ESTRO 2024 - Abstract Book

S2371

Clinical - Urology

ESTRO 2024

881

Digital Poster

Prostate volume change during 1.5T MR-guided adaptive SBRT and correlation with treatment toxicity

Luca Nicosia, Paolo Ravelli, Michele Rigo, Niccolò Giaj-Levra, Rosario Mazzola, Edoardo Pastorello, Francesco Ricchetti, Andrea Gaetano Allegra, Ruggero Ruggieri, Filippo Alongi

IRCCS Sacro Cuore Don Calabria Hospital, Cancer Care Center, Advanced Radiation Oncology Department, Negrar, Italy

Purpose/Objective:

The introduction of MR-linac technology might allow the safe organs at risk (OAR) sparing, potentially reducing toxicity. When comparing this treatment modality with conventional radiotherapy, little is known regarding prostate volume variation during treatment and how this may affect the accuracy of dose to target and OARs, and toxicity. In fact, radiation-induced inflammation might lead to prostate swelling which can reduce treatment accuracy if small target-to-PTV margins are used. The present study aims to evaluate prostate volume change during daily-adaptive prostate SBRT on 1.5T MR-linac and to correlate it with treatment toxicity.

Material/Methods:

a series of patients affected by low- and intermediate-risk prostate cancer was treated by 5-fraction SBRT at our Institution within a prospective study (Prot. n° 23748). The total dose was 35 Gy and 36.25 Gy delivered every day or on alternate days. Treatment toxicity was recorded with the following patient-reported outcomes (PROMs): IPSS, ICIQ-SF, and EPIC-26.

Results:

overall 254 patients were included in the analysis. The baseline (i.e., before treatment) median CTV volume was 55 cc (IQR 40.1-65.2; range 15.3-163.3). Mean values from fraction 1 to fraction 5 were: 58.9 cc, 60.5 cc, 61.8 cc, 62.5 cc, and 62.7 cc, with an absolute mean volume increase of 3.8 cc. At the univariate analysis, CTV at baseline significantly changes over treatment (p=<0.0001). Moreover, we found prostate swelling (>5% volume increase, mean 15.4% increase) in 50% of cases, stable volume in 39% of patients, and prostate shrinkage in 11% of cases (>5% volume reduction, mean 12.2% reduction). Patients with baseline CTV volume >55 cc showed a trend towards higher CTV shrinkage (-10.5% versus -14.5%; p=0.052). We found no correlation between CTV change and PROMs. Prostate swelling was generally compensated by the planned PTV expansion, even though the setup margins dropped from mean of 47.4 cc at baseline to 38.9 cc at the last fraction, with few cases not covered by initial setup margins.

Conclusion: