ESTRO 2023 - Abstract Book

S1725 ESTRO 2023 for all plans on a voxel-level using the McNamara model with an / ratio of 2 Gy. D0.3cm ³ in the brainstem and brainstem core was evaluated for a constant RBE of 1.1 and the McNamara RBE. LETd0.3cm ³ was evaluated in the overlap between the 95% isodose and the brainstem/brainstem core. Results The maximum LETd in the brainstem varied considerably between plans. The interquartile range of LETd0.3cm ³ at the 95 % cutoff was 0.8 keV/ µ m, the maximum was 4.4keV/ µ m (figure 2), and the median was 3.5 keV/ µ m (figure 1b). LETd0.3cm ³ in the brainstem core tended to be slightly lower with a median of 3.3keV/ µ m. Taking variable RBE into account, D0.3cm ³ increased from 53.2 GyRBE (RBE 1.1) to 59.2 GyRBE (McNamara RBE). Brainstem core median D0.3cm ³ rose from 52.5 GyRBE (RBE 1.1) to 58 GyRBE (McNamara RBE). For five patients, brainstem D0.3cm ³ was greater than 60 GyRBE with the McNamara RBE (figure 1c). All of these patients were below the age of six years at treatment. In two of them, the clinical target volume (CTV) was posterior to the brainstem, in two, the CTV was superolateral to the brainstem, and in one, the CTV wrapped around the brainstem. In all five patients, the CTV overlapped partly with the brainstem. Four of them had three-field plans, but one of them had five fields. Digital Posters

Conclusion Even when field configurations are chosen to mitigate elevated LETd, there are still regions in the brainstem in some patients with relatively high LETd and dose. There is no significant clinical evidence that this increases the risk of brainstem toxicity. However, new strategies for reducing LETd in the brainstem and other high-risk organs for PBS should be investigated.

PO-1962 Respiratory gated proton therapy for liver cancer: Setup accuracy and respiratory motion variabilty

E. Worm 1 , S. Nankali 2 , J.B. Thomsen 2 , L. Stick 2 , M. Høyer 2 , B. Weber 2 , H.R. Mortensen 2 , P.R. Poulsen 2