Magnetic resonance imaging of the low rectum: defining the radiological anatomy

Low rectal cancer provides a particular surgical challenge of local tumour control and sphincter preservation. Histopathological studies have shown that an involved circumferential resection margin (CRM) and depth of extramural invasion are independent markers of poor prognosis and correlate with high local recurrence rates due to residual microscopic disease [1]. Recent data suggests that a CRM at risk of tumour involvement can be reliably seen on the pre-operative magnetic resonance imaging (MRI) scan with good correlation with the histological specimen [2-5]. In published series, low rectal cancers have a higher incidence of involved resection margins, with rates up to 30% for abdomino-perineal excision (APE) vs 10% for low anterior resection (LAR) [6-9]. This has been attributed to narrow surgical planes deep within the pelvis as the mesorectum becomes narrowed and tapered, forming a bare muscle tube at the level of the anal sphincter complex. The challenge for the surgeon is to undertake careful removal of a cylinder of tissue beyond the rectal wall without perforating the tumour. An overall local recurrence rate of 10% after APE for all stages of rectal cancer has been reported and this low rate was attributed to the surgical technique that included a wide peri-anal dissection and lateral division of the levator ani. The abdominal dissection was stopped above the tumour, taking care to avoid separation of the tumour from the levator ani to reduce the risk of inadvertent tumour cell spillage [8]. Therefore, rates of involved surgical margins from APE specimens may be reduced when a cuff of levators is taken compared with standard resection. In this review, we will discuss how MRI of the low rectum can aid in the staging and optimization of the best treatment strategy for low rectal cancer.