Session Item

R₂^* mapping can be used to characterize tumour hypoxia, which is associated with therapy resistance. Acquiring R₂^* maps during MR-guided radiotherapy can potentially help in treatment adaptation by escalating the dose to resistant sub-volumes. This study aims to evaluate R₂^* changes in patients with prostate cancer undergoing radiotherapy on an MR-Linac. 

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 R₂^* mapping was acquired twice per week before treatment delivery using a radial multi-echo gradient-echo sequence with the following parameters: TR = 48 ms, ΔTE=5ms, FOV = 400x400x180 mm, and 1.5x1.5x4 mm³ acquisition voxel size, TA= 7.56 minutes). R₂^* maps were calculated from the magnitude images in MATLAB (The Mathworks, Natick, MA, USA) using a linear operation on the R₂^* exponential decay curve. Prostate was delineated manually and mean R₂^* 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. 

Figure 1a shows the mean R₂^* values during the course of radiotherapy.  Baseline mean R₂^* values varied across different patients, which could be attributed to the inter-patient variability. Repeated measure ANOVA showed no statistically significant difference in R₂^* values across different fractions (p=0.24). This variability can be seen visually in R₂^* maps of two patients across different fractions (Figure 1b). Figure 2 shows the comparison of R₂^* values during fraction 0 (before radiotherapy) and R₂^* of all the last five (15-20) fractions. There is a decrease in R₂^* values in five out of seven patients as compared to baseline. The difference between R₂^* 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 R₂^* values (r=0.82, p=0.008).

This was an exploratory study to evaluate the weekly changes in R₂^* values for prostate cancer patients undergoing radiotherapy on an MR-Linac. There was no persistent change in R₂^* values in all patients with an overall decrease in R₂^* in five out of seven patients at the end of treatment. This decrease in R₂^* values could be indicative of the fibrosis or apoptotic changes in the prostate. This work focused only on the changes within the whole prostate in a limited number of patients and future work will evaluate the voxel-level changes in the tumour.  The correlation of R₂^* with clinical and quantitative parameters (e.g diffusion) may help characterise the nature of these changes and can be useful for adaptive radiotherapy.