MRI Evaluation of Rectal Cancer: Staging and Restaging

https://doi.org/10.1067/j.cpradiol.2016.11.011Get rights and content

Magnetic resonance imaging (MRI) plays an important role in the staging and restaging of rectal cancer. Multiplanar high-resolution (≤3-mm section thickness) T2-weighted images are the primary sequences used for rectal cancer staging. No preprocedural bowel cleansing regimen, intravenous contrast material, nor endorectal coil is necessary. MRI is highly accurate for differentiating T1-T2 disease from T3 and T4 disease, an important distinction as patients with T3 and T4 tumors typically undergo preoperative neoadjuvant chemoradiation before resection. At MRI, the muscularis propria appears as a thin black line encircling the outer wall of the rectum, and tumor extension through this line indicates T3 disease. Further tumor extension into adjacent organs indicates T4 disease. Endorectal ultrasound is generally preferred to differentiate T1 (submucosal involvement) from T2 (extension into but no disruption of muscularis propria) disease. MRI is also accurate in the assessment of tumor involvement of the mesorectal fascia. Tumor involvement of the mesorectal fascia increases the likelihood of recurrence following resection. MRI is less accurate for determination of lymph node status, though heterogeneous signal intensity and irregular margins are suggestive of node positive disease. Approximately 10%-30% of patients who undergo preoperative chemoradiation experience a complete pathologic response that is defined as no residual tumor found at histopathologic analysis of the resected specimen. The addition of diffusion-weighted images to T2-weighted images improves the accuracy of restaging examinations for determination of complete pathologic responders.

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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