ESTRO 2023 - Abstract Book

S1856

Digital Posters

ESTRO 2023

Results Figure 2a shows the number of times each sequence was scored higher for each structure examined on the DWI and ADC images.

The Bland-Altman analysis (Figure 2b) shows the results of the quantitative comparison between the two sequences.

Conclusion The modified sequence performed as well as or better than the consortium sequence, in the qualitative image quality assessment for 92% (DWI & ADC) of comparisons for whole prostate, peripheral/transitional zone and tumour visualisation. The modified sequence performed as well as or better that the consortium sequence for 77% (DWI) /69% (ADC) of comparisons for seminal vesicle visualisation. These results indicate that the modified sequence is comparable to, or improves, images quality. In the comparison of ADC, the modified sequence measured median prostate ADC on average 5% lower than the consortium sequence, indicating a systematic difference between the two sequences. ADC measurements from the modified sequence should be directly compared to those obtained from the consortium sequence. Further work will be undertaken to use the modified sequence to measure ADC throughout treatment to detect ADC changes. 1 Odense University Hospital, Laboratory of Radiation Physics, Department of Oncology, Odense, Denmark; 2 University of Southern Denmark, Department of Clinical Research, Odense C, Denmark; 3 Odense University Hospital, Department of Oncology, Odense, Denmark; 4 University of Southern Denmark, Department of Clinical Research, Odense, Denmark Purpose or Objective The apparent diffusion coefficient (ADC), derived from diffusion-weighted magnetic resonance imaging (DWI) is a potential radiotherapy (RT) response biomarker. The aim of this study was to investigate if treatment response can potentially be measured during RT using longitudinal ADC measurements in patients with pancreatic tumours. Materials and Methods The study included 37 patients with pancreatic tumours receiving 50 Gy in five fractions on a 1.5 T MR-linac (Unity, Elekta). DWI scans were acquired at each fraction, and ADC values were calculated both within the gross tumour volume (GTV) (ADC_GTV) and the kidney (ADC_kidney) that was far outside the high dose region. Calculations were based on the median DWI signal at b-values 150 and 500 s/mm2. A visual inspection of the images was performed to check for artifacts in or near the relevant regions. For each patient, ADC values were normalized to the value at the first fraction, and the time dependency was measured as the slope of a linear fit of the normalized ADC as a function of fraction number. For each fit, PO-2076 Longitudinal ADC measured with MR-linac in patients with pancreatic tumours A. Bisgaard 1,2 , U. Bernchou 1 , A.S. Bertelsen 1 , T. Schytte 3,4 , C. Brink 1,4 , M.W. Ejlsmark 3,4 , F. Mahmood 1,4