ESTRO 2021 Abstract Book

S516

ESTRO 2021

OC-0648 Dose escalation is feasible in short course radiotherapy of rectal cancer – a planning study L. Devlin 1 , B. Hunter 2 , H. Chemu 1 , P. McLoone 3 , R. Valentine 2 , P. Cornelious 2 , A. Duffton 1 , S. O'Cathail 4 1 Beatson West of Scotland Cancer Centre, Radiotherapy, Glasgow, United Kingdom; 2 Beatson West of Scotland Cancer Centre, Department of Radiotherapy Physics, Glasgow, United Kingdom; 3 Institute of Health & Wellbeing, University of Glasgow, Glasgow, United Kingdom; 4 Institute of Cancer Sciences,, University of Glasgow, Glasgow, United Kingdom Purpose or Objective Short course radiotherapy (SCRT) with 5x5Gy followed by chemotherapy is a new standard of care for locally advanced rectal cancer 1 . Given the low biological dose of this prescription; the existence of a dose response; and the moderate α/β ratio (~5) of rectal cancer, we hypothesise that hypofractionated dose escalation would have significant radiobiological advantages. We assessed the in-silico feasibility of dose escalation to the primary tumour up to 35Gy using a simultaneous integrated boost (SIB), conserving OAR constraints. Materials and Methods Patients immobilised in supine position had a planning CT (2-2.5mm, L2/3 to mid-femur) with bowel and bladder preparation and intravenous contrast. GTV was macroscopic tumour including rectal lumen. CTVA was GTV + 10mm margin. CTVB was elective nodes (mesorectal, presacral, obturator and internal iliac). PTV_Low was created from CTVF (CTVA + CTVB) + 7mm margin. PTV_High (SIB) was created from GTV + 5mm margin. OAR (bladder, small bowel loops, bowel cavity) were delineated as per RTOG guideline definitions. All structures were delineated by Clinical Oncologists and peer reviewed. Each patient had 4 plans produced at increasing dose levels to the SIB volume (PTV_High) 27.5Gy, 30Gy, 32.5Gy and 35Gy. The dose prescribed to PTV_Low was 25Gy in 5 fractions. Plans were inverse planned with energy of 6MV on a Varian Truebeam (2.7) in Eclipse v15.5 using PO v15.5 for inverse planning, Acuros v15.5 for final calculation and DVH estimation algorithm v15.5. Feasibility was defined as 90% of plans achieving the planning objectives at 32.5Gy dose level (EQD2 53.2Gy). Results 19 rectal cancer SCRT patients were anonymised and included in the study. Patient median age was 70 (IQR 60.75-77), F (n=4), M (n=15). Rectum level was low (n=10), mid (n=4) and upper (n= 5). A total of 95 treatment plans were analysed. The mean volume was 130cm 3 (SD 81.5) and 769.6cm 3 (SD 241.1) for PTV_High and