ESTRO 2024 - Abstract Book

S2544

Clinical - Urology

ESTRO 2024

Material/Methods:

This is a multi-institutional analysis of 238 patients with histologically diagnosed PCa treated with 5 fractions SBRT with the following doses: 35 Gy and 36.25 Gy delivered to low/favourable intermediate, and unfavourable intermediate/high risk, respectively. 120 (55.5%) patients were treated with MRgRT and 118 (49.5%) with Cyberknife at two different Institutions. The primary end-point was acute toxicity comparison. Patients were included in the analysis if they had at least 6 months of follow-up for the acute toxicity end-point evaluation. Toxicity assessment was performed according to CTCAE v5.0 scale. Secondary end-point was late toxicity. A PSM analysis was performed by matching patients 1:1 by the following characteristics: prostate volume (threshold 50 cc), IPSS before SBRT (threshold 7), and total treatment dose (35 Gy versus 36.25 Gy).

Results:

The population after PSM was represented by 150 individuals. The median PSA before SBRT was 7.27 ng/ml (range 0.49-41 ng/ml). The median prostate volume was 50 cc (range 20-113.5). The PTV margin was 5 mm in all directions and 3 mm posteriorly in both groups. In the CyberKnife system, the median maximal dose administered to the CTV was 44.3 Gy (range 42.6-46.7), compared to 38.7 Gy in MRgRT. The median duration of beam-on time for all therapy sessions combined was 25 minutes (range 13-56 minutes) for the CyberKnife system and the overall treatment duration for MRgRT (baseline imaging, contouring, planning, pretreatment verification and treatment delivery) was 38 minutes (range 34-39 minutes). Globally, acute G1, G2, and G3 genitourinary (GU) toxicity occurred in 49.3%, 12%, and 2.7% cases, respectively. Acute G1, and G2 gastrointestinal (GI) toxicity occurred in 18%, and 6%, cases, respectively. In the univariate analysis, MRgRT patients reported a significantly lower acute G1 GU toxicity (G1: 37.3% versus 61.3%,) (p=0,007), whileG2 (10.7% versus 13.3%), and G3 toxicity (2.7% in both groups) did not differ significantly. Acute GI toxicity did not differ significantly (p=0.28). Late grade 2-3 GU toxicity did not differ significantly between groups (G2: 9.3% versus 10.6%, G3 2.7% versus 0%; p=n.s.). There was no difference between groups in late GI toxicity (p=0.39).

In the multivariate analysis (MVA), treatment technique was the only factor associated with acute (OR 2.645, 95%CI 1.099-6.365; p=0.02) GU toxicity. No factors were associated with acute and late GI toxicity in the MVA.

Conclusion:

The results of the present study suggest that MRgRT using homogenous planning might be associated with lower grade 1 genitourinary toxicity. However, no significant differences in clinically relevant moderate-to-high-grade acute and late GU and GI toxicities were reported in both groups. This might be explained by different prescription modality. Both techniques, prostate SBRT with 1.5TMR-linac or CyberKnife, exhibit a satisfactory late toxicity profile with rare occurrences of severe adverse events.. Prospective evidence on a larger population is needed to confirm these data.

Keywords: Prostate cancer, adaptive radiotherapy, salvage