MRI diagnosis of rectal cancer will be reviewed regarding 1) Anatomy of the rectum and per-rectal structure, 2)MR imaging technique for preoperative assessment of rectal cancer, 3) Important MR findings in preoperative assessment of rectal cancer, 4) Diagnostic accuracy of MR in rectal cancer.
Rectal cancer accounts for about one-third of colorectal cancer with the five-year survival rate of around 60-70% which varies depending on tumor advancement as; 100% for Tis, 90% for T1 or T2 and 70% for T3 or T4 tumor unless node involvement is present. It, however, drops to 40% in cases with lymph node metastases and to 5% with distant metastases. Vascular invasion also affects the survival. Local recurrence which ranges from 3% to 32% is mainly caused by incomplete removal of lateral spread of the tumor that influences on the patients’ quality of life and prognosis, as well. Standard treatment for rectal cancer is surgical resection and chemo-radiation therapy is added depending on the tumor extension and further aggressive management is necessary for more advanced tumor. Current surgical technique has been standardized by the total mesorectal excision (TME) which removes tissues in the mesorectal fascia as a block where lymph nodes, vessels, nerves and fat tissue exist. Therefore, precise assessment of relation between tumor and the mesorectal fascia is important for appropriate management to achieve complete tumor resection. In fact, variation of recurrent rate is largest in T3 tumors depending on the distance between tumor and the mesorectal fascia.MRI is considered as the most reliable imaging test for pre-operative assessment of rectal cancer since it provides excellent anatomical information as well as tumor extent with its high special and soft tissue contrast resolution. In this presentation, 1) to 4) below will be discussed.
1) Anatomy of the rectum and per-rectal structure: Rectal wall consists of mucosal layer, muscular layer of mucosa, submucosal layer, proper muscular layer which demonstrates low, low, high and low signal on T2WI of MRI, respectively. The mesorectal fascia is demonstrated as a low signal linear structure surrounding the rectum and peri-rectal fat tissue containing lymph nodes, vessels and nerves. Intra-pelvic muscular anatomy is also well demonstrated.
2) MR imaging technique for preoperative assessment of rectal cancer: Neither intraluminal or intravenous contrast media is not used. Anti-peristaltic agent may be applied. Mainly, T2 weighted MR images are obtained with phased array coils. Relatively small FOV (160mm) and thin-section thickness (3-5mm) are used to provide high resolution images. Axial, coronal and/or sagittal images and images of perpendicular plane to the longitudinal axis of the rectum are obtained. Diffusion images are also obtained for detection of tumor and metastatic lymph nodes.
3) Important MR findings in preoperative assessment of rectal cancer: Tumor is usually demonstrated as slightly higher signal than proper muscular layer and as lower signal than submucosal layer and fat tissue within the mesorectal fascia. Relation between "tumor and proper muscular layer", and "tumor and mesorectal fascia" are important in the assessment of local tumor extension. Assessment of vascular invasion (extramural venous invasion) is also important for the evaluation of patients' prognosis. Lymph node metastases are assessed from shape and intra-nodal signal as well as signal brightness on diffusion weighted images.
4) Diagnostic accuracy of MR in rectal cancer: Diagnostic accuracy in T staging ranges between 65% and 86%, and limitation exists in distinction between T2 and border-line T3 lesions since peri-rectal fat stranding caused by fibrosis alone and by fibrosis containing tumor cells look similar on MRI. While, accuracy in distinction between T3 and T4 ranges 80 and 86%.