ESTRO 2020 Abstract Book

S404 ESTRO 2020

and 72 minutes. The latter due to a technical failure. Rest of the timings is summarized in table 1. A CTV coverage of >99% was achieved every fraction. In 2.5% of fractions PTV coverage was below the target of 97% and the V26.75 Gy was above 5 cc in 0,8%. However, both violations were accepted after visual inspection of the treatment plan.

precession sequence was acquired during the simulation and every day of the MRgRT treatment using a 0.35 T MR scanner. GTV contouring was shared between 2 radiation oncologists and blinded with respect to any information regarding the TRG. The physical doses were converted in Biologically Effective Dose (BED) to compensate the different radiotherapy schemes adopted in the centres, considering α/β equal to 10, recovery factor equal to 0.6Gy/day and proliferation delay time equal to 7 days. ERI TCP was calculated considering the fractions corresponding to a BED of 23Gy. Table 1 summarises the treatment details of the different centres and the fractions analysed to obtain the same BED levels. To investigate the optimal timing to predict pCR, different ERI values were estimated considering as V mid the volumes at different BED levels: 12, 36 and 45Gy.

Conclusion In conclusion, an online adaptive workflow with full replanning for hypofractionated radiotherapy in patients with rectal cancer on a 1.5T MR-linac is feasible, well tolerated and currently takes about 48 minutes per fraction.

Poster Highlights: Poster highlights 22 PH: Outcome modelling

PH-0715 External validation of ERITCP as response predictor in rectal cancer using MR-guided Radiotherapy D. Cusumano 1 , L. Boldrini 1 , P. Yadav 2 , Y. Gao 3 , G. Chiloiro 1 , A. Piras 4 , S. Broggi 5 , J. Lenkowicz 4 , L. Placidi 1 , H. Musunuru 2 , N. Dinapoli 1 , B. Barbaro 1 , L. Azario 1 , M.A. Gambacorta 1 , M. De Spirito 1 , M. Basetti 2 , Y. Yang 3 , C. Fiorino 5 , V. Valentini 1 1 Fondazione Policlinico Universitario A.Gemelli IRCCS, Dipartimento di Diagnostica per immagini- Radioterapia Oncologica ed Ematologia, Roma, Italy ; 2 School of Medicine and Public Health- University of Wisconsin- Madison, Department of Human Oncology, Madison, USA ; 3 University of California, Department of Radiological Sciences, Los Angeles, USA ; 4 Università Cattolica del Sacro Cuore, Istituto di Radiologia, Roma, Italy ; 5 San Raffaele Scientific Institute, Medical Physics, Milan, Italy ) is a radiobiological parameter that showed promising results in predicting pathological complete response (pCR) on T2-weighted 1.5T MR images of patients affected by Locally Advanced Rectal Cancer (LARC). It models the early rectal cancer regression combining volumetric tumour information obtained during treatment simulation (V pre ) and at mid therapy (V mid ) (Formula in Fig.1). This study aims to validate ERI TCP in the context of low tesla MR-guided Radiotherapy (MRgRT), using MR images acquired with different magnetic field strength and imaging sequence. Furthermore, the optimal timing of MR imaging for pCR prediction was estimated, calculating the ERI index at different dose levels and quantifying its predictive ability in terms of AUC. Material and Methods 43 patients were enrolled from 3 institutions and subject to a MRgRT treatment delivered in 25-28 fractions with concomitant chemotherapy. Tumour regression grade (TRG) was defined according to Mandard and pCR was defined as TRG=1. For each patient, a true fast imaging with steady state Purpose or Objective Early Regression Index (ERI TCP

Results 7 patients showed pCR. ERI TCP showed the highest predictive performance, showing an AUC of 0.96 (95%CI:0.93-1). In particular, using the threshold of 13.1 previously suggested from the analysis of the training cohort, this parameter correctly classified 41/43 cases (accuracy=95%), showing good results in terms of sensitivity (86%), specificity (97%), NPV (97%) and PPV (86%), as shown in Fig.1. The value of 23Gy represents the optimal BED level to perform prediction; ERI estimated at different BED levels showed lower AUC: 0.69 (0.41-0.93) for ERI 12Gy , 0.87 (0.73-0.98) for ERI 36Gy and 0.78 (0.56-0.96) for ERI 45Gy