CyberKnife Ultra-Hypofractionated SBRT for Localized Prostate Cancer with Dose Escalation to the Dominant Intraprostatic Lesion: In Silico Planning Study

The aim is to evaluate the feasibility of ultra-hypofractionated (UH) SBRT with CyberKnife® (CK) radiosurgery (Accuray Inc., Sunnyvale, California, USA) for localized prostate cancer (PCa) with a concomitant focal boost to the dominant intraprostatic lesion (DIL). Patients with intermediate/highrisk PCa, with at least one visible DIL on multi-parametric MRI, were included. For each, two CK-SBRT in silico plans were calculated using 95% and 85% isodose lines (CK-95%, CK-85%) and compared with the UH-DWA plan delivered with VERO®. All plans simulated a SIB prescription of 40 Gy to PTV-DIL and 36.25 Gy to the whole prostate (PTV-prostate) in five fractions every other day. Fifteen patients were considered. All plans reached the primary planning goal (D95% > 95%) and compliance with organs at risk (OARs) constraints. DVH metrics median values increased (p < 0.05) from UH-DWA to CK-85%. The conformity index of PTV-DIL was 1.00 for all techniques, while for PTV-prostate was 0.978, 0.984, and 0.991 for UH-DWA, CK-95%, and CK-85%, respectively. The CK-85% plans were able to reach a maximum dose of 47 Gy to the DIL while respecting OARs constraints. CK-SBRT plus a focal boost to the DIL for localized PCa appears to be feasible. These encouraging dosimetric results are to be confirmed in upcoming clinical trials such as the phase-II “PRO-SPEED” IEO trial.