Rectal Cancer Staging
Endorectal ultrasound of a T2 cancer. The submucosa has been breached (arrows) but the muscularis propria layer is intact
CT of the chest, abdomen and pelvis is routine in colorectal cancer staging to exclude pulmonary or hepatic metastases, and to show any lymphadenopathy or peritoneal involvement. However, rectal cancer presents present greater technical problems in resection and preventing local recurrence than colonic tumours, and understanding the spread of the tumour allows a rationale decision to be made regarding pre-operative chemoradiotherapy and the type of surgery needed. Multidetector CT has considerably improved the ability of CT to visualise the primary tumour, but its poor soft tissue resolution in comparison to MRI. MRI therefore remains the examination of choice for local staging, and this has been borne out by the MERCURY study (1), to answer practical management issues:
1. Does the tumour involve the mesorectum or peritoneum?
2. Are there involved lymph nodes outside the mesorectum, i.e. in the side wall of the pelvis?
If the answer is yes to any of these questions, then the favoured surgical procedure of total mesorectal excision will leave tumour behind, and to avoid inevitable local recurrence pre-operative chemoradiotherapy is required.
A typical MRI scan for rectal cancer staging does not require any patient preparation or contrast injection. Initially a T2w sagittal sequence is taken through the pelvis to identify the tumour and plan a series of high definition thin section T2w images using phased array pelvic coils at right angles to the longitudinal axis of the tumour, to see the extent of the tumour outside the rectal wall and relate this to the mesorectum. A standard series of T2w images is also taken through the pelvis to the iliac bifurcation to look for lymph nodes. With low rectal cancers a high definition series in the coronal plane helps show if there is invasion of the levators or anal canal. Lymph node involvement is based mainly on the size of the node, but the appearance of the node on MRI may also help distinguish infiltrative from reactive changes.
One of the side effects of colorectal cancer screening will be an increase in the number of early rectal cancers detected. Whether a cancer is T1 or T2 may affect management, and there is always the problem of being sure that large villous adenomas are not malignant in part. CT does not resolve the bowel wall layers, MRI can but with less precision than endosonography. Rectal endosonography is really a complementary examination to MRI (2) as it is better at looking at tumour extension within the rectal wall, and MRI outside the rectal wall. With a rigid endoprobe only tumours in the lower rectum (<12cms) can be visualised, higher lesions require insertion of the probe through a rectoscope. The rectum must be prepared. A disposable Fleet enema 20 min before works well. Acoustic contact is established using water filled balloon system. The distinction between T0 and T1 requires careful examination to demonstrate loss of the lamina propria at the site of invasion. T2 involvement is more robust as the submucosal reflective layer is lost, but inflammatory changes around T2 cancers may lead to overstaging as a T3.
Conclusion:
. MRI for local staging
. CT (chest, abdomen, pelvis) for distant metastases
. Rectal endosonography if local excision is being considered
(1) Brown G, Daniels IR. Preoperative staging of rectal cancer: the MERCURY research project. Recent Results Cancer Res 2005; 165:58-74.
(2) Bartram C, Brown G. Endorectal ultrasound and magnetic resonance imaging in rectal cancer staging. Gastroenterol Clin North Am 2002; 31(3):827-839.