ESTRO 2021 Abstract Book

S294

ESTRO 2021

Results 70 scans from 7 patients were analysed in total. 7 scans were excluded due to incorrect anatomical location of DWI. The mean ADC values increased from 1.36 10 -3 mm 2 /2 (SD ± 0.27) at fraction 1 to 1.66 10 -3 mm 2 /s (SD ± 0.16) at fraction 30 (p = 0.0312). Absolute ADC and relative ADC (percent) changes are seen throughout treatment with greatest difference seen between baseline and week 4-5.

Conclusion CONCLUSION: DWI signal change during rectal cancer radiotherapy can be successfully measured on the MR- linac. The ADC change seen is consistent with treatment response as measured on diagnostic MRI[1]. Ongoing work compares pathological outcome to the measured MR-linac ADC parameters in order to validate DWI as a response biomarker. [1] Sun YS, Zhang XP, Tang L, Ji JF, Gu J, Cai Y, et al. Locally advanced rectal carcinoma treated with preoperative chemotherapy and radiation therapy: Preliminary analysis of diffusion-weighted MR imaging for early detection of tumor histopathologic downstaging. Radiology 2010;254:170–8. https://doi.org/10.1148/radiol.2541082230. [2] Kooreman ES, van Houdt PJ, Keesman R, Pos FJ, van Pelt VWJ, Nowee ME, et al. ADC measurements on the unity MR-linac – a recommendation on behalf of the elekta unity MR-linac consortium. Radiother Oncol 2020. https://doi.org/10.1016/j.radonc.2020.09.046. OC-0398 Stability of multiparametric MR imaging biomarker-derived dose prescriptions for glioblastoma C. Brighi 1,2 , D. Waddington 1,2 , A. Walker 2,3,4,5 , L. Holloway 3,2,4 , F. Aly 2,3,4 , E. Koh 3,4,2 , P. Keall 1,2 1 University of Sydney, ACRF Image X Institute, Sydney, Australia; 2 Ingham Institute for Applied Medical Research, Medical Physics, Liverpool, Australia; 3 Liverpool and Macarthur Cancer Therapy Centre, Radiation Oncology, Liverpool, Australia; 4 University of New South Wales, South Western Clinical School, Sydney, Australia; 5 University of Wollongong, Centre for Medical Radiation Physics, Wollongong, Australia Purpose or Objective Glioblastoma multiforme (GBM) is the most aggressive primary brain cancer carrying a poor prognosis, characterised by tumour recurrence only months after surgical resection and chemo-radiation using standardised target volume delineation for all patients. Tailored adaptive radiotherapy (RT) plans incorporating multiparametric MR (mpMRI) biomarkers may improve outcomes by identifying regions of infiltrating tumour (IT) via a tumour probability (TP) map. This would potentially allow selective RT dose escalation to biologically active tumour and healthy tissue sparing utilising a dose-painting (DP) approach. This