ESTRO 2024 - Abstract Book

S1494

Clinical - Lower GI

ESTRO 2024

Early prediction of tumour response to neoadjuvant treatment for rectal cancer patients could enable a more personalised approach. Although the evaluation through functional magnetic resonance imaging (MRI) has demonstrated its potential as a predictive biomarker, the advent of the 1.5T MR-Linac Unity® (Elekta Ltd, Stockholm, Sweden) opens the possibility to investigate intravoxel incoherent motion diffusion-weighted imaging (IVIM-DWI) on a daily basis, thereby enabling a better understanding of the evolution of tumour biology during radiotherapy. As radiotherapy induces very early biological changes (from seconds to minutes), these effects can now be studied in vivo in humans through the MR-Linac. The objective of this study is therefore to analyse the inter- and intra-fractional dynamic evolution of the diffusion coefficient (D) and perfusion fraction (p-frac) during short-course radiotherapy in patients with rectal cancer.

Material/Methods:

We conducted an analysis on 15 patients with rectal cancer treated on a 1.5T MR-Linac with short course radiotherapy (5x5Gy). Patients underwent T2-weighted (T2w) MRI and IVIM-DWI before (pre) and after (post) the irradiation at each fraction (Fx), using b-values 0,150 and 500 s/mm2. Tumour delineation was performed manually using MIM® software based on the highest b-value DWI and the apparent diffusion coefficient (ADC) map with the T2w as guidance. For each fraction, D and p-frac were computed from IVIM-DWI. These values were then stratified into two groups based on the patients’ tumour regression grade (TRG) obtained from reassessment MRI: responders with a TRG ≤ 3 and non-responders with a TRG> 4. Wilcoxon signed rank tests were used to assess variation between Fx1 and Fx5. Differences below p<0.05 were deemed statistically significant.

Results:

All patients completed their MR-Linac treatment and tolerated it well. Among them, eight were classified as responders, four as non-responders and three are awaiting response reassessment. Out of 150 acquisitions, 143 were analysed, with seven images missing post-irradiation. Over the entire treatment course, D and p-frac of the responders' cohort significantly increased between Fx1 and Fx5, (9.5% (p=0.016) and 3.9% (p=0.016), respectively), while for non-responders similar changes were observed without reaching statistically significant level (5.7% (p=0.630) and 3.8% (p =0.375), respectively) (see fig.1). Concerning the intrafraction variation (pre-post), preliminary results revealed an escalation of D for both groups, while a tendency of decrease in p-frac. Additionally, we observed a difference in baseline, with higher values in responders compared to non-responders. Specifically, responders exhibited a 3% higher p-frac and a 10% higher D at baseline, as demonstrated in the visual representation highlighting an increase in perfused zones and their specific localisations (see fig. 2).