ESTRO 2025 - Abstract Book

S2819

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2025

plans differed by ≤ 0.7Gy for mean doses and ≤ 1.8Gy for maximum doses. Pareto surface generation was not repeatable, and plan navigation using only sliders led to suboptimal PTV coverage.

Conclusion: MCO effectively balances the TO between PCM sparing and PTV54 coverage, offering increased OAR sparing for minimal reduction in coverage. It may be a valuable tool in the context of personalised care.

Keywords: Multicriteria optimization

References: 1. Nutting C, Finneran L, Roe J, et al. Dysphagia-optimised intensity-modulated radiotherapy versus standard radiotherapy in patients with head and neck cancer (DARS): a phase 3, multicentre, randomised, controlled trial. Lancet Oncol . 2023;24(8):868-880. DOI: 10.1016/S1470-2045(23)00265-6.

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Digital Poster Full Monte Carlo Implementation in "Island Tumors": Exploring Dosimetric Impact and Significant Discrepancies SANTIAGO VELAZQUEZ-MIRANDA 1,2 , Carlos Manuel Sequeira-Ribeiro-Nunes 3 , ANA Ureba-Sanchez 3 , Maria Gonzalez Vizuete 4 , Antonio Leal-Plaza 5 1 Medical Physics, HOSPITAL UNIVERSITARIO VIRGEN DEL ROCIO, SEVILLE, Spain. 2 GI.TER. GI., INSTITUTO BIOMEDICINA SEVILLA, SEVILLE, Spain. 3 Fisiología Médica y Biofísica, UNICERSIDAD DE SEVILLA, SEVILLE, Spain. 4 Medical Physics, HHOSPITAL DE MERIDA, SEVILLE, Spain. 5 Fisiología Médica y Biofísica, UNIVERSIDAD DE SEVILLA, SEVILLE, Spain Purpose/Objective: Island tumors are generally defined as tumors smaller than the depth of maximum dose deposition, resulting in re build-up effects at the interface between lung tissue and the tumor. These tumors present challenges in radiotherapy due to discrepancies between treatment planning system (TPS) dose calculations and Full Monte Carlo (MC) simulations. The re-build-up region prevents electronic equilibrium, complicating dose calculations and leading TPS algorithms to misrepresent dose distributions, particularly at high-density interfaces. Monte Carlo simulations using the CARMEN MC TPS provide more accurate dose distributions, highlighting these discrepancies. This study quantifies TPS-MC differences for island tumors, emphasizing clinical implications and proposing a robust SBRT procedure to improve accuracy. Material/Methods: SBRT treatments were administered to thirteen patients with Island Tumors. Treatment regimens followed topographic criteria from M. Scorsetti and F. Alongi's guidelines. Patients had a functional status of 0–2 and were simulated using the ExaCradle stereotactic system (Anatge) and MoldCareBR-3 (Alcare). Dose calculations were performed with the CCC algorithm in Pinnacle (version 16.1) and delivered on a TrueBeam linear accelerator equipped with a 5 mm Millennium™ MLC. A 30 mm diameter sphere centered on the ITV barycenter was used for treatment planning (Figure 1), prescribing the dose directly to the GTV. Four key aspects were evaluated: 1. Margin expansion to improve robustness. 2. Increased secondary radiation component for better alignment between calculated and measured doses. 3. Low fluence modulation to reduce the likelihood of interplay effects, significant given the GTV's diameter comparable to the MLC leaf width. CARMEN MC TPS in verification mode calculations to analyze discrepancies with the TPS.