ESTRO 2023 - Abstract Book

S768

Monday 15 May 2023

ESTRO 2023

Conclusion Delivery of a focal boost with SBRT is feasible, with the trend in urinary toxicity comparable to contemporary SBRT series without a boost. The incidence of GU toxicity has predominance over GI toxicity and further dosimetric analysis is planned to help identify the determinants of urinary toxicity in prostate SBRT. OC-0922 Once or twice a week, What's best?: Joint acute toxicity analysis of the phase II hypo-FLAME trials L. De Cock 1 , C. Draulans 1 , F.J. Pos 2 , S. Isebaert 3,1 , R. De Roover 3 , U.A. van der Heide 2 , R.J. Smeenk 4 , M. Kunze-Busch 4 , J. van der Voort van Zyp 5 , H. de Boer 5 , L.G. Kerkmeijer 4,5 , K. Haustermans 3,1 1 KU Leuven, Oncology, Leuven, Belgium; 2 The Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands; 3 University Hospitals Leuven, Radiation Oncology, Leuven, Belgium; 4 Radboud University Medical Centre, Radiation Oncology, Nijmegen, The Netherlands; 5 University Medical Centre, Radiation Oncology, Utrecht, The Netherlands Purpose or Objective Prostate stereotactic body radiotherapy regimens (SBRT) vary in dose, fractionation and overall treatment time (OTT). Finding the optimal OTT is challenging as on the one hand a shorter OTT increases disease control in conventional fractionated regimens, while, on the other hand, a shorter interfraction time interval could influence the toxicity rates negatively. We investigated the impact of an OTT reduction from 29 days, using a once weekly (QW) schedule (hypo-FLAME trial), to 15 days, using a semi-weekly (BIW) schedule (hypo-FLAME 2.0 trial), on acute toxicity for whole prostate gland SBRT including a simultaneous integrated focal boost. Materials and Methods Patients with intermediate- or high-risk prostate cancer (PCa) were enrolled in either the phase II prospective multicenter hypo-FLAME or hypo-FLAME 2.0 trial. All patients were treated QW or BIW with SBRT delivering 35 Gy in 5 fractions to the whole prostate gland with an iso-toxic boost up to 50 Gy to the multiparametric MRI-defined tumor(s). The primary endpoint of both trials was radiation-induced acute genitourinary (GU) and gastrointestinal (GI) toxicity measured using the CTCAE criteria. Acute toxicity results of both schedules were compared using a multivariate binary logistic regression model, adjusting for potentially confounding factors as age, T-stage, cardiovascular disease, diabetes, hormone therapy and baseline toxicity. Results One hundred and one hundred twenty-four patients were enrolled in the hypo-FLAME and hypo-FLAME 2.0 trial, respectively. The majority of patients (70.5%) were classified as high-risk PCa according to the EAU risk classification. No grade ≥ 3 GU or GI toxicity was reported independent of the treatment schedule. The 90-days cumulative incidence of grade 2 GU toxicity was significant lower in the QW treated group with 34.0% compared to the BIW treated cohort with 47.5% (p = 0.011). The prevalence of grade 2 GU toxicity increased during treatment and reached a maximum at week 3 (34.5%) for the BIW schedule and at week 5 (25.5%) for the QW schedule (Figure 1). There was no significant difference in 90-days cumulative incidence of grade 2 GI toxicity with 7.4% and 5.0% for the BIW and QW scheme, respectively (p = 0.290).