ESTRO 2025 - Abstract Book

S4357

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

constraints V30Gy≤65% and V40Gy≤45% were achieved in 100% and 93.3% of cases respectively. Nonetheless, conformity rate for bladder V50Gy and V60Gy were slightly lower: 88.8% for both dose constraints. Bowel bag V45Gy and V50Gy constraints for grade 1 GI toxicity were the most challenging dose constraints. Specifically, V45Gy≤78 cc and V50≤17cc were breached in 82% and 95% respectively. Nonetheless, mean bowel bag V45Gy and V50Gy were 129cc and 50.07cc, meeting grade 2 GI toxicity dose constraints in ≥90% of cases. The mean bowel bag V55Gy, V60Gy and V65Gy were respectively 0.62cc, 0.28cc and 0.10cc, easily achieving grade 1 GI toxicity dose constraints. Conclusion: This study showed that hypofractionated whole pelvic VMAT using the POP-RT trial dose constraints was feasible for high-risk prostate cancer, achieving optimal target volume coverage and adhering to most OAR dose constraints. While dose constraints for femoral heads, bladder, and rectum were largely achievable, meeting bowel bag constraints for grade 1 GI toxicity was challenging. Nonetheless, grade 2 GI toxicity constraints were consistently met in the majority of cases, suggesting the potential for safe and effective implementation of this protocol with careful planning and monitoring. Mini-Oral Impact of anatomical inter-fraction variations on dose distribution in robustly optimized proton therapy plans for HN patients: a preliminary study Maria Giulia Vincini, Martina Marcandalli, Luca Bergamaschi, Michela Onza, Martina Persiani, Mattia Zaffaroni, Marianna Alessandra Gerardi, Floriana Pansini, Marco Liotta, Stefania Comi, Massimo Sarra Fiore, Daniela Alterio, Barbara Alicja Jereczek-Fossa IEO, European Institute of Oncology, IRCCS, IEO, European Institute of Oncology, IRCCS, Milan, Italy Purpose/Objective: One of the main challenges in proton therapy (PT) is dealing with errors with respect to the simulation CT, which can greatly impact the dose distribution. Robust optimization allows the physicist to consider the above uncertainties during the plan optimization process. However, no consensus exists regarding which parameters are most suitable for robust optimization. This retrospective study aims to evaluate the effect of anatomical inter-fraction variations on the dose distribution of robustly optimised PT plans in HN patients undergoing PT at the IEO (European Institute of Oncology, IRCCS, Milan, Italy). Material/Methods: HN patients treated with PT at IEO were retrospectively considered. Planning was performed in RayStation v11a. The delivered plan was robustly optimized using simulation CT as the nominal scenario and including error scenarios in the planning process (a shift in all directions of 0.3 cm and a range uncertainty of 3.5%, for at least 21 calculated scenarios). For each patient, the plan was then recalculated on the re-evaluation CT of the second, forth, and last week of treatment, and changes in dosimetric indices were collected. Results: A total of five patients were included in the analysis. Prescribed doses ranged from 66Gy to 70 Gy in 30-33 fractions. The percentage change of the parameters D98%, D95% and D1% for high risk clinical target volume (CTV) were between -3.82% and 0.58%, -2.56% and 0.39%, 0.09% and 3.03%, respectively ( Figure 1 ). Concerning OARs, the percentage of organs (17 total considered)for each patient for which dosimetric constraints were always met was between 53% and 88%, however, in the majority of the cases, the constraint was not met already in the nominal scenario ( Table 1 ), pointing out a complex anatomy with the organ close to the target. The organ that presented the Keywords: hypofractionated radiotherapy, prostate cancer 2466