ADAPTIVE STRATEGY FOR EXTERNAL BEAM RADIOTHERAPY IN PROSTATE CANCER: MANAGEMENT OF THE GEOMETRICAL UNCERTAINTIES WITH ROBUST OPTIMIZATION.

Abstract Objective We aim to develop and validate a new adaptive method (O-ART) for prostate cancer (PCa) radiotherapy (RT), using an off-line strategy to improve treatment personalization by modelling the internal target volume (ITV) on individual basis and account for the residual set-up uncertainties by robust optimization. Methods and Materials 20 patients with intermediate-high PCa treated with radical RT were enrolled. The first step of the O-ART strategy is the identification of a patient specific ITV based on the KV-cone beam CT (kV-CBCT) datasets acquired during the first five fractions. The deformable image registration (DIR) algorithm ANACONDA was used to propagate the clinical target volumes (CTV) from the reference planning CT to the CBCTs; these contours were assessed by a radiation oncologist. In the second step the ITV was used to re-plan the treatment using a min-max robust algorithm based on the worst scenario optimization. The CTV coverage and organs at risk (OAR) sparing achieved with the robust plan (RP) were analyzed and compared with the original standard plan (SP) calculating the dose distributions on the residual CBCTs. Results RP showed to achieve optimal coverage of CTV even in the worst scenario and significant less dose to rectum and bladder. CTV coverage of RP was statistically better than SP in terms of D99 (p=0.008) and D98 (p=0.02). Statistically significant mean dose reduction and D2 reduction was noted for rectum (p Conclusions This adaptive strategy in prostate cancer radiotherapy is feasible and safe; it may be used to adapt the treatment with better target coverage and OARs sparing than standard PTV based planning.