ESTRO 2021 Abstract Book

S1113

ESTRO 2021

hand, no real constraints for rectum were found, but, on the basis of the very few rectal/intestinal major toxicity events registered, one can assume the median values of the patients’ dose-volume points as a safe rectum dose-volume histogram. Finally, patients’ comorbidities status needs to be taken in a careful consideration in salvage SBRT patient selection and planning. PO-1356 Dose constraints validation in moderate hypofractionation in localized prostate cancer X. Maldonado 1 , M. Altabas 1 , J. Garre 1 , M. Vazquez 1 , S. Magriña 1 , A. Delgado 1 , N. Docampo 1 , A. Geng 1 , M. Hermida 2 , D. Sanchez-Artuño 2 , J. Giralt 1 1 HU Vall d'Hebron, Radiation Oncology, Barcelona, Spain; 2 HU Vall d'Hebron, Physics, Barcelona, Spain Purpose or Objective In September 2019 our department adopted the CHIPP hypofractionated scheme for the treatment of localized prostate cancer. We have assessed and compared the acute toxicity to validate the new critical organ dose constraints designed in terms of isotoxicity when moving from conventional to moderate hypofractionation. Materials and Methods Acute toxicity (graded according to CTCAE 4.0) from 40 men with localized prostate cancer (T1-T3N0M0) treated with moderate hypofractionation schedule (HF) (60-62 Gy in 20 fractions over 4 weeks) and adapted dose constraints (Table 1) is compared to a retrospective paired sample (n=42) treated with conventional fractionation (CF) regime (72 Gy in 30 fractions over 6 weeks). PTV design was comparable as well as the use of androgen deprivation. Both populations were treated by Halcyon Accelerator or True Beam Linac with a VMAT technique. Daily CBCT or fiducial marker triggered with KV images was used as IGRT method. Acute data were prospectively collected using physician/nurse questionnaires: baseline, weekly the first 6 weeks and monthly from the end of treatment. The χ² test and t-student test were used to compare baseline characteristics of the two populations and the overall worst degree of toxicity.

Results When comparing the two populations, median age was 74 years old (p=0,24), median Corporal mass index 35 (p=0.39), 30% presented diabetes(p=0.9), 24% on oral anticoagulants (p=0.36). ISUP was grade 2/3 in 71% of patients (p=0.61), median PSA 11 ng/ml (p=0,2). Use of ADT for 6 month in 35% and 24 month in 33% (p=0,75). IPSS score 0-7 in 50,6% and 8-19 in 49,4% (p=0,53). There weren’t statistical differences in patients characteristics between populations except for tobacco use (23% in CF and 0% in HF, p=0.001), median Charlson score (0-5 73% in CF and 45% in HF, p=0.013) and clinical T (T1 78% in CF and 21% in HF, p=0,024). When the T stage was mpMR assessed there was no difference (4% and 5% for CF and HF respectively). Main acute GU and GI toxicity were grade 0-1 with no grade 3-4. Acute GU toxicity from week 1 to 6, at 1 month and at 3 month is displayed in Table 2. There were statistical differences in weeks 1 and 2, against HF and at 1 month against CF. Acute G2 GU toxicity was more frequent in CF (10%) than in HF (4,7%) (p=NS). Regarding acute GI toxicity it has been mild with no grade 2, 3 or 4. There were neither differences comparing groups nor during the follow up at 1 or 3 month (0 vs 7% and 0 vs 2%, p= 0.25).