ESTRO 2020 Abstract Book

S60 ESTRO 2020

Purpose or Objective Biochemical recurrence after radical prostatectomy (RP) is routinely managed with salvage radiation therapy (SRT) with curative purpose. However, patients with macroscopic local recurrence detected with positive metabolic imaging showed poor response to conventional SRT and may need treatment intensification. Dose escalated treatment may be used to achieve better disease control. Stereotactic radiotherapy for macroscopic prostate recurrence (SSRT) may be used to increase outcome in these patients, with potential advantages in terms of reduced treatment volumes and lower number of fractions. No direct comparisons between conventional prostate bed SRT and SSRT have been carried out. Thus, we retrospectively collected data from patients treated with these two approaches and compared their outcomes with a propensity score based matched analysis. Material and Methods Data from 185 patients treated in 6 Italian Centers for macroscopic prostate bed recurrence after RP were retrospectively collected and reported. Overall, 90 and 95 patients underwent SSRT and SRT, respectively (Fig.1). Patients in SSRT group were treated with a mean dose of 34.4 Gy (32.5-35 Gy) in 5 fractions, while patients in SRT cohort were treated with a mean dose of 72.4 Gy (60-79 Gy) in 28-38 fractions. Propensity scores were calculated for each patient using multivariate logistic models on the basis of PSA at recurrence, concomitant androgen deprivation therapy (ADT), Time to recurrence (TTR) and Gleason Score (GS). Biochemical recurrence was defined as a PSA increase above 0.2 ng/ml for patients with a PSA nadir < 0.2 ng/ml or 2 consecutive PSA increases >25% if compared to nadir in patients with a PSA nadir > 0.2 ng/ml. Cohort characteristics were compared after matching with Chi squared and Mann Whitney test to assess the distribution of baseline features in the groups. Comparison in terms of Biochemical recurrence free survival (BRFS) was performed using analysis of covariance to take in account different extent of follow- up in the cohorts. Rate of reported genitourinary (GU) and gastrointestinal (GI) G> 2 toxicity, according to CTCAE score v.4.03 was compared with Chi-squared test.

Conclusion Significantly lower toxicity was observed after SSRT while maintaining the same tumour outcome, when compared to SRT. The findings of our study suggest that SSRT may be considered a valid option for post-prostatectomy macroscopic local recurrence, offering short (convenient), cost-effective, well tolerated and efficacious approach. E. Gioscio 1 , T. Rancati 1 , B. Avuzzi 2 , L. De Cecco 3 , S. Morlino 2 , B. Noris Chiorda 2 , S. Villa 2 , T. Di Florio 1 , F. Badenchini 1 , T. Giandini 4 , A. Cicchetti 1 , E. Mancinelli 3 , M.S. Serafini 3 , A. Devecchi 3 , E. Orlandi 2 , R. Valdagni 1,5 1 Fondazione IRCCS Istituto Nazionale Tumori, Prostate Cancer Program, Milan, Italy ; 2 Fondazione IRCCS Istituto Nazionale Tumori, Radiation Oncology 1, Milan, Italy ; 3 Fondazione IRCCS Istituto Nazionale Tumori, Platform of Integrated Biology- Department of Applied Research and Technology Development, Milan, Italy ; 4 Fondazione IRCCS Istituto Nazionale Tumori, Medical Physics, Milan, Italy ; 5 Università degli Studi di Milano, Department of Oncology. Milan, Italy Purpose or Objective A mono-institutional trial was set up in 2017 to investigate the modulations of gut microbiota (MB) after prostate cancer (PCa) radiotherapy (RT). We here explore how RT can modulate MB species abundances, but also the possibility that urinary (GU) and intestinal (GI) toxicity (tox) is associated to di ff erent modulations in MB Material and Methods Consecutive patients (pts) receiving conventional (70-78Gy @2Gy/fr) or moderately hypofractionated (65-67.5Gy @2.6Gy/fr) VMAT+IGRT were enrolled. Gut MB was measured pre-RT and at RT end. Stool samples were collected using gut-OMNIgene devices, DNA extracted using the QIAamp-DNA-Stool-Mini-Kit, bacterial 16S ribosomal-RNA reads analyzed with QIIME software and pooled in Operational Taxonomic Units (OTU) with Uclust software. Pre-RT and post-RT OTUs at genus and species taxonomic levels were considered. Modulation was evaluated by Wilcoxontest, both on the whole population and after stratification with respect to acute GU and GI tox (CTCAE): Grade 0 (G0) vs Grade 1 (G1) vs Grade 2 (G2) Results 135 pts were included, 157 genera and 412 bacterial species were identified. 59/157 genera and 132/412 species were found in at least 10% of pts and included in the analyses. 7 genera (Bilophila, Faecalibacterium, Flavonifractor, Gemmiger, Phascolarctobacterium, Roseburia, Sutterella) and 4 species (Blautia producta, Faecalibacterium prausnitzii, Roseburia faecis, Ruminococcos bromii) were significantly modulated by RT (p<0.01 and <0.001, for genera and species, respectively), details in fig 1. Of interest, Ruminococcus bromii is a keystone species for the degradation of resistant starch in the colon and a possible relationship between Roseburia faecis and irritable bowel syndrome was recently proposed. When considering GI tox, 29 (22%), 30 (22%) and PH-0119 Modulations of gut microbiota following radiotherapy for prostate cancer

Results After matching, 39 and 40 patients in the SRT and SSRT groups were selected (table 1). Mean BRFS was 33.8 and 39.6 months in the SRT and SSRT groups, respectively (p=0.18). Overall, acute and late GU toxicity in the SRT versus SSRT groups was reported in 45 versus 23% (p ˂ 0,0001) and 20 versus 5 % (p=0.04) of patients, respectively. Acute and late GI toxicity was reported in 40 versus 10.3 % (p=0.0002) and 17.5 versus 3% (p=0.002), respectively.