MRI Evaluation of Rectal Cancer: Staging and Restaging
Introduction
Colorectal cancer is the third leading cause of cancer worldwide, and rectal cancer accounts for approximately 30%-35% of cases of colorectal cancer.1, 2, 3 Advancements in surgical techniques, chemotherapy, and radiation therapy regimens have resulted in decreased patient morbidity, decreased rates of local recurrence, and decreased mortality.4 For example, development of the total mesorectal excision (TME) resection technique in the late 1970s that includes removal of the rectum, perirectal fat, and surrounding mesorectal fascia has resulted in decreased rates of local recurrence.5, 6, 7, 8, 9 Today in some centers, carefully selected patients with small superficial tumors may be offered a minimally invasive resection technique such as transanal endoscopic microsurgery or transanal minimally invasive surgery.10, 11, 12 Multiple studies have also reported reductions in rates of local recurrence and in some instances improved survival with preoperative chemotherapy and radiation.13, 14, 15, 16, 17 Currently, preoperative chemoradiation is typically administered to individuals who have tumors that extend beyond the rectal wall or node-positive disease.
Given the variety of treatment options available—options which vary based on tumor stage—accurate tumor staging is critically important. Magnetic resonance imaging (MRI) plays an important role in the staging and restaging of rectal cancer. This article reviews MRI technique as well as MRI staging and restaging of rectal cancers.
Section snippets
Technique
High-resolution T2-weighted imaging, defined as section thickness ≤3 mm, is the primary sequence used for local staging of rectal cancer. Small voxel size (eg, 1.0 × 0.7 × 3 mm) is important for high-resolution local staging sequences.18 Sequences are typically acquired in the axial, coronal, and sagittal planes. Axial T2-weighted images should be acquired perpendicular to the rectal tumor axis. Coronal T2-weighted images should be acquired parallel to the tumor axis or anal canal. Axial images
Initial Staging
At initial evaluation, the role of MRI is to determine the stage of the rectal cancer, assess for involvement of the CRM, and evaluate the relationship of the tumor to the anal sphincter.29 Rectal cancer is staged based on the T, N, M staging system where T stage refers to the local extent of tumor spread, N stage refers to regional lymph nodes, and M stage refers to distant metastatic disease.30 Additionally, the location of the rectal tumor (eg, lower third, middle third, or upper third of
Restaging
MRI is also the modality of choice for restaging rectal malignancies. As with initial staging, high-resolution T2-weighted imaging is the primary sequence for morphologic assessment of rectal malignancy. However, a pitfall of T2-weighted imaging following chemoradiation is that posttreatment fibrosis can be difficult to differentiate from residual tumor. A meta-analysis found a relatively poor mean sensitivity (50%) but a good mean specificity (91%) for MR restaging of rectal cancer after
Author Contributions
C.C. Moreno, P.S. Sullivan, and P.K. Mittal wrote and edited this manuscript.
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