Rectal cancer, although sharing many of the features of generic colorectal carcinoma (CRC), has some features that make it unique. These are predominantly related to its anatomical location which has implications in both preoperative imaging assessment and surgical technique.
Rectal cancer is generally considered a disease of the elderly, but the incidence of cases in patient <50 has been increasing. There is a slight male predilection which is not seen in cancers of the rest of the large bowel.
Patients often present with altered bowel habits or rectal bleeding 3.
Although CT can make the diagnosis in more advanced cases, due to better soft tissue contrast, MRI has become the fundamental imaging modality for evaluation.
MRI may be performed for
- diagnosis / locoregional staging
- helps evaluate which patients may benefit from neoadjuvant therapy and for evaluating poor prognostic factors
- helps surgical planning
- assessment of the effectiveness of neoadjuvant therapy
- monitoring for recurrence post therapy
MRI is able not only to assess tumor stage but other important prognostic features such as local lymph node involvement and extramural venous invasion (EMVI). The key sequences are T2-weighted images in short and long axis to the rectum. A key component to successful MR staging is an understanding of the local anatomy and how it appears on MRI (see MR anatomy for assessment of rectal cancer). MRI is usually performed without rectal distension. Rectal distension can be considered for assessment of smaller or polypoid lesions, or in post-op recurrence, and can be done by infusion of warm water or warm ultrasound gel into the rectum before the scan.
For more information, see MRI protocol for assessment of rectal cancer
Stages T1 and T2
Distinguishing T1 from T2 is difficult on MRI. With increasing interest in minimally invasive surgery to remove early cancers, these cancers are increasingly being staged using endorectal ultrasound.
T3 tumors are those that extend beyond the wall of the rectum and into the perirectal fat without reaching the mesorectal fascia or adjacent organs. Although the tumor may be well circumscribed, the muscularis propria cannot be traced in the region of involvement. Often the interface between fat and tumor is blurred representing local invasion. It is important to remember however that desmoplastic reaction can mimic this due to fibrosis.
The distance from the invasive margin of the tumor to the mesorectal fascia is important in guiding resection and need for preoperative chemo-radiotherapy.
These tumors extend into the pelvic wall or adjacent organs/structures.
Factors to assess on MRI and describe on MRI report
T stage assessment
- morphology of primary tumor: annular/ulcerating/polypoidal/villous/eroding/mucinous/signet/can not be assessed
- distance of distal edge from anal verge
- distance of distal edge from puborectalis sling
- longitudinal extent
- whether it lies above or below peritoneal reflection with approximate measurements
- invading edge of tumor (e.g. x o'clock to y o'clock)
- whether it is confined to or extends through muscularis propria
- extent of extramural spread (mm)
- distance from mesorectal fascia
- extramural venous invasion (EMVI)
N (locoregional) stage assessment
- size is not a reliable indicator of nodal involvement
- irregular or spiculated margin and heterogeneous signal intensity
- number of nodes at level of tumor (in mm, signal and border)
- number of nodes above level of tumor (in mm, signal and border)
- closest circumferential resection margin (CRM)
- distance of CRM from anal verge
- pelvic sidewall nodes: none/benign looking/malignant looking
- evidence of peritoneal involvement
Treatment and prognosis
The mainstay of treatment is surgical excision, however pre-operative down-staging with either radiotherapy alone (more common in Europe) or combined chemo-radiotherapy (more common in the US) is employed in T3 and/or N1 disease (see rectal cancer staging) 1.
Depending on the stage at the time of resection local recurrence rates vary from 3-32% 1 with overall good 5-year survival in T1 and T2 tumors (85-100% and 70% respectively) 1,2,5. Some focal T1 lesions are candidates for transanal endoscopic microsurgery.
The majority of local recurrences occur within 20-36 months 2,5, have positive resection margins, and have a major impact on prognosis, with 80-90% of these patients succumbing to the disease within 5 years 4.
Some authors suggest that a CRM of less than 1 or 2 mm (the exact cutoff is debated) confers a poorer prognosis and patients should be considered for neoadjuvant treatment 6.
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