ESTRO 2024 - Abstract Book

S2372

Clinical - Urology

ESTRO 2024

the present study reported a significant prostate volume change during prostate SBRT on 1.5T MR-linac. We observed either prostate swelling in half of the cases or a few cases of prostate shrinkage. No correlations were found with PROMs in this population treatment with daily-adaptive strategy.

Keywords: prostate cancer, adaptive radiotherapy, SBRT

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Two- vs five-fraction SBRT for prostate cancer: feasibility of randomization and toxicity

Juan P Canales 1 , Gabriela P Antúnez 1 , Cristian J Salazar 2 , Tomás R Merino 1 , Paula S Reyes 1 , Esteban A Barnafi 2

1 Pontificia Universidad Católica de Chile, Hemato-oncology, Santiago, Chile. 2 Pontificia Universidad Católica de Chile, Medical School, Santiago, Chile

Purpose/Objective:

Evaluate feasibility of randomizing and assessing acute toxicity of patients with low- and intermediate-risk prostate cancer favorable into a two- versus five-fraction SBRT regimen. Additionally, lay the foundations to evaluate late toxicity and biochemical control.

Material/Methods:

Randomization was to prostate in 26 Gy in 2 fractions versus 36.25 Gy in 5 fractions. The primary outcome was recruitment (Percentage of screened patients eligible recruited in six months). Recruitment success was defined as 70%. We use Lewis method to calculate sample size [1]. Secondary outcomes were acute toxicity (acute genitourinary (GU) and gastrointestinal (GI) adverse events according to CTCAE v4.03 [2]). Ordinal regression [3] was employed to analyze acute toxicity, and beta regression [4] with random effects was performed to longitudinally model IPSS; both were adjusted for age effects.

Results:

From August 2021 to May 2023, 42 patients met the inclusion criteria and twelve declined to participate. 30 patients were randomized, 15 patients to 2-fractions and 15 patients to 5-fractions. The recruitment rate was 71%. The patients characteristics are summarized in table 1. Four and three patients in 5- fractions experienced acute grade ≥ 2 GU and GI toxicity, respectively. Six and one patients in 2- fractions experienced acute grade ≥ 2 GU and GI toxicity, respectively. No grade 3 or higher acute toxicity was reported. In the regression analysis, an increased risk of acute GI toxicity is evident in the 5-fraction group (Beta = -3.1, p = 0.017). In the analysis of acute GU toxicity, no difference was observed in both groups (Beta = 0.47, p = 0.54).