Kavanaugh, James

Abstract Background Highly modulated radiotherapy delivery demands functional multileaf collimator (MLC) quality assurance (QA) with the ability to measure very low doses during short (<1 s) temporal segments of beam delivery. A radiochromic film with adequate sensitivity in the few‐cGy range is therefore needed. LD‐V1 film was designed for 2–20 cGy measurements in diagnostic kV energy ranges, but its performance for megavoltage (MV) beam qualities has not been well characterized. Purpose To demonstrate the feasibility of LD‐V1 film for low‐dose MV measurements and characterize its signal dependence under conditions relevant for clinical QA. Methods A 6 MV calibration curve was generated using eight dose levels between 0 and 40 cGy and compared with EBT4 film irradiated under identical conditions. Film characteristics evaluated included dose response, scanner and film signal uniformity, scan repeatability, energy dependence, post‐exposure signal evolution, scanning‐orientation dependence, and one‐dimensional (1‐D) profile visualization. Results LD‐V1 demonstrated higher net optical density (OD) and significantly less measurement uncertainty compared to EBT4 at dose levels less than 40 cGy. Significant energy dependence between kV and MV beam qualities was seen. Average net OD differences of 6.3% were seen between landscape and portrait scanning orientations. Post‐irradiation signal growth followed a logarithmic trend, stabilizing at ∼24 h. Scan‐to‐scan variation was <0.1%. Film‐signal uniformity was within 3.9%. 1‐D profiles showed discernible dose fall‐off for doses as low as 10 cGy. Conclusions LD‐V1 demonstrates suitability for low‐dose MV measurements and exhibits improved low‐dose sensitivity relative to EBT4, indicating its potential value for MLC QA in highly dynamic treatment deliveries. Nonetheless, its pronounced energy and scanning‐orientation dependence underscores the need for rigorous and consistent calibration and handling procedures.

AbstractIntroductionUsing diagnostic CT for radiotherapy (RT) planning can bypass traditional CT simulation but introduces challenges in patient positioning and Hounsfield unit (HU) fidelity, affecting dose accuracy. Here a Varian Ethos adaptive‐driven CT direct‐to‐treatment (DtT) multi‐fraction stereotactic body radiation therapy (SBRT) workflow is presented.MethodsThis study employed institutional diagnostic PET‐CT images to generate an initial reference Ethos treatment plan. HU and dosimetric accuracy were validated for PET‐CT, Ethos CBCT images (with and without Hypersight (HS), and the gold‐standard helical CT simulators). Following the SBRT reference plan creation on the low dose diagnostic PET‐CT, the first fraction was delivered with a newly generated adaptive plan calculated on the HS CBCT (Ethos) images. For multi‐fraction treatments, the first day CBCT images and adaptive plan become the reference for subsequent IGRT treatments. This study includes workflow validation and initial three patient experience.ResultsThe DtT adaptive SBRT workflow was successfully implemented, with initial end‐to‐end testing demonstrating feasibility. In‐house solutions were introduced to facilitate the adaptive to IGRT plan conversion. The Ethos system, especially with HS, maintained HU fidelity and dose calculation accuracy comparable to helical CTs. On‐table adaptive sessions were within 37–51 min, aligning with single‐fraction palliative studies. Subsequent non‐adaptive IGRT fractions were efficiently completed within 7–27 min.ConclusionsThis study demonstrates the feasibility of DtT adaptive‐driven multifraction SBRT using diagnostic PET‐CT. This approach reduces the need for on‐site patient presence prior to treatment, expands planning lead times, and improves overall efficiency in radiotherapy for well selected patients.

Abstract Objective . An algorithm was developed for automated positioning of lattice points within volumetric modulated arc lattice radiation therapy (VMAT LRT) planning. These points are strategically placed within the gross tumor volume (GTV) to receive high doses, adhering to specific separation rules from adjacent organs at risk (OARs). The study goals included enhancing planning safety, consistency, and efficiency while emulating human performance. Approach . A Monte Carlo-based algorithm was designed to optimize the number and arrangement of lattice points within the GTV while considering placement constraints and objectives. These constraints encompassed minimum spacing between points, distance from OARs, and longitudinal separation along the z -axis. Additionally, the algorithm included an objective to permit, at the user’s discretion, solutions with more centrally placed lattice points within the GTV. To validate its effectiveness, the automated approach was compared with manually planned treatments for 24 previous patients. Prior to clinical implementation, a failure mode and effects analysis (FMEA) was conducted to identify potential shortcomings. Main results. The automated program successfully met all placement constraints with an average execution time (over 24 plans) of 0.29 ±0.07 min per lattice point. The average lattice point density (# points per 100 c.c. of GTV) was similar for automated (0.725) compared to manual placement (0.704). The dosimetric differences between the automated and manual plans were minimal, with statistically significant differences in certain metrics like minimum dose (1.9% versus 1.4%), D5% (52.8% versus 49.4%), D95% (7.1% versus 6.2%), and Body-GTV V30% (20.7 c.c. versus 19.7 c.c.). Significance. This study underscores the feasibility of employing a straightforward Monte Carlo-based algorithm to automate the creation of spherical target structures for VMAT LRT planning. The automated method yields similar dose metrics, enhances inter-planner consistency for larger targets, and requires fewer resources and less time compared to manual placement. This approach holds promise for standardizing treatment planning in prospective patient trials and facilitating its adoption across centers seeking to implement VMAT LRT techniques.

OBJECTIVE The internal high-dose volume varies widely for a given prescribed dose during stereotactic radiosurgery (SRS) to treat brain metastases (BMs). This may be altered during treatment planning, and the authors have previously shown that this improves local control (LC) for non–small cell lung cancer BMs without increasing toxicity. Here, they seek to identify potentially actionable dosimetric predictors of LC after SRS for melanoma BM. METHODS The records of patients with unresected melanoma BM treated with single-fraction Gamma Knife RS between 2006 and 2017 were reviewed. LC was assessed on a per-lesion basis, defined as stability or a decrease in lesion size. Outcome-oriented approaches were utilized to determine optimal dichotomization for dosimetric variables relative to LC. Univariable and multivariable Cox regression analysis was implemented to evaluate the impact of collected parameters on LC. RESULTS Two hundred eighty-seven melanoma BMs in 79 patients were identified. The median age was 56 years (range 31–86 years). The median follow-up was 7.6 months (range 0.5–81.6 months), and the median survival was 9.3 months (range 1.3–81.6 months). Lesions were optimally stratified by volume receiving at least 30 Gy (V30) greater than or equal to versus less than 25%. V30 was ≥ and < 25% in 147 and 140 lesions, respectively. For all patients, 1-year LC was 83% versus 66% for V30 ≥ and < 25%, respectively (p = 0.001). Stratifying by volume, lesions 2 cm or less (n = 215) had 1-year LC of 82% versus 70% (p = 0.013) for V30 ≥ and < 25%, respectively. Lesions > 2 to 3 cm (n = 32) had 1-year LC of 100% versus 43% (p = 0.214) for V30 ≥ and < 25%, respectively. V30 was still predictive of LC even after controlling for the use of immunotherapy and targeted therapy. Radionecrosis occurred in 2.8% of lesions and was not significantly associated with V30. CONCLUSIONS For a given prescription dose, an increased internal high-dose volume, as indicated by measures such as V30 ≥ 25%, is associated with improved LC but not increased toxicity in single-fraction SRS for melanoma BM. Internal dose escalation is an independent predictor of improved LC even in patients receiving immunotherapy and/or targeted therapy. This represents a dosimetric parameter that is actionable at the time of treatment planning and warrants further evaluation.