S2496
Physics - Radiomics, functional and biological imaging, and outcome prediction
ESTRO 2026
References: 1. Nomura M et al. Predictive factors for radiation pneumonitis in oesophageal cancer. Br J Radiol. 2012;85:e603-e608. doi:10.1259/bjr/143864852. Tonison JJ et al. Radiation pneumonitis after intensity- modulated radiotherapy for esophageal cancer. Sci Rep. 2019;9:1851. doi:10.1038/s41598-018-38414-53. van Hagen P et al. Preoperative chemoradiotherapy for esophageal or junctional cancer. N Engl J Med. 2012;366:2074-2084. doi:10.1056/NEJMoa1112088 Keywords: dose distribution, V5/V20 ratio Digital Poster 4969 Temporal ADC dynamics on a 0.35 T MR-Linac as an early response biomarker in rectal cancer Matteo Nardini 1 , Giuditta Chiloiro 1 , Giulia Panza 1 , Matteo Galetto 1 , Angela Romano 1,2 , Carlo Guglielmo Cattaneo 2 , Marco Valerio Antonelli 1 , Marco Rapisarda 1 , Claudio Votta 1 , Andrea Bevacqua 1 , Luca Boldrini 1,2 , Lorenzo Placidi 1 1 Diagnostic Imaging and Oncological Radiotherapy Department, Fondazione Policlinico Universitario A. Gemelli, IRCCS, Rome, Italy. 2 Department of Radiological and Hematological Sciences, Catholic University of the Sacred Heart, Rome, Italy Purpose/Objective: Quantitative MRI (qMRI) provides objective and reproducible measurements of tumour characteristics, which support the assessment of neoplasia and the capacity to stratify patients into different prognostic groups, guiding therapeutic decisions. qMRI acquired on MR-Linac systems enables treatment-response monitoring within online adaptive radiotherapy workflows. We investigated whether the apparent diffusion coefficient (ADC) measured serially on a 0.35 T MR-Linac could discriminate responder patients (R) from non-responder patients (NR) in rectal cancer patient cohort. Material/Methods: Twenty patients treated on a MR-Linac (0.35 T/6 MV) were retrospectively analysed. Treatment consisted of a 25 fractions SIB2 55/45 Gy to the mesorectum (CTV1) and nodes (CTV2). At 10th fraction, patients were reassessed using the Early Regression Index1(ERI) and classified as R or NR; NR patients received in total a 5.1 Gy dose escalation to the GTV within an online adaptive MRIgRT protocol2. Diffusion-weighted imaging (DWI-b=50, 800 s/mm ² ) of pelvic region was acquired pre-treatment and weekly during therapy. Mono-exponential fitting was used to calculate ADC maps. After rigid registration of morphological images (TrueFISP) to ADC maps, the GTV was contoured on TrueFISP images and mean ADC of GTV ROI extracted at each timepoint(Fig.2). Mann–Whitney test was used
to assess between-group differences (RvsNR) in mean ADC at each timepoint.
Fig.1-GTV (green line) on Trufi (a) and reported on relative ADC map(b). SIB3 planning volumes and isodoses(c). Results: The cohort comprised 9 R and 11 NR. Mean ADC of GTV differed significantly between R and NR for almost all fractions (mean p=0.095), except the 25th (p=0.94) (Fig.2). The largest differences were observed at fraction 5 (p=0.056). The temporal ADC profile exhibited a relative peak at fraction 10 where we have the maximum increase of ADC compared to the simulation (226.0*10-6 mm2/s (NR) and 241.7*10- 6 mm2/s (R)). The average increase in ADC during treatment was 213.0*10-6 mm2/s for R and 194.41*10-6 mm2/s for NR.
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