Hypofractionated Simultaneous Integrated Boost (SIB) on Dominant Intra-prostatic Lesions (DILs) By Biologically Based IMRT Optimization

Abstract

Materials/Methods: Seven intermediate/high risk prostate cancer patients (pts) were submitted to T2WI, T1WI and DWI MRI: imaging showed evidence of one DIL in four pts and two DILs in three pts in the peripheral zone. PTVDIL was obtained by 5 mm isotropic expansion of DIL. A favorable pts (one DIL without overlap between PTVDIL and rectum, urethra and bladder, pts 1), and the worst one (two DILs, overlap rectum-PTVDILs: 24.3 cm3, overlap urethra-DILs:0.1 cm3, pts 2) were selected for this study. Based on the assumption that local failures are mainly due to highly radioresistant clonogens within the DILs, we considered different parameters for DILs and the prostate outside DILs (CTV). In the Poisson-like model used during optimization, we assessed the following values: TD50 = 57.3 Gy and 68.3 Gy for CTV and TD50 = 82.3 Gy and 95 Gy for DIL. Values of g50 ranging between 4 and 8 and a/b = 10 were considered, consistently with the hypothesis that inter-patient dose-effect variability is mainly due to the presence of resistant DILs. Different combinations of these parameters were tested with the aim of maximizing TCP while constraining NTCPs below 5%. A noncoplanar 6 MV 9-field arrangement was used and the number of fractions was fixed to 28, following our hypofractionated protocol: planning were optimized using the recently available biological optimization and evaluation module of the Varian Aria-Eclipse system; NTCP/TCP calculations were also compared with the corresponding results using an independent software (Bioplan).