S2765
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
Material/Methods: Treatment plans for 15 patients with locally advanced oropharyngeal cancer from The Cancer Imaging Archive were prepared using both 5-field and 6-field PBS techniques. For the 5-field technique, gantry angles of 205, 300, 0, 60, and 155 were used (Fig 1). Couch rotation was not applied for any of these fields. A range shifter was used only for the anterior field, and blocking structures were employed to control the delivery of radiation doses from each beam. For the 6- field technique, gantry angles of 250, 310, 345, 15, 50, and 110 were used (Fig 2). For gantry angles 250 and 110, a 20-degree couch tilt from the cranial direction was applied. Range shifters were used for all treatment fields. Different field-specific target volumes were used for each field to control radiation dose delivery from each beam. Mandible dose metrics (V30GyE to V70GyE, Dmean, D0.03cc, D1%, D2%), other organ-at-risk (OAR) doses, and target coverage were recorded. Paired t-tests assessed dose differences, with statistical significance set at p<0.05.
Conclusion: The 5-field PBS technique offers superior mandible dose reduction at intermediate dose levels and better OAR sparing, suggesting a potential decrease in ORN risk and other radiation-induced toxicities in proton therapy for oropharyngeal cancer. Further studies with clinical outcomes are needed to confirm these dosimetric advantages. Keywords: Osteoradionecrosis, Proton Therapy, Technique Impact of Aperture Shape Controller Settings on VMAT Plan Complexity and Dosimetric Quality in Hippocampus-Sparing Whole Brain Radiotherapy Shu-Sheng Li 1 , Ti-Hao Wang 1 , JinHuei Ji 2 1 Department of Radiation Oncology, China Medical University Hospital, Taichung, Taiwan. 2 Research and Development, EverFortune.AI Co., Taichung, Taiwan Purpose/Objective: Hippocampus-sparing whole brain radiotherapy (HS- WBRT) requires highly conformal dose delivery to preserve neurocognitive function. However, VMAT planning often introduces high multileaf collimator (MLC) modulation and elevated monitor units (MU). This study evaluates the influence of Aperture Shape Controller (ASC) settings, implemented in Varian Eclipse v16.1, on plan complexity and dosimetric performance for HS-WBRT delivered on a Varian TrueBeam linear accelerator. Material/Methods: Five HS-WBRT clinical cases were retrospectively Digital Poster 610
Results: The 5-field technique significantly reduced mandible V30GyE (27.38% vs. 36.75%, p<0.001), V40GyE (20.10% vs. 26.98%, p<0.001), V50GyE (13.06% vs. 16.27%, p<0.001), and Dmean (19.74 GyE vs. 25.85 GyE, p<0.001) compared to the 6-field technique, but increased V70GyE (1.20% vs. 0.94%, p=0.005) and D2% (65.12 GyE vs. 64.64 GyE, p=0.040). The 5-field technique also lowered doses to most OARs, including oral cavity, parotid glands, and brainstem (p<0.001) (Table 1), with comparable spinal cord sparing and target coverage.
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