ESTRO 2022 - Abstract Book

S509

Abstract book

ESTRO 2022

gland prostate SBRT (30 Gy in 5 fractions to PTV; 33 Gy in 5 fractions to CTV). SBRT was delivered over 2 weeks on conventional linac (CL) or MRLinac (MRL). Rectal spacer was placed at the discretion of the treating physician. Primary endpoint was local control. Toxicity and QoL assessments included CTCAE v4.0 and EPIC questionnaire at baseline, 1, 3 and 6 months, and 1, 2, 3, and 5 years. Results Ninety-five patients enrolled between 2016 and 2021 with >6 month follow-up were included in this analysis; 83% had intermediate-risk and 17% had high-risk disease. Median age was 70 years (range; 56 – 79). Seventy-five patients were treated on CL and 20 on MRL. Rectal spacer was placed in 68 patients (57 – CL; 11 – MRL). Acute G1 and G2 GU toxicity were 53% and 25%, respectively; G1 and G2 GI toxicity was 24% and 4%, where 74% had a rectal spacer. All patients (4) with acute G2 GI toxicity were treated on CL. Mean percentage change in EPIC urinary score at 1 month was -9.3; -11.8 on CL and 0.5 on MRL. At 6 months this improved to -0.6. Mean EPIC bowel score reduced by -4.4 at 1 month; -5.3 on CL and -0.5 on MRL. Six month bowel score was 0.7. Presence of rectal spacer did not appear to influence the bowel score. Conclusion An approach of HDR focal boost with whole gland prostate SBRT is feasible and well tolerated with minimal acute toxicity. Short-term QoL mirrors that seen in other radiotherapy techniques with initial reduction followed by recovery by 6 months. Further follow up will determine local response, late toxicity and QoL outcomes using this treatment approach. 1 University Medical Center Freiburg, Department of Radiation Oncology, Freiburg, Germany; 2 German Cancer Consortium (DKTK), Partner Site Freiburg, Freiburg, Germany; 3 Faculty of Medicine - University of Freiburg, Berta-Ottenstein- Programme, Freiburg, Germany; 4 University Medical Center Freiburg, Division of Medical Physics - Department of Radiation Oncology, Freiburg, Germany; 5 University Medical Center Freiburg, Department of Nuclear Medicine, Freiburg, Germany; 6 University Medical Center Freiburg, Department of Radiology, Freiburg, Germany; 7 European University of Cyprus, German Oncology Center, Limassol, Cyprus Purpose or Objective The arm A of the HypoFocal phase II trial aims to evaluate the safety of focal dose escalated external beam radiotherapy (EBRT) on combined positron emission tomography targeting prostate specific membrane antigen (PSMA-PET) and multiparametric magnet resonance tomography (mpMRI) defined boost volumes. This approach results in significantly lager boost volumes compared to the usually used mpMRI-only approach. Despite adherence to rectal dose constraints in the initial planning CT, interfractional differences of rectum filling might be particularly relevant for rectal dose metrics in this setting. To prevent increased gastrointestinal (GI) toxicities we developed an offline-adaptive planning strategy to account for various rectum fillings. Here we presents results as part of a planned safety analysis after 6 months. Materials and Methods Patients treated in arm A of the HypoFocal phase II trial according to the offline-adaptive planning strategy were analyzed. Patients were treated with 60 Gy in 20 fractions to the prostate and up to 75 Gy to mpMRI- and PSMA-PET defined boost volumes. Dose constraints according to the CHHiP trial were applied. The 3D images acquired during the first three fractions were imported into the RT planning software. After manual co-registration with the planning CT, rectum was delineated in each 3D image (CBCTs) and contours were transferred to the planning CT. Based on the average rectum volume and position in CBCTs, a rectum_adapt volume was created. RT plans were re-evaluated considering the additional rectum contours. If ≥ 1 mandatory constraint was not met in more than 1 volume a re-planning with optimization on rectum_adapt was performed. GI toxicities were assessed according to CTCAE v5.0. Dose metrics were evaluated and association with GI toxicities analyzed with univariate binary logistic regression. Results 23 of 25 patients in arm A were treated according to the adaptive planning strategy. Median boost volume was 10.2 ml (range 4-28). Median mean dose to the boost volume was 70 Gy. In 4 patients re-planning was performed. Median volumes receiving 60 Gy (V60) and V50 were significantly different between rectum and rectum_adapt, but not V40 and V30 (figure 1). Dose metrics were not significantly different between rectum and rectum based on 3D images (CBCT1-3). Cumulative grade 0, 1 and 2 GI toxicity at 6 months FU were 13%, 74% and 13%, respectively. Rectum V60, V50, and V40 but only rectum_adapt V40 were significantly associated with ≥ 2 GI toxicity (p=0.027-0.046). Figure 1: median volumes and 95% CI of rectum in the initial planning CT, CBCTs of the first three fractions and rectum_adapt receiving 60-30 Gy (V60-30). * indicates p<0.05 PD-0581 Offline-adaptive planning of mpMRI- and PSMA-PET- based dose escalation: results of a phase II trial S. Spohn 1,2,3 , M. Gainey 4,2 , M. Mix 5 , J. Ruf 5 , M. Benndorf 6 , D. Baltas 4,2 , A. Grosu 1,2 , C. Zamboglou 1,2,3,7