dose coverage, still achieving V100%RX of the GTV≥99% and V100%RX of the ITV≥98%. In proton plans for patients with TPDs>10 mm, inadequate target dose coverage was observed. More specifically, 8 fractions of proton-1st plans and 4 fractions of proton-2nd had V100%RX of the GTV<99% and V100%RX of the ITV<98%. Conclusions:Adequate tumor dose coverage was achieved in photon SBRT for magnitude of TPDs up to 20 mm. TPDs had greater impact in proton SBRT and adaptive planning was needed when the magnitude of TPDs>10 mm to provide adequate tumor dose coverage.

PURPOSE:To investigate whether there is a volume threshold in target volume of brain metastases below which a small cone size and sharp penumbra in Gamma Knife (GK) may provide improved plan quality when compared to Volumetric Modulated Arc Therapy (VMAT)-based stereotactic radiosurgery (SRS).METHODS:For patients treated on GK SRS for brain metastases in 2018-2019 in our institution, 121 patients with two and three targets were identified. Twenty-six patients with two or three brain metastases (total of 76 lesions) were selected for this study. Two VMAT plans, SmartArc (Pinnacle) and HyperArc (Eclipse), were generated retrospectively for each patient. Plan quality was evaluated based on RTOG conformity index (CI), Paddick gradient index (GI), normal tissue (NT) V12Gy and V4.5Gy. By using the receiver operating characteristic (ROC) curve for both VMAT plans (SmartArc and HyperArc) and metrics of RTOG CI and NT V12Gy, we compared GK plans to SmartArc and HyperArc plans separately to determine the threshold volume.RESULTS:For SmartArc plans, both ROC curve analyses showed a threshold volume of 0.4 cc for both CI and NT V12Gy. For HyperArc plans, the threshold volumes were 0.2 cc for the CI and 0.5 cc for NT V12Gy. GK plans produced improved dose distribution compared to VMAT for targets ≤0.4 cc, but HyperArc was found to have competing results with GK in terms of CI and NT V12Gy. For targets > 0.4 cc, both SmartArc and HyperArc showed better plan quality when compared to the GK plans.CONCLUSIONS:Target volumes ≤0.4 cc may require a small cone size and sharp penumbra in GK while for target volumes >0.4 cc, VMAT-based SRS can provide improved overall plan quality and faster treatment delivery.

Abstract A high‐resolution diode array has been comprehensively evaluated. It consists of 1013 point diode detectors arranged on the two 7.7 × 7.7 cm2 printed circuit boards (PCBs). The PCBs are aligned face to face in such a way that the active volumes of all diodes are in the same plane. All individual correction factors required for accurate dosimetry have been validated for conventional and flattening filter free (FFF) 6MV beams. That included diode response equalization, linearity, repetition rate dependence, field size dependence, angular dependence at the central axis and off‐axis in the transverse, sagittal, and multiple arbitrary planes. In the end‐to‐end tests the array and radiochromic film dose distributions for SRS‐type multiple‐target plans were compared. In the equalization test (180° rotation), the average percent dose error between the normal and rotated positions for all diodes was 0.01% ± 0.1% (range −0.3 to 0.4%) and −0.01% ± 0.2% (range −0.9 to 0.9%) for 6 MV and 6MV FFF beams, respectively. For the axial angular response, corrected dose stayed within 2% from the ion chamber for all gantry angles, until the beam direction approached the detector plane. In azimuthal direction, the device agreed with the scintillator within 1% for both energies. For multiple combinations of couch and gantry angles, the average percent errors were −0.00% ± 0.6% (range: −2.1% to 1.6%) and −0.1% ± 0.5% (range −1.6% to 2.1%) for the 6MV and 6MV FFF beams, respectively. The measured output factors were largely within 2% of the scintillator, except for the 5 mm 6MV beam showing a 3.2% deviation. The 2%/1 mm gamma analysis of composite SRS measurements produced the 97.2 ± 1.3% (range 95.8‐98.5%) average passing rate against film. Submillimeter (≤0.5 mm) dose profile alignment with film was demonstrated in all cases.

A commercial semi-empirical volumetric dose verification system (PerFraction [PF], Sun Nuclear Corp.) extracts multi-leaf collimator positions from the electronic portal imaging device movies collected during a pre-treatment run, while the rest of the delivered control point information is harvested from the accelerator log files. This combination is used to reconstruct dose on a patient CT dataset with a fast superposition/convolution algorithm. The method was validated for single-isocenter multi-target SRS VMAT treatments against absolute radiochromic film measurements in a cylindrical phantom. The targets ranged in size from 0.8 to 3.6 cm and in number from 3 to 10 per plan. A total of 17 films rotated at different angles around the cylinder axis were analyzed. Each of 27 total targets was intercepted by at least one film, and 2-4 different films were analyzed per plan. Film dose was always scaled to the ion chamber measurement in a high-dose, low-gradient area deliberately created at the isocenter. The planar dose agreement between PF and film using 3%(Global dose-difference normalization)/1 mm gamma analysis was on average 99.2 ± 1.1%. The point dose difference in the low-gradient area in the middle of every target was below 3%, while PF-reconstructed and film dose centroids for individual targets showed submillimeter agreement when measured on a well aligned accelerator. Volumetrically, all voxels in all plans agreed between PF and the primary treatment planning system at the 3%/1 mm level. With proper understanding of its advantages and shortcomings, the tool can be applied to patient-specific QA in routine radiosurgical clinical practice.

A superposition/convolution GPU-accelerated dose computation algorithm (the Calculator) has been recently incorporated into commercial software. The algorithm requires validation prior to clinical use. Three photon energies were examined: conventional 6 MV and 15 MV, and 10 MV flattening filter free (10 MVFFF). For a set of IMRT and VMAT plans based on four of the five AAPM Practice Guideline 5a downloadable datasets, ion chamber (IC) measurements were performed on the water-equivalent phantoms. The average difference between the Calculator and IC was -0.3 ± 0.8% (1SD). The same plans were projected on a phantom containing a biplanar diode array. We used the forthcoming criteria for routine gamma analysis, 3% dose-error (global (G) normalization, 2 mm distance to agreement, and 10% low dose cutoff). The γ (3%G/2 mm) average passing rate was 98.9 ± 2.1%. Measurement-guided three-dimensional dose reconstruction on the patient CT dataset (excluding the Lung) resulted in a similar average agreement rate with the Calculator: 98.2 ± 2.0%. The mean γ (3%G/2 mm) passing rate comparing the Calculator to the TPS (again excluding the Lung) was 99.0 ± 1.0%. Because of the significant inhomogeneity, the Lung case was investigated separately. The calculator has an alternate heterogeneity correction mode that can change the results in the thorax for higher-energy beams (15 MV). As this correction is nonphysical and was optimized for simple slab geometries, its application leads to mixed results when compared to the TPS and independent Monte Carlo calculations, depending on the CT dataset and the plan. The Calculator vs. TPS 15 MV Guideline 5a IMRT and VMAT plans demonstrate 96.3% and 93.4% γ (3%G/2 mm) passing rates respectively. For the lower energies, which should be predominantly used in the thoracic region, the passing rates for the same plans and criteria range from 98.6 to 100%. Overall, the Calculator accuracy is sufficient for the intended use.

Purpose: Despite improvements in optimization and automation algorithms, the quality of radiation treatment plans still varies dramatically. A tool that allows a priori estimation of the best possible sparing (Feasibility DVH, or FDVH) of an organ at risk (OAR) in high‐energy photon planning may help reduce plan quality variability by deriving patient‐specific OAR goals prior to optimization. Such a tool may be useful for (a) meaningfully evaluating patient‐specific plan quality and (b) supplying best theoretically achievable DVH goals, thus pushing the solution toward automatic Pareto optimality. This work introduces such a tool and validates it for clinical Head and Neck (HN) datasets. Methods: To compute FDVH, first the targets are assigned uniform prescription doses, with no reference to any particular beam arrangement. A benchmark 3D dose built outside the targets is estimated using a series of energy‐specific dose spread calculations reflecting observed properties of radiation distribution in media. For the patient, the calculation is performed on the heterogeneous dataset, taking into account the high‐ (penumbra driven) and low‐ (PDD and scatter‐driven) gradient dose spreading. The former is driven mostly by target dose and surface shape, while the latter adds the dependence on target volume. This benchmark dose is used to produce the “best possible sparing” FDVH for an OAR, and based on it, progressively more easily achievable FDVH curves can be estimated. Validation was performed using test cylindrical geometries as well as 10 clinical HN datasets. For HN, VMAT plans were prepared with objectives of covering the primary and the secondary (bilateral elective neck) PTVs while addressing only one OAR at a time, with the goal of maximum sparing. The OARs were each parotid, the larynx, and the inferior pharyngeal constrictor. The difference in mean OAR doses was computed for the achieved vs. FDVHs, and the shapes of those DVHs were compared by means of the Dice similarity coefficient (DSC). Results: For all individually optimized HN OARs (N = 38), the average DSC between the planned DVHs and the FDVHs was 0.961 ± 0.018 (95% CI 0.955–0.967), with the corresponding average of mean OAR dose differences of 1.8 ± 5.8% (CI −0.1–3.6%). For realistic plans the achieved DVHs run no lower than the FDVHs, except when target coverage is compromised at the target/OAR interface. Conclusions: For the validation of VMAT plans, the OAR DVHs optimized one‐at‐a‐time were similar in shape to and bound on the low side by the FDVHs, within the confines of planner's ability to precisely cover the target(s) with the prescription dose(s). The method is best suited for the OARs close to the target. This approach is fundamentally different from “knowledge‐based planning” because it is (a) independent of the treatment plan and prior experience, and (b) it approximates, from nearly first principles, the lowest possible boundary of the OAR DVH, but not necessarily its actual shape in the presence of competing OAR sparing and target dose homogeneity objectives.

The original helical ArcCHECK (AC) diode array and associated software for 3D measurement-guided dose reconstruction were characterized and validated; however, recent design changes to the AC required that the subject be revisited. The most important AC change starting in 2014 was a significant reduction in the overresponse of diodes to scattered radiation outside of the direct beam, accomplished by reducing the amount of high-Z materials adjacent to the diodes. This change improved the diode measurement accuracy, but in the process invalidated the dose reconstruction models that were assembled based on measured data acquired with the older version of the AC. A correction mechanism was introduced in the reconstruction software (3DVH) to accommodate this and potential future design changes without requiring updating model parameters. For each permutation of AC serial number and beam model, the user can define in 3DVH a single correction factor which will be used to compensate for the difference in the out-of-field response between the new and original AC designs. The exact value can be determined by minimizing the dose-difference with an ionization chamber or another independent dosimeter. A single value of 1.17, corresponding to the maximum measured out-of-field response difference between the new and old AC, provided satisfactory results for all studied energies (6X, 15X, and flattening filter-free 10XFFF). A library of standard cases recommended by the AAPM TG-244 Report was used for reconstructed dose verification. The overall difference between reconstructed dose and an ion chamber in a water-equivalent phantom in the targets was 0.0% +/- 1.4% (1SD). The reconstructed dose on a homogeneous phantom was also compared to a biplanar diode dosimeter (Delta(4)) using gamma analysis with 2% (local dose-error normalization) / 2 mm / 10% cutoff criteria. The mean agreement rate was 96.7% +/- 3.7%. For the plans common with the previous comparison, the mean agreement rate was 98.3% +/- 0.8%, essentially unchanged. We conclude that the proposed software modification adequately addresses the change in the dosimeter response.