Analysis of rectal dose during prostate stereotactic body radiotherapy in MR-guided radiotherapy.

Abstract

242 Background: The Unity MR-Linac combines a 7-MV Linac with 1.5T magnetic resonance (MR) imaging capability and it enables adaptive radiotherapy, whereby the target and organs at risk are recontoured and a plan is optimised daily. During treatment a session MR image is taken first, on which the target and organs-at-risk are contoured, and a plan created. A verification image is taken prior to dose delivery to identify intra-fractional changes. If present, the daily treatment plan is shifted to reflect the anatomy. A post-treatment image is acquired at the end of treatment. This study evaluates the dosimetric changes to the rectum caused by intra-fractional changes during treatment delivery for prostate stereotactic body radiotherapy (SBRT) calculated on the verification and post-treatment images. Methods: The first five patients treated on the MR-Linac with 5-fraction SBRT to the prostate are included in this study. For each patient, the rectum was contoured on the verification and post-treatment MR images for each of the five fractions. The dose delivered to the rectum with the original treatment plan was then calculated on each image and the V36Gy rectal dose constraint was noted. Results: Out of the 25 fractions, a post treatment image was not performed in one fraction; 24 fractions were therefore analysed in total. The rectal V36Gy dose constraint exceeded the mandatory target of 2cc on 50% of the verification images and 46% of the post-treatment images. In 6 fractions the rectal V36Gy was greater than 2cc on both the verification and post-treatment images suggesting this rectal constraint was exceeded throughout treatment. In 17% of patients, the volume of rectum receiving 36Gy increased at each timepoint an image was taken during the treatment workflow. Conclusions: The rectal V36Gy dose constraint is susceptible to minor changes in rectal filling, which may often lead to higher than the accepted dose constraint. Thus, a single planning CT scan is unlikely to be representative of dose delivered. Adaptive radiotherapy can reduce this uncertainty somewhat, but intra-fraction dose re-optimisation would be required to ensure the rectal V36Gy remains acceptable at all times.