ESTRO 2022 - Abstract Book

S397

Abstract book

ESTRO 2022

Purpose or Objective To understand the role of relative biological effectiveness (RBE) modeling and dose-averaged linear energy transfer (LET d ) distribution in the treatment of sacral chordoma (SC) patients with carbon ion radiotherapy (CIRT). Materials and Methods We analyzed 50 SC patients consecutively treated before August 2018, with a local effect model (LEM)-based optimization, following a purely sequential boost schedule of 16 fractions (4.4 – 4.6 Gy(RBE) per fraction), with target shrinkage after 9 fractions. With a minimum follow-up of 12 months, 26 were classified as progressive disease; while 24 were reported as stable disease or partial regression and populated the control group for the analysis. To investigate patterns of failure, the relapse volume was contoured on the corresponding follow-up diagnostic sequence and described as in-field, field edge or out-of-field. Treatment plans were recalculated with the modified microdosimetric kinetic RBE model (mMKM) and target prescription dose (D RBE|50% ), near-to-minimum- (D RBE|95% ) and near-to-maximum- (D RBE|2% ) doses were compared, between the two cohorts, in both RBE systems. LET d distribution was evaluated for in-field relapsed cases with respect to the control group. A subset of cases was mMKM-optimized to test feasibility of a new treatment protocol, aiming at the improvement of the therapeutic ratio. Finally, the variation of LET d evaluators in relation to the RBE model used for plan optimization was quantified. Results Half of the relapse volumes were located in a well-covered high D LEM region, where D MKM and LET d resulted sub-optimal (Figure 1). Recalculated target D MKM|50% and D MKM|95% were respectively 10% and 18% lower than what we aimed at. Dosimetric evaluators showed no significant difference, in neither of the RBE models, between relapsed and control sets. On average, over these cases, median target LET d was significantly lower than the control cohort mean value (27 vs 30 keV/mm) (Figure 1b). Most notably, the volume receiving dose from high-LET particles (>50 keV/mm) lay substantially below recently reported data on the Japanese experience. mMKM-optimization generated plans with LET d distributions comparable to LEM- based, with no statistically significant difference in neither of the considered criteria.

Figure 1 Optimized D LEM (a), recalculated D MKM (b) and corresponding LET d (c) distributions of a high-dose relapsed case. The colorwash scale normalization values are: a) D LEM = 70.4 Gy (RBE), b) D MKM = 67.2 Gy (RBE) and c) LET d = 50 keV/mm. Plotted contours indicate the low dose - CTV (9 fractions - yellow), high dose - CTV (16 fractions - orange), GTV (red) and relapse (light blue) volumes. Conclusion Multi model RBE-evaluation and LET-based optimization could play a key role in the enhancement of the therapeutic ratio of CIRT for large radioresistant tumors such as sacral chordomas.

OC-0453 Proton arc therapy with optimized settings reduces toxicity for head and neck cancer patients

B.A. de Jong 1 , C. Battinelli 2 , E. Engwall 2 , G. Janssens 3 , J.A. Langendijk 1 , E.W. Korevaar 1 , S. Both 1

1 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands; 2 RaySearch Laboratories AB, Research and Development, Stockholm, Sweden; 3 Ion Beam Applications SA, Research and Development, Louvain-la-Neuve, Belgium Purpose or Objective Proton arc therapy has dosimetrical gains over conventional intensity modulated proton therapy (IMPT) for diverse indications. A relationship exists between normal tissue complication probability (NTCP), and the number of energy layers (ELs) -and beams in a proton arc plan. This work aims to investigate the number of ELs and beams required for optimal plan quality and impact on NTCP for oropharyngeal cancer patients selected for IMPT. Materials and Methods The RaySearch energy layer reduction algorithm iteratively selects ELs from beams equidistantly spaced over a 360 degree arc. The number of ELs and beams were varied, to determine their relationship with NTCP and the estimated delivery time for three patients. The delivery time was estimated for a “step and shoot” type delivery, in which the gantry remains stationary while the fields are delivered. Proton arc plans with optimized EL reduction settings were generated for ten oropharyngeal cancer patients previously treated with IMPT. Proton arc and clinical IMPT plans were compared in terms of integral dose and NTCP for dysphagia and xerostomia, while target coverage was robust.

Results