Session Item

To compare dose distribution plans between ultra-hypofractionated (UH) IMRT with VERO system (BrainLab AG, Tokyo, JP) and UH-SBRT with CyberKnife (CK) radiosurgery system (Accuray Inc., Sunnyvale, CA) using image-guided virtual fiducial markers tracking, for localized prostate cancer (PCa) with concomitant focal boost to the dominant intraprostatic lesion (DIL).

Fifteen patients affected by intermediate or high-risk PCa, with at least one visible DIL on previously performed mpMRI, were included in this in-silico planning study.

Dosimetric results were compared between fifteen delivered VERO IMRT and thirty respective in-silico CK SBRT plans (15 CK plans with prescription to 95% isodose line (IDL) plus other 15 CK plans with prescription to 85% IDL), aiming to perform UHRT of 40 Gy in 5 fractions (8 Gy/fr) to the DIL while treating the whole prostate to 36.25 Gy in 5 fractions (7.25 Gy/fr) every other day, using a 5mm isotropic expansion margin, except 3mm posteriorly, for prostate gland planning target volume (PTV-p) and 3 mm isotropic margin for PTV of the DIL (PTV-d).

A total of 45 plans were compared for this study. Both in-silico SBRT and delivered IMRT plans were able to create the dose gradient (+10.34%) needed for the SIB prescription and have reached the primary planning goal of D95% > 95% for both PTVs in every plans. However, the comparison of dose coverage objectives established that statistically significant difference exists.

Comparing delivered IMRT vs in-silico SBRT plans with prescription to 85% IDL, showed better coverage of PTV-p and, especially, of PTV-d, where a significant increase of median D95%, D2%, D0.03cc, D50% V100% and V110% was observed respect to delivered plans.

The same difference for PTVs dose coverage, was maintained in a lower degree when comparing the median value of delivered IMRT plans versus in-silico SBRT plans with prescription to 95% IDL.

Concerning the organs at risk (OARs), the three techniques maintained the dose distribution well below all the prescribed OARs dose constraints in every plan. Overall, the in-silico SBRT CY plans with prescription to 85% IDL resulted to be the best technique able to reach a maximum dose of 47 Gy to the DIL while respecting all the OARs constraints applied in IMRT-delivered VERO plans. An example of dose distribution of the three tecniques can be found in Figure 1.

Based on growing experience in the literature, adding a high focal boost to the DIL, identified through a mpMRI, could improve oncological outcomes in localized PCa patients. In this setting, SBRT delivery technique is required to improve tumor control probability while respecting OARs dose constraints.

This in-silico planning comparison proved that CK UH-SBRT plus a focal boost to the DIL for localized PCa appears to be feasible. These encouraging dosimetric results need to be confirmed in phase II/III clinical trial such as the upcoming mono-institution phase II “PRO-SPEED” IEO trial.