ESTRO 2021 Abstract Book

S293

ESTRO 2021

Conclusion Change in tumour BF during neoadjuvant radiation may be a prognostic biomarker of patient outcome in rectal cancer and may assist in identifying high-risk patients for postoperative therapy.

OC-0397 Quantitative analysis of diffusion weighted imaging in rectal cancer during MR-Linac radiotherapy M. Ingle 1 , M.D. Blackledge 2 , I. White 3 , S. Lalondrelle 1 , S. Hafeez 1 , S. Bhide 1 1 Institute Cancer Research/Royal Marsden Hospital, Radiotherapy and Imaging, Sutton, United Kingdom; 2 Institute Cancer Research, Radiotherapy and Imaging, Sutton, United Kingdom; 3 Guys and St Thomas NHS Trust, Clinical Oncology, London, United Kingdom Purpose or Objective Achieving complete pathological response in rectal cancers has proven to improve long term outcome of patients. Pathological response has association with diffusion weighted imaging (DWI) signal change. An increase in the apparent diffusion coefficient (ADC) is seen in those achieving pathological response [1]. With the advent of integrated magnetic resonance imaging and linear accelerator systems (MR-linac) serial DWI can be taken during treatment, potentially enabling tailoring of treatment based on response. The aim of this exploratory study was to determine feasibility of detecting DWI signal change on the MR-linac during the treatment course. Materials and Methods Seven consecutive patients with T3-4 N0-2 M0 rectal adenocarcinoma undergoing long course chemoradiotherapy in two phase treatment recruited prospectively to ethics approved protocol were analysed. Phase I consistent of 9Gy/5# to the tumour boost volume delivered on the MR-linac (Unity, Elekta AB, Stockholm Sweden). This was followed by phase II; 45Gy/25# to the pelvis delivered on C-linac arm. Anatomical imaging and DWI was acquired on the MR-linac daily during phase I and weekly during phase II. DWI was acquired using Elekta MR-linac consortium consensus guidance and included b values 0, 30, 150, 500 [2]. Region of interest was contoured on each b500 image (GTV b500 ), where signal intensity of tumour was best demonstrated. The corresponding T2 weighted image was utilised for anatomical reference. Using an in-house designed ADC calculation tool, the mean ADC value of the GTV b500 tumour volume was computed for each MR- linac treatment fraction.