Optic nerve constraints for carbon ion RT at CNAO - Reporting and relating outcome to European and Japanese RBE.

Abstract

Until now, carbon ion RT (CIRT) dose constraints for the optic nerve (ON) have only been validated and reported in the NIRS RBE-weighted dose (D_NIRS). The aim of this work is to improve CNAO's RBE-weighted dose (D_LEM) constraints by analyzing institutional toxicity data and by relating it to D_NIRS. A total of 65 ONs from 38 patients treated with CIRT to the head and neck region in the period 2013-14 were analyzed. The absorbed dose (D_Abs) of the treatment plans was reproduced and subsequently both D_LEM and D_NIRS were applied, thus relating CNAO clinical toxicity to D_NIRS. Median FU was 47 (26-67) months. Visual acuity was preserved for the 56 ONs in which the old constraints were respected. Three ONs developed visual decline at D_LEM|1% ≥71 Gy(RBE)/D_LEM|20% ≥68 Gy(RBE), corresponding to D_NIRS|1% ≥68 Gy(RBE)/D_NIRS|20% ≥62 Gy(RBE). Dose recalculation revealed that NIRS constraints of D_NIRS|1% ≤40 Gy(RBE)/D_NIRS|20% ≤28 Gy(RBE) corresponded to D_LEM|1% ≤50 Gy(RBE)/D_LEM|20% ≤40 Gy(RBE). Reoptimization of treatment plans with these new D_LEM constraints showed that the dose distribution still complied with NIRS constraints when evaluated in D_NIRS. However, due to uncertainties in the method, and to comply with the EQD2-based constraints used at GSI/HIT, a more moderate constraint relaxation to D_LEM|1% ≤45 Gy(RBE)/D_LEM|20% ≤37 Gy(RBE) has been implemented in CNAO clinical routine since October 2018. New D_LEM constraints for the ON were derived by analyzing CNAO toxicity data and by linking our results to the experience of NIRS and GSI/HIT. This work demonstrates the value of recalculating and reporting results in both D_LEM and D_NIRS.