ESTRO 2024 - Abstract Book

S3609

Physics - Dose prediction, optimisation and applications of photon and electron planning

ESTRO 2024

CyberKnife UH-SBRT for localized PCa: a preliminary report from the PRO-Speed prospective Trial

Giovanni Carlo Mazzola 1 , Giulia Marvaso 1,2 , Giulia Corrao 1 , Karl Amin 1,2 , Dario Zerini 1 , Stefano Durante 1 , Andrea Vavassori 1 , Maria Giulia Vincini 1 , Mattia Zaffaroni 1 , Elena Rondi 3 , Sabrina Vigorito 3 , Giuseppe Ronci 3 , Federica Cattani 3,2 , Francesco Alessandro Mistretta 4 , Stefano Luzzago 4 , Gennaro Musi 4,2 , Ottavio De Cobelli 4,2 , Barbara Alicja Jereczek-Fossa 1,2 1 European Institute of Oncology, Division of Radiation Oncology, Milan, Italy. 2 University of Milan, Department of Oncology and Hematology-Oncology, Milan, Italy. 3 European Institute of Oncology, Unit of Medical Physics, Milan, Italy. 4 European Institute of Oncology, Division of Urology, Milan, Italy

Purpose/Objective:

To compare dose distributions and dosimetric data between previously in-silico generated and new patient delivered plans of ultra-hypofractionated (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).

Material/Methods:

Dosimetric data of the first 15 patients treated in the IEO PRO-Speed Trial, affected by localized PCa with at least one visible DIL on previously performed mpMRI, were included in this analysis.

Dose distribution plans coming from 15 patient-delivered plans treated from December 2022 to October 2023 were compared with 15 previously generated in-silico CK SBRT plans from our previous dosimetric planning study (both in-silico and patient-delivered plans with prescription to 85% isodose line). A simultaneous boost of 40 Gy in 5 fractions (8 Gy/fr) was prescribed to the DIL while a total dose of 36.25 Gy (7.25 Gy/fr) was prescribed to the whole prostate plus proximal part of seminal vesicles 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).

Results:

A total of 30 plans were compared for this preliminary study. Age, comorbidity, PCa stage, DIL position and PTV volumes (PTV-p and PTV-d) of the two populations compared were similar.

Every in-silico and patient-delivered SBRT plans reached the dose gradient (+10.34%) needed for the SIB prescription and fully achieved the primary planning goal of D95% > 95% for both PTVs. However, the comparison of dose coverage objectives established that small statistically significant difference exists. Compared to the previous in-silico generated SBRT plans, the SBRT-delivered plans showed a slightly reduction in dose coverage for PTV-d and of PTV-p ( Figure 1 ). Particularly for PTV-d, the median values of D95% decreased by 1,1% (101,4% vs 100,3%) while also V100% decreased by 2% (98,5% vs 96,5%) and V110% decreased by 5,1% (26,1% vs 21,2%). In regard to PTV-p, the D0.03cc decreased by 1,8% (114,8% vs 113%), the D2% decreased by 1,5% (110,5% vs 109%) while, as for the PTV-d, the V100% showed a major reduction of 5.5% (81,6% vs 76,1%) and the V110% decreased by 2,5% (3,4% vs 0,9%). For both PTV-d and PTV-p, the median values of D98%, D50% and V95% showed no differences in the comparison. Concerning the organs at risk (OARs), both in-silico and delivered plans maintained the dose distribution well below all the prescribed OARs dose constraints, but small differences were noticed. Summary of the main differences regarding the main OARs is reported in Table 1 . Comparing the rectum constraints for the in-silico plans vs the patient-delivered ones, an increase in the median values for the V18Gy by