Image-guided Radiotherapy for Rectal Cancer - A Systematic Review

Radiotherapy for rectal cancer is becoming more conformal. Both the rectum and the mesorectum are mobile structures and the use of image-guided radiotherapy techniques may improve treatment delivery. Studies up to 2008 have previously been reviewed; rectal motion was mostly studied in bladder and prostate cancer cases. Large variations were seen in both the rectal volume and rectal wall displacement during the treatment course. We reviewed the literature on primary rectal cancer. A systematic review was conducted using Medline and Embase databases using the keywords 'rectal, radiotherapy, IGRT, image guided, organ motion, internal margin, target shape/volume'. Nine studies looked at both inter- and intrafractional motion of the gross tumour volume, rectum, mesorectum and the clinical target volume using a variety of imaging modalities. There was significant movement in the upper mesorectum. There was a strong relationship between rectal filling and mesorectal motion. Differences according to gender and body mass index have been reported. One study showed adequate dose to the rectum despite rectal motion and deformation. Current margin recipes may not apply to deformable structures. Suggested margins for the clinical target volume to planning target volume expansion are between 1 and 3.5 cm. There may be a role for re-imaging and re-planning during a treatment course. From the available data, electronic portal imaging devices should continue to be used to match for bony anatomy. Additional information on internal motion can be obtained by cone beam computer tomography or tomotherapy and if available its use should be considered. Individualised anisotropic margins may be required. Further work is required to assess the optimal imaging modality, whether to match to bone or soft tissue, and to assess if internal motion affects treatment outcome.