Dose prescription in carbon ion radiotherapy: a planning study to compare NIRS and LEM approaches with a clinically-oriented strategy

In carbon ion radiotherapy there is an urgent clinical need to develop objectivetools for the conversion of relative biological effectiveness (RBE)-weighteddoses based on different models. In this work we introduce a clinically orientedmethod to compare NIRS-based and LEM-based GyE systems, minimizingdifferences in physical dose distributions between treatment plans. Carbon ionplans were optimized on target volumes of cubic and spherical shapes, for RBEweighted dose prescription levels ranging from 3.6 to 4.4 GyE. Plans werecalculated for target sizes from 4 to 12 cm defining three beam geometries:single beam, opposed beam and orthogonal beam configurations. The twotreatment planning systems currently employed in clinical practice were used,providing the NIRS-based and LEM-based GyE calculations. Physical dosedistributions of NIRS-based and LEM-based treatment plans were compared.LEM-based prescription doses that minimize differences in physical dosedistributions between the two systems were found. These doses were comparedwith the mean RBE-weighted dose obtained with a Monte Carlo code (FLUKA)interfaced with LEM I. In the investigated dose range, LEM-based RBEweightedprescription doses, that minimize differences with NIRS plans, shouldbe higher than NIRS reported prescription doses. The optimal dose depends ontarget size, shape and position, number of beams and dose level. The opposedbeam configuration resulted in the smallest average prescription dose difference(0.45 +/- 0.09 GyE). The second approach of recalculating NIRS RBE-weighteddose with a Monte Carlo code interfaced with LEM resulted in no significantdifference with the results obtained from the planning study. The delivery of avoxel by voxel iso-effective plan, if different RBE models are employed, is notfeasible; it is however possible to minimize differences in a treatment plan withthe simple approach presented here. Dose prescription ultimately represents aclinical task under the responsibility of the radiation oncologist, the presentedanalysis intends to be a quantitative and objective way to assist the clinicaldecision