ESTRO 2021 Abstract Book

S1139

ESTRO 2021

by means of 1.5T MRI-linac.

Materials and Methods Between October 2019 and April 2020, twenty consecutive castration sensitive oligorecurrent prostate cancer patients were enrolled in an ethical committee approved prospective observational study (Protocol n. XXXX) and treated with PSMA-PET/CT guided SBRT by means of 1.5T MRI-linac (Unity, Elekta AB, Stockholm, Sweden). The mean delivered dose was 35 Gy in 5 fractions. Clinicians reported toxicity was prospectively collected according to Common Terminology Criteria for Adverse Events v5.0. Quality of life (QoL) assessment was performed using EORTC-QLQ C30 questionnaires administered at baseline, end of treatment and at first follow-up. Results Twenty-five lesions in 20 castration sensitive oligorecurrent patients were treated: the most commonly treated anatomic sites were nodal (n=16) and pelvic bone (n=9). Median PSA-value pre-MRI guided SBRT was 1.16 ng/mL (range, 0.27–8.9), whereas median PSA value at first follow-up after SBRT was 0.44 ng/mL (range, 0.06 – 8.15). At first follow-up, for 16 patients showing detectable PSA, PSMA-PET/CT was performed detecting respectively in 6 cases partial response and in 10 cases complete response. In the remaining cases PSA-value was undetectable after SBRT. Radiotherapy treatment was safe and well tolerated according to the PROMs. No acute G2 or higher toxicities were recorded. Conclusion The current series represent the largest one exploring the feasibility and patient-reported outcomes following PSMA-PET/CT guided SBRT by means of 1.5T MRI-linac. The preliminary findings here reported are encouraging in terms of effectiveness and tolerability. PO-1389 Interrater agreement in contouring the neurovascular bundle and internal pudendal artery for MRgRT F. Teunissen 1 , R. Wortel 2 , F. Wessels 3 , S. van de Pol 1 , M. Rasing 1 , A. Claes 1 , J. de Boer 1 , R. Meijer 2 , H. van Melick 4 , H. Verkooijen 5 , J. van der Voort van Zyp 1 1 University Medical Center Utrecht, Radiation Oncology, Utrecht, The Netherlands; 2 University Medical Center Utrecht, Urology, Utrecht, The Netherlands; 3 University Medical Center Utrecht, Radiology, Utrecht, The Netherlands; 4 St. Antonius Hospital, Urology, Nieuwegein, Utrecht, The Netherlands; 5 University Medical Center Utrecht, Imaging and Oncology Division, Utrecht, The Netherlands Purpose or Objective It is hypothesized that radiation damage to neural and vascular tissue, such as the neurovascular bundles (NVBs) and internal pudendal arteries (IPAs), during radiotherapy for prostate cancer (PCa) contributes to erectile dysfunction. Neurovascular sparing MR guided adaptive radiotherapy (MRgRT) aims to preserve erectile function after treatment. However, the NVBs and IPAs are not routinely contoured in current radiotherapy practice. Before neurovascular sparing MRgRT for PCa in the setting of an MR-Linac can be implemented, the agreement of the contouring of the NVBs and IPAs on pre-treatment MRI needs to be assessed. Materials and Methods The guidelines for reporting reliability and agreement studies (GRRAS) recommendations were followed. Four radiation oncologists independently contoured the prostate, the left and right NVB and the left and right IPA in an unselected consecutive series of 15 PCa patients treated with 5x7.25 Gy MRgRT on an MR-Linac. For each patient contouring was done on a single pre-treatment T2-weighted 1.5T MRI. Dice similarity coefficients (DSCs) for pairwise interrater agreement of contours were calculated. DSC = 0 indicating no spatial overlap and DSC = 1 indicating complete spatial overlap between contours. A subset of the caudal half of the contours was made, covering the midgland to apex part of the prostate. For this anatomic region the NVB lies in closest proximity to the prostate and conflict between dose coverage of the prostate and dose sparing of the NVB is Median overall interrater DSC for the prostate was 0.91 (interquartile range (IQR): 0.88 – 0.92). For the left and right NVBs the median overall interrater DSC was 0.60 (IQR: 0.54 – 0.67) and 0.62 (IQR: 0.54 – 0.69) respectively (figure 1) and for the left and right IPAs 0.58 (IQR: 0.52 – 0.63) and 0.58 (IQR: 0.51 – 0.63) respectively (figure 2). Subset analysis of the caudal half of the NVBs resulted in a median overall interrater DSC of 0.67 (IQR: 0.59 – 0.73) for the left and 0.66 (IQR: 0.61 – 0.70) for the right side. Figure 1: Representative case of contours of 4 raters of the NVBs highest. Results