Rectal cancer (staging)

Staging strongly influences 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.

TNM staging

See TNM staging system for a general description.

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

• 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, the desmoplastic reaction has a spiky and sharp configuration whereas tumour usually has a nodular and lumpy configuration.

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.

• 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

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)
  • may be contiguous or non-contiguous
  • non-contiguous deposits reflect N1c
  • imparts poor prognosis as a predictor of haematogenous spread
• circumferential resection margin (CRM)
  • represented by the mesorectal fascia (MRF)
  • 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
• preoperative post-chemoradiation rectal cancer response assessment