S1706
Physics - Dose prediction/calculation, optimisation and applications for particle therapy planning
ESTRO 2026
(for physical and biological dose) and LETd-volume histograms were collected for each plan. Select dose and LETd statistics were additionally investigated for possible correlations with the percentage of viable tumor cells after treatment. Results: All the original plans showed comparable target coverage according to their respective RBE models (Figure 1), with carbon ions providing slightly steeper dose gradients. However, conventionally optimized CIRT plans demonstrated a significant loss in coverage after dose recalculation with mMKM, with median D95% in the GTV decreasing by 10.3%. Conversely, for LET-optimized mMKM-based plans, recalculation with LEM-I resulted in a difference of less than 3%. For comparison, the McNamara model for the proton plans yielded an increase of approximately 6%. No statistically significant correlations were found between any biological dose, physical dose or LETd statistics and the percentage of viable tumor cells.
Khamfongkhruea C, Tattenberg S, Zarifi M, Verburg J, Richter C. Inter-center comparison of proton range verification prototypes with an anthropomorphic head phantom. Phys Med Biol. 2024;69:225010. Keywords: proton therapy,prompt gamma-ray timing,verfication Digital Poster 823 RBE model uncertainties in carbon ion radiotherapy for soft tissue sarcoma: dosimetric evaluation and the role of LET Natalia Sycheva 1,2 , Katharina Kozyra 1,3 , Hanna Waldsperger 1,3 , Semi B Harrabi 1,3 , Jürgen Debus 1,2 , Andrea Mairani 1,4 , Thomas Tessonnier 1,2 1 Heidelberg Ion-Beam Therapy Center (HIT), Department of Radiation Oncology, Heidelberg University Hospital, Heidelberg, Germany. 2 Clinical Cooperation Unit Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany. 3 National Center for Tumor Diseases (NCT), Heidelberg University Hospital, Heidelberg, Germany. 4 Clinical Cooperation Unit Translational Radiation Oncology, German Cancer Research Center (DKFZ), Heidelberg, Germany Purpose/Objective: In the neoadjuvant radiotherapy of soft tissue sarcoma of the extremities, particle therapy offers unique advantages due to its superior dose distributions and, in the case of carbon ion radiotherapy (CIRT), higher linear energy transfer (LET). However, uncertainties remain in the calculation of relative biological effectiveness (RBE) and corresponding biological dose in CIRT, with the Local Effect Model I (LEM-I) and the modified microdosimetric kinetic model (mMKM) producing different predictions at lower LET. In this study, we assessed RBE-related uncertainties in CIRT using different planning strategies and compared them to those observed in proton beam therapy. Material/Methods: Clinical treatment plans for a total of 46 patients with soft tissue sarcoma who received either proton beam therapy (n = 22) or CIRT (n = 24) were analyzed. All patients received a total dose of 39 Gy in 13 fractions.1 Among CIRT plans, 21 were optimized without any dose-averaged LET (LETd) objectives using LEM-I, while 3 most recent plans were optimized using mMKM with an additional minimum LETd objective of 45-55 keV/ μ m in the target and adjusted to fit LEM-I organ- at-risk constraints. To assess the differences in dose predictions from the radiobiological models, LEM-I plans were recalculated with mMKM, and the proton plans, originally designed with a fixed RBE of 1.1, were recalculated using the McNamara model. Dose-volume
Conclusion: In the CIRT of soft tissue sarcoma of the extremities, the use of LEM-I without LETd escalation strategies results in large RBE uncertainties and may lead to inferior target coverage compared to proton beam therapy. LETd optimization can help reduce this uncertainty and increase the efficacy of CIRT. References: 1. Brügemann D, Lehner B, Kieser M, Krisam J, Hommertgen A, Jaekel C, Harrabi SB, Herfarth K, Mechtesheimer G, Sedlaczek O, Egerer G, Geisbüsch A, Uhl M, Debus J, Seidensaal K. Neoadjuvant irradiation of extremity soft tissue sarcoma with ions (Extrem- ion): study protocol for a randomized phase II pilot trial. BMC Cancer. 2022;22:538. doi:10.1186/s12885- 022-09560-x Keywords: carbon ion radiotherapy, LETd optimization, RBE
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