Rectal cancer (staging)

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 tumour deposits ¹¹.

TNM staging

See TNM staging system for a general description, and this article lists the various abbreviations used in rectal cancer staging.

Primary tumour staging (T)

Strictly speaking TNM staging, such as the American Joint Committee on Cancer (AJCC) 8^th edition, does not subclassify T3. However, this subclassification does have the treatment and prognostic significance ^7,8; tumours with a stage T3b or less confer a 5-year cancer-specific survival rate of 85%, whereas tumours with a stage T3c or greater have a 54% survival rate,.

• Tx: primary tumour cannot be assessed
• T0: no evidence of primary tumour
• Tis: carcinoma in situ: intraepithelial or invasion of lamina propria
• T1: tumour invades submucosa
• T2: tumour invades muscularis propria
  • MRI does not yet have the resolution capable of enabling differentiation of T1 and T2 lesions
• T3: tumour invades through the muscularis propria into the subserosa or into non-peritonealised perirectal tissues without reaching the mesorectal fascia or adjacent organs
  • T3a: tumour extends <1 mm beyond muscularis propria ⁴
  • T3b: tumour extends 1-5 mm beyond muscularis propria ⁴
  • T3c: tumour extends 5-15 mm beyond muscularis propria ⁴
  • T3d: tumour extends 15 mm beyond muscularis propria ⁴
• T4: tumour invades directly into other organs or structures and/or perforates visceral peritoneum
  • T4a: tumour penetrates to the surface of the visceral peritoneum
  • T4b: tumour 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 tumour 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 tumours due to apparent transgression of tumour 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 tumour (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 tumour.

• 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: tumour 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.

Metastases (M)

• 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 groupings

• stage 0: Tis N0 M0
• stage I: T1-2, N0 M0
• stage II
  • IIa: T3, N0, M0
  • IIb: T4a, N0, M0
  • IIc: T4b, N0, M0
• stage III
  • 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 tumour 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 haematogenous spread
• tumour deposits ¹¹
  • distinct from lymph node metastases
  • found along path of vessels, and interrupt vessels
  • may see vessel 'tail' into deposit
  • separate from primary tumour
  • 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 tumour, 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 tumour

Special consideration should be given to low rectal tumours 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 tumours (apart from mucinous tumours) 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 tumour staging ¹⁰

  • Stage 1: tumour confined to bowel wall with intact outer muscularis propria
  • Stage 2: tumour replaces muscularis propria but does not extend into intersphincteric plane
  • Stage 3: tumour invades intersphincteric plane or lies within 1 mm of levator muscles
  • Stage 4: tumour invades external anal sphincter and is within 1 mm and beyond levators with or without invading adjacent organs

• Early rectal tumour staging ⁹

  • T0/early T1sm1: no submucosa (sm) disruption evident with entire thickness of submucosal stripe preserved
  • T1sm2: at least 1 mm of submucosa preserved
  • T1sm3/early T2: less than 1 mm of submucosa preserved but full thickness of muscularis propria preserved
  • T2 early: more than1 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)

See also

• rectal cancer
• rectal cancer response assessment
• terminology of rectal cancer staging