S2766
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
selected. For each patient, six VMAT plans were created using the following ASC settings: None, Very Low, Low, Moderate, High, and Very High. All plans were generated in Eclipse v16.1 for Halcyon and optimized using identical clinical constraints, ensuring compliance with hippocampal dose limitations. Plan quality was evaluated using total MU, multiple MLC modulation metrics, and a Plan Quality Index (PQI) integrating PTV coverage and organ-at-risk sparing. The clinical implications of complexity reduction on plan robustness were also reviewed. Results: Increasing ASC strength yielded a consistent decline in MLC modulation complexity. Total MU progressively decreased as ASC increased from None to High, indicating enhanced delivery efficiency. However, a marked MU rise occurred at the Very High ASC setting due to excessive aperture restriction. Target coverage demonstrated a mild but steady reduction with stronger ASC application, indicating a conformity trade-off. OAR doses—including hippocampal sparing metrics—remained largely unchanged across Low to High ASC levels and within clinically acceptable limits. Notable PQI deterioration and increased dose deviation were occasionally observed only under Very High ASC conditions. Conclusion: ASC is an effective optimization tool for reducing VMAT plan complexity in HS-WBRT on Halcyon. Low to Moderate ASC settings provide a balanced improvement in modulation efficiency and total MU without degrading dosimetric quality, whereas Very High ASC may introduce plan robustness concerns. These findings support the clinical use of appropriately selected ASC levels to enhance treatment efficiency while maintaining the stringent dosimetric requirements of hippocampus-sparing radiotherapy. Keywords: Hippocampus-Sparing Whole Brain Radiotherapy, ASC Digital Poster Highlight 715 Clinical Implementation of RapidArc Dynamic for Bi-lateral Head and Neck Cancer Lauren Weinstein 1 , Lauren Johnson 2 , Catherine Laferlita 3,4 , Kenton Thompson 3,4 , Vanessa Panettieri 3,4 , Sandro Porceddu 3,4 , Matthew Skinner 1 1 Radiation Oncology, Kaiser Permanente, South San Francisco, USA. 2 Advanced Oncology Solutions, Varian Medical Systems, Palo Alto, USA. 3 Department of Radiation Oncology, Peter MacCallum Cancer Centre, Melbourne, Australia. 4 Sir Peter MacCallum Department of Oncology, The University of Melbourne, Melbourne, Australia
Purpose/Objective: RapidArc Dynamic (RAD) (Varian Medical Systems, Palo Alto, USA) is a new treatment solution that combines Volumetric Modulated Arc Therapy (VMAT) and static angle modulated ports (STAMP) with a dynamic collimator rotation and a deterministic optimization algorithm. Early reports have indicated RAD's potential to improve plan quality for a range of anatomical sites, including head and neck. The aim of this study was to evaluate if a RAD approach shown to improve plan quality for advanced nasopharyngeal carcinoma in a retrospective comparative study1 could be translated to other head and neck sites and implemented clinically. Material/Methods: Five head and neck patients (2 nasopharynx, 2 oropharynx, 1 larynx) with 2-4 dose levels were planned using Eclipse 18.1 MR1(Varian Medical Systems, Palo Alto, USA) using both VMAT and RAD. All plans were optimized to meet clinically acceptable dose constraints. and calculated with AcurosXB 18.1.1, reporting dose-to-medium for TrueBeam with 6 MV and a Millennium MLC. VMAT plans used 3 full, coplanar arcs while RAD plans were created using a previously reportedbeam arrangement of 2 full arcs with 4 STAMPs per arc2,3. RAD weighting varied between balanced (0), with manual collimator optimization between STAMPs, and static (+1), with automatic collimator optimization. Organs of Interest (OOI), monitor units (MUs), and beam on time were compared for each patient’s VMAT and RAD plan. Results: Mean improvements in OOI sparing for RAD relative to VMAT are shown in Table 1. The larynx demonstrated the largest improvement in dose to an OOI using RAD, a mean reduction of 17.5% (503 cGy) (Figure 1). RAD plans also demonstrated mean dose reductions of 12.8%, 10.3%, 9.2%, 7.7% and 6.7% compared to VMAT for the brainstem, left and right parotids, esophagus and spinal cord, respectively. Monitor units increased with RAD plans by an average of 43.6% as did beam on time, which increased by 14.8%. Following physician peer review, all RAD plans were selected for treatment.
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