ESTRO 2023 - Abstract Book

S1755

Digital Posters

ESTRO 2023

For the considered beam configuration, the LETd increased towards the beam’s distal edge, being the LET_D98% in the GTV higher than 44.0 keV/µm for only two patients (median in the cohort: 40.6 keV/µm). Due to the recognized LEM-I overestimation of the RBE for lower-LET particles, D_Bio|HIT was higher than D_Bio|NIRS at the beam’s entrance channel, and therefore in the GI tract. The NIRS constraint to the GI tract D_2cm3= 46.0 Gy[RBE]_NIRS led to a HIT possible constraint of D 2cm3= 49.1 Gy[RBE]_HIT, instead of the HIT current standard D_max = 43.2 Gy[RBE]_HIT.

Conclusion The results suggested that a more relaxed constraint to the GI tract may be considered without leading to toxicity, and therefore improving tumor coverage. Due to differences in the beam configuration, a lower LETd was observed in the GTV in comparison to the Japanese experience. However, a higher D_Bio|NIRS observed in the tumor may be linked to the preservation of the tumor LC currently observed within the PACK-cohort. Future analyses of other clinical endpoints are foreseen alongside the progression of the PACK-trial.

PO-1985 The importance of spot distribution and its influence on the LEM/MKM model relationship in CIRT

J. Gora 1 , P. Fossati 1,2 , M. Mumot 1 , M. Shamshad 1 , M. Stock 1,2 , A. Carlino 1

1 MedAustron Ion Therapy Center, Medical Department, Wiener Neustadt, Austria; 2 Karl Landsteiner University of Health Sciences, -, Krems, Austria Purpose or Objective Carbon ion radiotherapy (CIRT) has been clinically employed in different centers with different RBE models (mostly LEM or MKM). Although many LEM/MKM translation strategies are already used in clinical routine to convert dose prescriptions and OARs dose constraints, there are many factors (not only model, but also treatment planning related) that can influence the LEM/MKM relationship. This study focus on spot distribution strategies and its influence on the translation between the models. Materials and Methods 20 patients with various indications and tumor sites, previously treated with CIRT were selected. They followed protocols derived from the Japanese experience and adjusted for the different RBE model (4.1-4.8 Gy RBE/fx). Prescription and OAR constraints were translated from MKM RBE-weighed doses (obtained from clinical data) into LEM. All clinical plans had between 2-4 beams, spaced at least by 30 ◦ . They were clinically optimized with LEM model, implemented in RayStation11B (RaySearch Laboratories) and subsequently recomputed with the MKM (the physical doses remain identical). Additionally, the LETd distribution was evaluated. 2 main strategies of treatment planning were considered: a) Blocking: placement of the spots was not allowed in the certain regions (forbidding beams going through OARs to reach the contralateral portion