ESTRO 2022 - Abstract Book

S1553

Abstract book

ESTRO 2022

R 2 * mapping can be used to characterize tumour hypoxia, which is associated with therapy resistance. Acquiring R 2 * maps during MR-guided radiotherapy can potentially help in treatment adaptation by escalating the dose to resistant sub- volumes. This study aims to evaluate R 2 * changes in patients with prostate cancer undergoing radiotherapy on an MR-Linac. Materials and Methods Seven patients with prostate cancer received radiotherapy after androgen deprivation therapy with a dose of 60Gy/48.6Gy to prostate and seminal vesicles in 20 fractions. Patients were treated on a 1.5T MR-Linac (Elekta AB. Stockholm, Sweden) and R 2 * mapping was acquired twice per week before treatment delivery using a radial multi-echo gradient-echo sequence with the following parameters: TR = 48 ms, Δ TE=5ms, FOV = 400x400x180 mm, and 1.5x1.5x4 mm 3 acquisition voxel size, TA= 7.56 minutes). R 2 * maps were calculated from the magnitude images in MATLAB (The Mathworks, Natick, MA, USA) using a linear operation on the R 2 * exponential decay curve. Prostate was delineated manually and mean R 2 * values within the whole prostate were compared across all fractions. The first day of treatment (fraction 0) was considered baseline and the last five fractions (15-20) were considered as an end of treatment. Statistical analysis was performed using Graphpad Prism. Results Figure 1a shows the mean R 2 * values during the course of radiotherapy. Baseline mean R 2 * values varied across different patients, which could be attributed to the inter-patient variability. Repeated measure ANOVA showed no statistically significant difference in R 2 * values across different fractions (p=0.24). This variability can be seen visually in R 2 * maps of two patients across different fractions (Figure 1b).

Fi gure 2 shows the comparison of R 2 * values during fraction 0 (before radiotherapy) and R 2 * of all the last five (15-20) fractions. There is a decrease in R 2 * values in five out of seven patients as compared to baseline. The difference between R 2 * values within the prostate of fraction 0 and fraction 15-20 was not statistically significant (paired t-test, p=0.2 ). There was a significant correlation between fraction 0 and fraction 15-20 R 2 * values (r=0.82, p=0.008 ).

Conclusion This was an exploratory study to evaluate the weekly changes in R 2 * values for prostate cancer patients undergoing radiotherapy on an MR-Linac. There was no persistent change in R 2 * values in all patients with an overall decrease in R 2 * in