ESTRO 2022 - Abstract Book

S1668

Abstract book

ESTRO 2022

Even with a 0mm PTV margin, the overall difference between both techniques is limited. For specific patients with an OAR close to the lymph node boost, the MR-Linac based sequential boost was beneficial over the SIB technique.

PO-1882 Impact of PTV margin reduction on organ at risk dose in short course radiotherapy of rectal cancer.

L. Devlin 1 , S. O'Cathail 2 , L. Grocutt 3 , A. Duffton 1

1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom; 2 Institute of Cancer Sciences, University of Glasgow, Glasgow, United Kingdom; 3 Beatson West of Scotland Cancer Centre, Department of Radiotherapy Physics, Glasgow, United Kingdom Purpose or Objective For rectal cancer dose to small bowel has been shown to be associated with acute toxicity. As PTV margin reduction results in smaller treatment volumes, dose to organs at risk (OAR) will also decrease. There is limited evidence of the effect of reduced PTV margins on dose to OAR in short course radiotherapy (SCRT). Aim-To evaluate the impact of reducing PTV margins on OAR dose in SCRT of rectal cancer. Materials and Methods Patients treated with SCRT between May-July 2020 were included in the analysis. A planning CT was acquired in supine position with bowel and bladder preparation. GTV was macroscopic tumour including rectal lumen. CTVA was GTV + 10mm margin. CTVB was elective nodes. PTV 5mm, PTV 7mm and PTV 10mm were created from CTVF (CTVA + CTVB) + 5mm, + 7mm and + 10mm respectively . OAR (bladder, small bowel loops) were delineated as per RTOG definitions. All structures were delineated by a Clinical Oncologist and peer reviewed by a multi-disciplinary team. For each plan the dose prescribed to PTV was 25Gy in 5 fractions. Treatment plans were created using Eclipse Treatment planning system (TPS) v15.5. Three plans were calculated for each patient with PTV 5mm , PTV 7mm and PTV 10mm . The same planning objectives and priorities were used for each plan within the photon optimiser (PO) and recalculated using ACUROS v15.5. All plans consisted of 2 VMAT full rotational arcs, 6 MV at 600 MU/min and delivered on a Varian Truebeam (2.7) [Varian Medical Systems, Palo Alto, Ca, USA]. Dose metrics were extracted from dose volume histograms (DVH) for all plans. Results 20 rectal cancer SCRT patients were included in the analysis. Median age was 70 [IQR 60.75 -77], female (n=5), male (n=15). Rectum level was low (n=11), mid (n=4) and upper (n= 5). A total of 60 treatment plans were analysed. The mean volume for PTV 5mm was 790.7 cm 3 (SD 239.7), PTV 7mm 930.1 cm 3 (SD 267.3), PTV 10mm was 1103 cm 3 (SD 296.5). The mean volume for small bowel and bladder was 206 cm 3 (SD 154.5) and 221.1 cm 3 (SD 154.5) respectively. All plans complied with the planning constraints for each structure (Table 1). Figure 1. Shows the population DVH for bladder and small bowel with bladder showing most variation towards higher dose. For small bowel, variation in dose was observed across the median volume for all PTV margins.