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extreme hypofractionation is gradually becoming a mainstay of treatment for prostate cancer (PCa). Patients’ (pts) tolerance to such RT regimes is undoubtedly under the spotlight. Herein, we report acute toxicity data and predictors in a single-center observational analysis.

this toxicity analysis pertains to a consecutive series of low-to-high-risk PCa pts treated with (LINAC-based VMAT) extreme-hypofractionation (42.7Gy/7 non-consecutive fractions-frs, 15-19 days) +/- ADT. Genitourinary (GU) and gastrointestinal (GI) acute toxicities were scored according to RTOG during the following timepoints: each RT fr, 1- and 3-months (mos) from RT end. Patient (risk-group, previous TURP, prostate cc ≤ or > 80cc, IPSS score ≤ or > 12), treatment (neoadjuvant-concomitant-adjuvant ADT use, overall treatment time-OTT ≤17 or >17 days) and dosimetric (V37Gy bladder ≤10cc, ≤15cc, ≤20cc, >20cc) variables were analyzed (logistic regression analysis) to investigate potential acute RTOG GU G2+ toxicity predictive/protective factors.

187 pts (mean age 74.5 years) were treated between May 2020 and October 2021. Most pts were classified as intermediated-risk (77.5%), followed by high-risk (18.2%) and low-risk (4.3%). ADT was administered to 56.1%. Baseline IPSS >12 was observed in 23.9%. Most pts had a prostate volume<80cc (83.9%). Previous TURP was reported for 38 pts, mainly more than 2 years before RT (31 pts). The most commonly recorded V37Gy bladder metric was 16-20cc(57.2%), followed by 11-15cc(21.4%), 0-10cc(12.3%) and >20cc(9.1%). Treatment was completed within 17 days for 66 pts, in >17 days for 121 pts. Acute RTOG GU G2+ events were observed in 23% of pts, peaking at the last fr(19.8%, 37 pts). 3 pts experiencing G2+ during the first 6 frs had <G2 at RT end. RTOG GU G2+ rate at 1-mo was 3.2% (with 2/6 experiencing G2+ for the first time) and 1.07% at 3-mos (2 pts, with 1 experiencing G2+ for the first time). Considering the 146 pts with IPSS data available at baseline, from the logistic regression analysis ADT emerged as a statistically significant GU G2+ protective factor (p<0.001; fig.1A).Excluding IPSS (due to 41 pts with missing baseline data), prostate volume>80cc resulted a statistically significant predictor(p=0.018), ADT a statistically significant protective factor (p<0.001), and the OTT>17 days a borderline significant protective factor (p=0.05; fig.1B).When employing continuous values for prostate volume cc, bladder V37Gy cc, and IPSS scores, ADT was significantly protective(p=0.001),while greater prostate volumes and higher IPSS scores resulted as significant predictors(p=0.01 and p=0.02, respectively)(fig.1C).RTOG GI G2+ rate at RT end was 2.1%, 1.07% at 1-mo, while no G2+ was recorded at 3-mos. 

extreme hypofractionation was well tolerated, with acute GU G2+ toxicity peaking at RT end and almost disappearing at 3-mos. Larger prostates and high IPSS scores resulted as GU G2+ predictors, while ADT as the main protective factor. GI G2+ was negligible.