|ECR 2018 / C-2109|
|Current staging of rectal cancer with MRI: what you need to know and be aware of|
Findings and procedure details
In this section, we review MRI technique, along with illustrations of important anatomic MRI landmark, key MRI findings for different local stages of rectal adenocarcinoma are described and illustrated with cases from our institution.
In our institution rectal cancer staging is performed in a 1.5T MR scanner, using a 10ch torso coil, with no prior patient preparation (unless fasting for the last 4 hours) and scopolamine intravenous administration. Our standard rectal cancer staging MRI protocol consists of T2 weighted-images in direct axial and sagittal planes. Two further high-resolution T2 weighted-images are performed (small field-of-view and slice thickness ≤3mm), perpendicular and parallel to the largest axis of the rectal tumor. In the low rectal tumors we add images directed to the anal canal. We also perform diffusion-weighted imaging, with 2 b-values acquisition (b=0, b=1000) and the corresponding ADC map.
Anatomical Landmarks in Pelvic MRI for rectal cancer
Anal verge - it is the most distal portion of the anal canal, where the non-keratinized stratified squamous epithelium turns into the keratinized epithelium of the perianal skin .
Dentate line - it is the line that divides the proximal two thirds and the distal third of the anal canal. It separates the columnar epithelium (proximal two thirds) from the non-keratinized stratified squamous epithelium (distal third) . It is not visible on MRI.
Anorectal angle - it is situated at the upper portion of the puborectalis muscle, constituting the upper portion of the surgical anal canal .
Internal sphincter - it is the inner muscular wall of the anal canal which is the direct continuation of the circular muscularis propria of the rectum .
External sphincter - it is composed of the inferior portion of the levator ani muscle, the puborectalis muscle and the deep, superficial, and subcutaneous external sphincter muscles .
Rectosigmoid junction - the anatomical definition of the rectosigmoid junction is based on haustrations which are present in the sigmoid colon and absent in the rectum, about 15 cm from the anal verge. On high-resolution MRI, the rectosigmoid junction is the location where the rectum becomes completely covered by peritoneum (the anterior peritoneal reflection) .
Anterior peritoneal reflection - the line where the peritoneum reflects from the rectovesical/rectovaginal pouch to cover the anterior face of the rectum. This anatomic landmark is consistently depicted on MRI, and it is considered the rectosigmoid junction .
Mesorectal fat - layer of fat surrounding the rectum .
Mesorectal fascia - visceral layer of the endopelvic fascia, which encircles the rectum and the mesorectal fat, nodes, and lymphatic vessels to form a distinct anatomic unit .
Pelvic organs - the structures most commonly involved by rectal cancer are the uterus, vagina, prostate gland, and seminal vesicles .
Pelvic sidewall - the structures along the pelvic sidewall in proximity to the rectum include the common, external, and internal iliac vessels, the ureters, the pyriformis foramen .
Tumor location 
- Low rectal tumor: Distal border is up to 5 cm from the anal verge.
- Mid rectal tumor: Distal border is 6-10 cm from the anal verge.
- High rectal tumor: Distal border is >10 cm up to 15 cm from the anal verge.
Key findings on MRI for treatment selection
Key findings on MRI for treatment selection include:
1. Local staging;
2. Nodal staging;
3. Mesorectal fascia (MRF) involvement;
4. Extramural venous invasion.
1. Local Staging
In the TNM classification for colorectal cancer, tumor staging is based on the local tumor transmural invasion (T), the number of metastatic nodes (N) and presence of distant metastases (M).
T1 and T2 tumors are confined to the rectal wall Fig. 2 : T1 tumors invade the submucosa and T2 tumors invade the muscularis propria, which is demonstrated as a thin lower signal layer in the rectal wall. For the specific differentiation between T1 and T2 tumors, endo-rectal ultrasound is recommended, since MRI is yet not able to consistently and accurately distinguish these stages .
T3 tumors grow through the rectal wall and infiltrate the mesorectal fat. With some exceptions, tumors tend to extend beyond the muscular layer at their centre, both in the longitudinal plane, parallel to the rectum and in the plane perpendicular to the rectum .
- T3a: < 1mm extension beyond muscularis propria Fig. 3.
- T3b: 1-5 mm extension beyond muscularis propria Fig. 4.
- T3c: 5 - 15 mm extension beyond muscularis propria Fig. 5.
T4 tumors growth into neighbouring organs.
- T4a: extension to the visceral peritoneum (usually the anterior peritoneal reflection) Fig. 7.
A tumor that invades the pelvic floor or pelvic sidewall muscles should be considered a T4 tumor Fig. 12 . Tumor perforation to the peritoneal cavity is also classified as T4 according to the TNM classification.
In low rectal tumors the report should include and describe the presence or absence of tumor invading the anal sphincter complex. If a tumor extends caudally into the internal sphincter, it may be considered a T3 tumor Fig. 13 . Involvement of the intersphincteric plane, external sphincter and levator musculature should be assessed, as this influences treatment planning, namely the surgical protocol Fig. 14. There is no consensus whether a tumor involving the external anal sphincter should be T3 or T4 . A detailed description of findings is crucial is this cases, for the adequate management.
2. Nodal Staging
At primary staging, MRI can differentiate between N0 and N+ tumors. Criteria for lymph node staging on T2-weighted sequences are signal intensity, border contour and shape. Morphologically suspicious criteria are round shape, irregular border and heterogeneous signal Fig. 15 .
Size is also a predictor, but there is no optimal inferior cut-off threshold for involved nodes.
Criteria for malignant node :
1. Short axis diameter ≥ 9 mm.
2. Short axis diameter 5–8 mm AND ≥ 2 morphologically suspicious characteristics.
3. Short axis diameter < 5 mm AND 3 morphologically suspicious characteristics.
4. All mucinous lymph nodes (any size).
TNM nodes classification:
- NX: Regional lymph nodes cannot be assessed
- N0: No regional lymph node metastases
- N1: Metastases in 1–3 regional lymph nodes
- N1a: Metastases in one regional lymph node
- N1b: Metastases in 2–3 regional lymph nodes
- N1c: Tumor deposit(s) in the subserosa, mesentery, or nonperitonealized pericolic or perirectal tissues without regional nodal metastasis
- N2: Metastases in 4 or more regional lymph nodes
- N2a: Metastases in 4–6 regional lymph nodes
- N2b: Metastases in 7 or more regional lymph nodes
Extra-mesorectal suspicious lymph nodes imaged by the pelvic MRI study should be specifically mentioned so that radiotherapy and surgery protocols can be adapted Fig. 16.
Regional lymph nodes include internal iliac, inferior mesenteric, pre and lateral sacral, sacral promontory, sigmoid mesenteric, superior, middle and inferior rectal lymph nodes.
3. Mesorectal fascia
MRF surrounds the mesorectal fat and is seen as a fine regular line of low signal intensity in high-resolution T2 images , contrasting with the hyperintense mesorectal fat. The distance to the mesorectal fascia is the single most important local prognostic factor . A distance of the tumor to the MRF of ⩽1 mm is considered involvement of fascia Fig. 1. In this case or if the tumor reaches the MRF, neoadjuvant therapy should be offered before surgery. In cases where the distance between the tumor and the MRF is >1 and ≤ 2mm, the MRF is considered at risk.
Although a suspicious lymph node spanning ⩽1 mm from the MRF should be assessed and reported, caution should be taken using this as a single finding defining MRF involvement .
4. Extramural vascular invasion (EMVI)
The macroscopic vascular invasion, as EMVI or discontinuous venous implants Fig. 17, should also be sook and reported. Although this feature is not mentioned in TNM classification, it is related to higher probability of metastatic disease, subsequent relapse and poor survival .
- Stranding into the mesorectal fat is an equivocal sign that may indicate either a T2 tumor with desmoplastic reaction Fig. 18 or a T3 tumor with tumoral strands Fig. 19. However, if on primary staging MRI, stranding extends from the tumor into the mesorectal fascia, it should be considered mesorectal invasion .
- Distinguishing EMVI from suspicious lymph nodes is sometimes difficult. An analysis in different planes may help the distinction: EMVI is likely if a vascular structure is in proximity or in continuity; or if the vessel is expanded, irregular or filled with tumor intensity material Fig. 20.
- Imaging in the optimal plane is crucial for correct interpretation, especially T staging. Incorrect plane obliquity leads to blurring of the muscularis propria or a pseudospiculated appearance that may lead to overstaging. It is crucial to know and identify tumor location. Diffusion-weighted imaging can assist in the localization of tumor. However, a significant limitation of diffusion-weighted imaging is that the primary tumor is sometimes obscured by susceptibility artefacts from bowel gas. In this case, endorectal filling with 60 ml of gel can be useful.
- One should avoid staging for lesions that are not definitely cancers. T-staging is only for rectal adenocarcinomas and not for benign polyps or other tumors.
- Squamous cell carcinomas of the anal canal have a staging system that is different from that of low rectal tumors. The same applies to metastases from other primaries to the rectum.
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