S2801
RTT - RTT contouring, target definition, and treatment planning
ESTRO 2026
PTV-V95%, PTV-V110%, treatment times, and online shifts were compared between the two configurations using an independent two-sample t-test, with a significance level set at 0.05.
health-related quality of life of breast cancer patients. European Journal of Surgical Oncology 2020;46:1848– 53.3. Jagsi R, et al. Impact of Radiotherapy on Complications and Patient-Reported Outcomes after Breast Reconstruction. J Natl Cancer Inst 2018;110:157–65.4. Boersma L, et al. Model-Based Selection for Proton Therapy in Breast Cancer: Development of the National Indication Protocol for Proton Therapy and First Clinical Experiences. Clin Oncol; 2022 Apr;34(4):247-257. Keywords: Proton therapy, Tissue expander, Evaluation of a simplified two-isocenter VMAT approach for lower extremity treatment in Total Marrow Irradiation Simone Antonetti 1 , Nicola Lambri 1 , Leonardo Nicotera 1 , Ciro Franzese 2 , Aurora Zucca 1 , Carmela Galdieri 1 , Marta Scorsetti 1,2 , Pietro Mancosu 1,2 1 Radiotherapy and Radiosurgery Department, IRCCS Humanitas Research Hospital, Rozzano Milan, Italy. 2 Department of Biomedical Sciences, Humanitas University, Milan, Italy Digital Poster 4473 Purpose/Objective: Total Marrow Irradiation (TMI) is a modern evolution of Total Body Irradiation (TBI) that allows selective irradiation of skeletal structures while sparing organs of interest. When delivered with VMAT, TMI provides high conformity but requires several isocenters and prolonged treatment sessions. In particular, the lower extremities often demand multiple overlapping fields, increasing treatment complexity and patient discomfort [1]. The role of RTTs in patient setup, immobilisation, and verification directly influences treatment accuracy and efficiency. Based on feedback from treated patients, the RTT team promoted this study to simplify workflow for the lower extremities and reduce patient discomfort. This study specifically investigates whether a two-isocenter configuration for the lower extremities can maintain setup accuracy while improving treatment efficiency and patient experience. Material/Methods: A total of 150 patients treated with TMI between July 2013 and September 2025 were retrospectively selected from our institutional database. VMAT delivery typically required up to five isocenters for the upper body and three isocenters for the lower extremities, with collimator angles set at 90°. Since October 2020, a new configuration has been implemented using two isocenters for the lower extremities, with collimator angles of 5°/355° (Figure 1). CBCT imaging was performed for each isocenter.
Results: Dose coverage was comparable between
configurations: PTV-V95% was 99.5±0.4% vs 99.4±0.2% (three vs two isocenters; p>0.05), while the PTV-V110% was 7±7% vs 8±8% (p>0.05). Average treatment times significantly improved from 88±32 min to 73±9 min (p<0.01) when using three versus two isocenters for the lower extremities, respectively. The mean absolute online translations in lateral, vertical, and longitudinal directions with three isocenters were 3±5 mm, 2±3 mm, and 2±3 mm, respectively. With the new two- isocenter configuration, the translations were 3±3 mm (p>0.05), 3±4 mm (p<0.02), and 2±5 mm (p>0.05). Distributions for the two groups are shown in Figure 2. Although the slight increase in vertical translations was significant, it remained clinically acceptable, with only 4% of applied shifts exceeding 1 cm. The reduction in treatment time variability and consistent magnitudes of online translations suggest decreased setup variations overall.
Conclusion:
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