S1946
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Digital Poster 3764 Dosimetric Comparison of Nasopharyngeal Carcinoma Radiotherapy Using Conventional VMAT vs VMAT with Modulated Ports and Dynamic Collimator Rotation Wai Lung Wong 1 , Hing Ming Hung 1,2 , Vincent Wan Shun Leung 2 1 Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong, China. 2 Department of Health Technology and Informatics, The Hong Kong Polytechnic University, Hong Kong, China Purpose/Objective: Nasopharyngeal carcinoma (NPC) poses unique challenges in radiation oncology due to its intricate anatomical location and proximity to critical organs of interest (OIs), necessitating highly conformal dose delivery. This study introduces a novel volumetric modulated arc therapy technique (VMATdyn) combined with static intensity modulated ports and dynamic collimator adjustment, to further refine dose sculpting and improve target coverage and normal tissue sparing. However, the clinical benefits of VMATdyn for NPC have not yet been reported. The objective of this study is to assess the dosimetric advantages and feasibility of this technique compared to conventional VMAT (VMATcon) plans, with a focus on target coverage and critical organ sparing. Material/Methods: Fifteen retrospective NPC cases were re-computed with a prescription dose of 70 Gy in 35 fractions to the primary nasopharyngeal tumor and involved lymph nodes and a simultaneous integrated boost of 63 Gy in 35 fractions to bilateral retropharyngeal lymph nodes (Levels II, III, and Va). For each case, two plans were generated: a VMATcon plan using four static full arcs (collimator angles fixed at 15°, 345°, 80°, and 85°) and a VMATdyn plan employing two dynamic full arcs and two static intensity modulated ports (gantry angles fixed at 35° and 325°) with algorithm-optimized dynamic collimator rotation. The planning goal was to maximize dose coverage to Planning Target Volumes(PTVs) without exceeding the neurological OIs dose guidance. Identical planning objectives, priorities, and normal tissue optimization parameters were applied to both techniques to ensure a consistent dose comparison. Dosimetric endpoints for PTVs, Gross Tumour Volume(GTVs) and OIs were analyzed statistically to quantify improvements.
and organ-at-risk (OAR) doses for lungs, kidneys, and lenses were extracted from Monaco treatment planning systems for VMAT and Accuray Precision for HT. Results: A total of 49 patients were treated between January 2022 and June 2025 (13 VMAT vs 36 HT). Patients were younger in the VMAT cohort compared to HT (mean 8.4 years vs 21.1 years; p <0.001). VMAT patients were exclusively paediatric ( ≤ 18 years), whereas HT treated both paediatric (44%) and adult (56%). HT achieved higher target coverage for V90 (99.6% vs 99.0%, p<0.001) and V95 (98.6% vs 97.8%, p=0.007) compared to VMAT, whereas VMAT achieved higher V100 (90.0% vs 85.4%; p < 0.001) compared to HT. D0.03cc was higher with VMAT (117.5% vs 110.5%; p < 0.001). Mean beam-on time for VMAT was shorter for VMAT when compared to HT (21 minutes vs 47 minutes, p<0.001). Mean lung doses was lower with VMAT (8.3 Gy vs 10.7 Gy; p < 0.001) while mean kidney doses were comparable between both techniques (9.4 Gy vs 9.6 Gy; p = 0.178). Lens D0.03cc doses were significantly lower with VMAT (8.6 Gy vs 12.4 Gy; p < 0.001). Findings were consistent in the paediatric subset, showing improved OAR sparing and shorter beam-on times with VMAT. Conclusion: Both VMAT and HT produced clinically acceptable TBI plans with adequate target coverage. Variations in OAR doses reflected differences in PTV definitions and dose constraints. Despite shorter beam-on time with VMAT, overall in-room time should be compared to assess true resource impact. The joint UMMC–HKL experience shows that advanced TBI can be effectively implemented in a middle-income Asian setting using existing infrastructure, supporting efforts towards national standardisation protocols. References: 1. Hoeben, B. A. W. et al. ESTRO ACROP and SIOPE recommendations for myeloablative Total Body Irradiation in children. Radiotherapy and Oncology 173, 119–133 (2022).2. Simiele E et al. Radiation as an Immune Modulator: Where We Are With Modern Total Body Irradiation. Seminars in Radiation Oncology. 2025 Jan;35(1):67-86. DOI: 10.1016/j.semradonc.2024.10.003. PMID: 39672644. 3. Kovalchuk, N. et al. The Stanford Volumetric Modulated Arc Therapy Total Body Irradiation Technique. Pract Radiat Oncol 12, 245–258 (2022). Keywords: Total body irradiation, paediatric, technique
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