Pre-operative staging of rectal cancer: MRI or ultrasound?

Optimal management of rectal cancer depends on obtaining accurate and detailed staging information at the time of diagnosis. The majority of this comes from radiological staging investigations such as computed tomography (CT), magnetic resonance imaging (MRI) and endoanal ultrasound (EAUS). Whilst there is little debate on the use of CT to assess distant spread of disease, there is still variation in the use of MRI or EAUS in the local staging of rectal cancer. Both techniques have their roles but MRI is better able to visualise the entire rectum and mesorectum as well as accurately identify the circumferential resection (CRM) margin in relation to the tumour edge. Breach of the CRM is one of the most important predictors of local recurrence and knowledge of its relationship to the tumour determines initial management. MRI has additional advantages in being able to identify other poor prognostic factors such as extramural venous invasion (EMVI) and mucin deposition, which further influence oncological treatment. It also provides the surgeon with accurate information on the relationship of the tumour to surrounding structures and the sphincter complex which is important for surgical planning. This review highlights the important determinants of local staging in rectal cancer and presents the evidence to answer the question as to which is a better imaging modality—MRI or EAUS?