Rectal MRI is a key imaging investigation in the diagnosis, staging and follow up of rectal cancer. An increase in the utility of rectal MRI as been driven by the recognition of the mesorectum as a distinct anatomic compartment containing and limiting the margins of the rectum, and forming a surgical target to enable complete removal of the tumour and associated nodes en bloc. Several parameters need to be reviewed and reported on, and the emphasis on which factors are important has altered with emerging evidence.
The approach below takes recommendations from various major working groups and societies 1-4, as well as personal experiences, into consideration. The key reporting parameters described below may form the basis of a template report.
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Clinical information
The request should indicate biopsy confirmation of rectal malignancy, usually obtained via endoscopy, and the size of the tumour. It should also indicate the distance relative to the anal verge. MRI is usually not undertaken for tumours for which the distal edge is more than 15 cm from the anal verge.
MRI protocol
Standardised imaging acquisition is important. A combination of T2 and DWI sequences are employed for staging and restaging. There is no role for T1 weighted imaging, with post-contrast T1 weighted imaging reserved for imaging of recurrent rectal cancers in order to plan pelvic exenteration. Please see the article on rectal cancer MRI protocol for more details.
Step-by-step approach
Adequacy of imaging
the sagittal T2 images are used to plan the small field-of-view axial and coronal images, which enable accurate determination of the tumour's relationship to the muscularis propria and the mesorectal fascia
check that the axial and coronal images are truly perpendicular and parallel to the path of the rectum bearing the tumour
when the tumour traverses a part of the rectum that is not truly straight, it is necessary to acquire further sets of axial and coronal images
the risk of incorrect setting of the planes is volume averaging leading to unsharpness of the muscularis propria being misinterpreted as tumour transgression through the muscularis propria, essentially an upstaging of disease
Tumour size, location and morphology
the length of the tumour is measured, either on the sagittal or coronal images
the distance from the distal edge to the anorectal junction is measured; this is approximately the site where the angle between the rectum and anal canal is identified
the distance from the distal edge of the tumour to the anal verge is measured
the definition of whether a tumour is in the low, mid or upper rectum is widely variable and multiple proposals co-exist
the upper limit of the rectum is recognised as the sigmoid take-off on a sagittal image as the point at which the bowel travels anteriorly away from the sacrum 10
the morphology is described as circumferential or annular, semi-annular, or polypoid
semi-annular tumours typically have bulky edges which are the heaped margins of the outer edge of the tumour mass, and the invasive front will be between these edges and described using a clockface terminology
the relationship of the tumour to the anterior peritoneal reflection is also described; the reflection is off the upper part of the rectum and below this level the rectum is extraperitoneal and the margin of excision is the mesorectal fascia
Relationship to muscularis propria (T stage)
nodular extension through the muscularis propria, contiguous with the luminal tumour, is defined as tumour achieving T3 status according to the TNM classification
low signal spicules are not tumour but small fibrous strands or vessels
tumour extending to make contact with the peritoneal reflection or other organs are defined as T4a or T4b respectively
Relationship to circumferential resection margin
where tumour contacts (less than 1 mm distance) the mesorectal fascia, the circumferential resection margin is said to be involved or threatened
the same principle applies for tumour deposits, but not lymph nodes which have a smooth outer margin
remember that the CRM is not mentioned if the tumour lies in a peritonealised portion of the rectum
Extramural venous invasion and tumour deposits
tumour signal tissue extending in a serpiginous manner alongside the rectum represents tumour in vessel, known as extramural venous invasion, a risk factor and predictor for metastatic disease
irregularly marginated nodules of tissue along the path of the vessels represent tumour deposits, an entity that confers high risk and said to be more important than mesorectal lymph nodes due to the fact that TDs occur via haematogenous spread
Mesorectal and pelvic sidewall lymph nodes
unlike in other parts of the body, the primary means of identifying abnormal mesorectal nodes is by signal and contour heterogeneity, rather than size
a quirk of the current TNM staging system (TNM 8) is that tumour deposits, which are now known to confer a greater risk than involved nodes, present a category of N1c, less than if there were no TDs but many nodes
pelvic sidewall nodes are considered by some to be part of the local nodal spread and therefore counted within the N system, but by many to be metastatic and therefore counted as M1
Mucinous cancers
mucinous cancers 8 are recognised due to their higher signal intensity on T2 imaging, with a preponderance for local perforation and a less optimal response to chemoradiation
Low rectal cancers
the anatomy of the low rectum means that tumours that have penetrated the muscularis propria by even 1 mm may lie very close to the adjacent levator muscle as it turns into the external sphincter, and a conventional surgical resection plane that follows the mesorectal fascia may lead to cutting through tumour and a risk of a local recurrence
these tumours, if surgically treated, are managed by operations with larger tissue resections such as AP resection or extralevator AP resection (ELAPE)
Restaging following chemotherapy +/- radiotherapy
a detailed guide to analysing post-chemoradiation MRI rectal studies is presented here
appearances vary due to a combination of fibrosis, oedema and mucinous degeneration of tumour
if the imaging suggests a near/complete response such as with the split scar sign, endoscopy must be performed to visualise and biopsy the scar to confirm response
Template report
Suggested items to include in a template report, based on the above pieces of data, are 2, 4:
length of tumour
distance from distal edge of tumour to anorectal junction
distance from distal edge of tumour to anal verge
tumour relationship to peritoneal reflection (above/straddling/below)
tumour morphology (annular/semi-annular/polypoid)
invasive margins (clock face terminology)
T stage (and which organs involved if T4b)
depth of tumour invasion (less or greater than 5 mm)
shortest distance to mesorectal fascia or levator muscle
extramural venous invasion
tumour deposits
mesorectal lymph nodes
extramesorectal lymph nodes
for low cancers, involvement of anal sphincter muscles or intersphincteric fat
summary stage: longitudinal and axial location, T, N, EMVI, TD, CRM
To sum it up
ensure that the small field-of-view axial and coronal images are truly perpendicular and parallel to the plane of the portion of the rectum bearing the tumour, or else the T stage may be overestimated
for low tumours, describe the relationship of tumour to the levator muscle complex and anal sphincters as this may determine the surgical approach to ensure clearance as a standard total mesorectal excision may not deliver a complete resection
remember the anatomy of the peritoneum when describing whether a margin is threatened - only the lower third of the rectum is completely non-peritonealised
identify the key parameters that define risk - low cancers, T stage, depth of invasion of tumour through muscularis propria, CRM status, EMVI and tumour deposits
MRI is generally poor at determining whether lymph nodes are involved, and the importance of lymph node involvement when fully contained within the mesorectal compartment has been questioned by recent evidence
when restaging tumours following chemoradiation, a variety of appearances may be seen due to the combination of fibrosis, oedema and mucinous degeneration, but endoscopic review of the tumour site is essential to confirm complete response
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