Staging of rectal cancer strongly predicts the success of and rate of local recurrence following rectal cancer resection. MRI is the modality of choice for the staging of rectal cancer, to guide surgical and non-surgical management options. MRI is used at diagnosis, following downstaging chemoradiotherapy, and in follow-up, if a non-operative approach is used.
Note that there has been recent controversy regarding the ability of TNM 8 to appropriately identify high-risk factors in rectal cancer, most notably due to the emphasis placed on lymph node metastases, and not enough regard paid to tumor deposits 11.
Primary tumor staging (T)
Strictly speaking TNM staging, such as the American Joint Committee on Cancer (AJCC) 8th edition, does not subclassify T3. However, this subclassification does have the treatment and prognostic significance 7,8; tumors with a stage T3b or less confer a 5-year cancer-specific survival rate of 85%, whereas tumors with a stage T3c or greater have a 54% survival rate.
- Tx: primary tumor cannot be assessed
- T0: no evidence of primary tumor
- Tis: carcinoma in situ: intraepithelial or invasion of lamina propria
- T1: tumor invades submucosa
T2: tumor invades muscularis propria
- MRI does not yet have the resolution capable of enabling differentiation of T1 and T2 lesions
T3: tumor invades through the muscularis propria into the subserosa or into non-peritonealised perirectal tissues without reaching the mesorectal fascia or adjacent organs
- T3a: tumor extends <1 mm beyond muscularis propria 4
- T3b: tumor extends 1-5 mm beyond muscularis propria 4
- T3c: tumor extends 5-15 mm beyond muscularis propria 4
- T3d: tumor extends 15 mm beyond muscularis propria 4
T4: tumor invades directly into other organs or structures and/or perforates visceral peritoneum
- T4a: tumor penetrates to the surface of the visceral peritoneum
- T4b: tumor directly invades or is adherent to other organs or structures
Assessing T3 disease, it is important to remember that desmoplastic reaction can mimic this due to fibrosis; however, desmoplastic reaction has a spiky and sharp configuration whereas tumor usually has a nodular and lumpy configuration.
The planes that the small field-of-view axial and coronal sequences have been acquired in should be carefully scrutinised; obliquity may lead to overstaging of tumors due to apparent transgression of tumor through the muscularis propria, but that is in fact an artifact of the plane and volume averaging.
The distance from the invasive margin of the tumor (T3 disease) to the mesorectal fascia is important in guiding resection and the need for preoperative chemo-radiotherapy.
Regional lymph nodes (N)
Signal heterogeneity and irregular contour are the most reliable signs of nodal involvement on MRI. Size is not considered to be a very helpful criterion; up to 40% of nodes under 5 mm contain tumor.
- Nx: regional nodes cannot be assessed
- N0: no regional lymph node metastases
N1: metastasis in 1-3 regional (perirectal) lymph nodes
- N1a: metastasis in 1 regional lymph node
- N1b: metastasis in 2-3 regional lymph nodes
- N1c: tumor deposit(s) in the subserosa, mesentery, or non-peritonealised pericolic or perirectal tissues without regional nodal metastasis
N2: metastasis in 4 or more regional lymph nodes
- N2a: metastasis in 4-6 regional lymph nodes
- N2b: metastasis in 7 or more regional lymph nodes
It should be noted that there is controversy over whether nodes in the pelvic sidewall are regional or non-regional nodes. A multicenter study found that nodes greater than 7 mm within the pelvic sidewalls rendered a greater possibility of recurrence if treatment was just chemoradiotherapy and rectal cancer surgery, and no pelvic sidewall dissection 12.
- Mx: cannot be assessed
- M0: no distant metastasis
M1: distant metastasis
- M1a: metastasis confined to one organ or site (for example, liver, lung, ovary, non-regional node) without peritoneal metastases
- M1b: metastases in more than one organ
- M1c: metastasis to the peritoneum with or without other organ involvement
- stage 0: Tis N0 M0
- stage I: T1-2, N0 M0
- IIa: T3, N0, M0
- IIb: T4a, N0, M0
- IIc: T4b, N0, M0
- IIIa: T1-2, N1, M0
- IIIb: T3-4, N1, M0
- IIIc: T3-4b, N2, M0
- stage IV: any T, any N, M1
Additional prognostic indicators
The following are significant prognostic indicators, and should be commented on when staging rectal cancer with MRI, alongside the TNM stage:
extramural venous invasion (EMVI)
- discrete serpiginous or tubular projections of tumor signal tissue into perirectal fat, following the course of a visible vein
- may be contiguous or non-contiguous
- non-contiguous deposits reflect N1c
- imparts poor prognosis as a predictor of hematogenous spread
tumor deposits 11
- distinct from lymph node metastases
- found along path of vessels, and interrupt vessels
- may see vessel 'tail' into deposit
- separate from primary tumor
- irregular contours compared to most lymph node metastases which have smooth contours
circumferential resection margin (CRM)
- represented by the mesorectal fascia (MRF) in the non-peritonealised portion of the rectum
- CRM positive if either tumor, involved lymph node, or EMVI (continuous or discontinuous) is within 1 mm of the mesorectal fascia
- peritoneal reflection does not constitute CRM, which if involved reflects at least stage T4a disease
Additional specific MRI imaging staging subsets of rectal tumor
Special consideration should be given to low rectal tumors as these carry a different prognosis from higher lesions. This is predominantly due to anatomical considerations including waist-like tapering of the mesorectum 10.
Early rectal cancer and significant polyps may also be subclassified, necessitating the use of good quality, high-resolution T2-weighted MR images. This relies on the recognition that most rectal tumors (apart from mucinous tumors) have intermediate T2 signal compared to hyperintense submucosa and hypointense muscularis layers 9. Such staging may be helpful in selecting less extensive surgical resection options.
Low rectal tumor staging 10
- stage 1: tumor confined to bowel wall with intact outer muscularis propria
- stage 2: tumor replaces muscularis propria but does not extend into intersphincteric plane
- stage 3: tumor invades intersphincteric plane or lies within 1 mm of levator muscles
- stage 4: tumor invades external anal sphincter and is within 1 mm and beyond levators with or without invading adjacent organs
Early rectal tumor staging 9
- T0/early T1sm1: no submucosa (sm) disruption evident with entire thickness of submucosal stripe preserved
- T1sm2: <1 mm of submucosa preserved
- T1sm3/early T2: <1 mm of submucosa preserved but full thickness of muscularis propria preserved
- T2 early: >1 mm muscularis propria preserved
- T2/T3a: 0 mm muscularis propria preserved / less than 1 mm microscopic invasion beyond muscularis propria (prognosis identical)
- T3b: 1-5 mm invasion beyond muscularis propria (still carries good prognosis)
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