ESTRO 2023 - Abstract Book

S1808

Digital Posters

ESTRO 2023

MR-only treatment planning has increased the number of available timeslots on CT, reduced the time patients spend being scanned, but for the department to get full benefits requires effort to optimize the scanning and delineation workflows and training.

PO-2036 Dosimetric Comparison of High Dose Rate Brachytherapy vs MR-Guided SBRT Boost for Cervical Cancer

N. Dincer 1 , G. Ugurluer 1 , T. Zoto Mustafayev 2 , A. Serkizyan 2 , G. Aydin 1 , G. Gungor 1 , B. Atalar 1 , E. Ozyar 1

1 Acibadem MAA University School of Medicine, Radiation Oncology, Istanbul, Turkey; 2 Acibadem Maslak Hospital, Radiation Oncology, Istanbul, Turkey Purpose or Objective High dose rate brachytherapy (HDR-BT) is an integral part of the treatment of locally advanced cervical cancer. HDR-BT enables high doses with a steep dose decline at the periphery but not all patients are amenable to BT due to patient or facility related reasons. This dosimetric study aims to compare HDR-BT and Magnetic Resonance Image Guided Stereotactic Body Radiotherapy (MRgSBRT) plans in locally advanced cervical cancer patients treated at our institution. Materials and Methods Four locally advanced cervical cancer patients who received definitive external beam radiotherapy (EBRT) followed by HDR-BT were selected for this study. All patients had MR scans in MR-Linac immediately before HDR-BT. Similar target volumes and organs at risk (OARs) were delineated in MRI and HDR-BT planning CT according to guidelines. A final total EQD2 dose of 75-96 Gy, 60-75 Gy were aimed for HR-CTV and IR-CTV, respectively in accordance with EMBRACE and GEC- ESTRO guidelines. EQD2 of D2cc of bladder, rectum and sigmoid were set as <80-90 Gy, <65-75 Gy and <70-75 Gy respectively. Two plans with a prescribed dose of 28 Gy in 4 fractions were generated for all patients in HDR-BT planning and MR-Linac planning system. In MRgSBRT plans, PTV was created with a margin of 3 mm from HR-CTV and no PTV was created for HDR-BT. The plans were compared in terms of HR-PTV D90, HR-CTV D90, IR-CTV D90, D2cc rectum, D2cc bladder and D2cc sigmoid colon. Results The mean PTV volume for MRgSBRT plans was 38.8 cc and the mean HR-CTV volume for HDR-BT plans was 21.4 cc. The mean EBRT dose was 47.7 Gy. Mean EQD2 values of HR-CTV D90 and IR-CTV D90 for BT were 79.1 and 66.0 Gy, respectively. For MR-Linac, the means of HR-PTV D90, HR-CTV D90, IR-CTV D90 were 82.9, 99.2, 70.5 Gy, respectively. Means of EQD2 D2cc of bladder, rectum and sigmoid colon in HDR-BT plans were 86.4, 70.9 and 65.7 Gy, respectively. Mean doses of EQD2 D2cc of bladder, rectum and sigmoid colon for MRgSBRT plans were 81.4, 71.0 and 73.6 Gy, respectively. All dose constraints both in terms of target coverage and OAR sparing were met for MRgSBRT plans.

HDR-Brachytherapy (Gy EQD2) MRgSBRT (Gy EQD2)

HR-PTV D90 HR-CTV D90 IR-CTV D90 Bladder D2cc Rectum D2cc

N/A 79.1 66.0 86.4 70.9

82.9 99.2 70.5 81.4 71.0 73.6

Sigmoid Colon D2cc 65.7

Conclusion This dosimetric study demonstrates that MRgSBRT can be safely applied in cases where HDR-BT is not available or ineligible with better target coverage as well as similar OAR sparing. As this study did not take into account the online daily adaptive RT, we believe that aforementioned results even can be better using MRgSBRT. Results of this study support that online adaptive brachy-like MRgSBRT may replace HDR-BT in the future.

PO-2037 Hydrogel Spacer in the treatment of prostate cancer: Feasibility and Rectal Dosimetry

J.A. Domínguez Rullán 1 , M. Valero Perena 1 , E. Centelles Hidalgo 1 , F. Mesa López 1 , F. López Campos 1 , M.T. Muñoz Miguelañez 1 , C. Vallejo Ocaña 1 , D. Sevillano García 2 , J.D. García Fuentes 2 , S. Sancho García 1 , A. Hervás Morón 1 1 Hospital Universitario Ramón y Cajal, Radiation Oncology, Madrid, Spain; 2 Hospital Universitario Ramón y Cajal, Medical Physics, Madrid, Spain Purpose or Objective The planning study aimed to assess the feasibility of injecting and distribution of hydrogel spacer (SpaceOARTM/SpaceOAR VueTM) and to report rectal dosimetry parameters and toxicity. Materials and Methods From July 2020 to June 2022, 87 consecutive patients with intermediate-risk prostate cancer were scheduled at our institution for hydrogel spacer and fiducial markers insertion under ultrasound guidance. A planning CT scan with full bladder and empty rectum was done 7-10 days after implantation in all patients treated with hypofractionated EBRT (60 Gy in 20 fractions using VMAT and daily CBCT) or SBRT (36.25 Gy in 5 fractions with Cyberknife system) and fused with T2- weighted MRI in case of SBRT with SpaceOARTM insertion. After planning was complete, data was collected pertaining to the V32, V28 and V18 of the rectum in SBRT cases and V40, V48, V52, V56 and V60 of the rectum in hypofractionated