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 TMNTNM staging, such as the American Joint Committee on Cancer (AJCC) 8th8^th edition, does not sub-classifysubclassify T3. However, this sub-classificationsubclassification does have 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
• T3: tumour invades through the muscularis propria into the subserosa or into non-peritonealised perirectal tissues
  • 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

Regional lymph nodes (N)

Signal heteroegenity 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 (peri-rectal(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 nonperitonealizednon-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, nonregional non-regional node)
  • M1b: metastases in more than one organ/site or the peritoneum

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 1mm1 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: Tumourtumour confined to bowel wall with intact outer muscularis propria
  • Stage 2: Tumourtumour replaces muscularis propria but does not extend into intersphincteric plane
  • Stage 3: Tumourtumour invades intersphincteric plane or lies within 1mm1 mm of levator muscles
  • Stage 4: Tumourtumour invades external anal sphincter and is within 1mm1 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 1mm1 mm of submucosa preserved
  • T1sm3/early T2: less than 1mm1 mm of submucosa preserved but full thickness of muscularis propria preserved
  • T2 early: more than1mm1 mm muscularis propria preserved
  • T2/T3a: 0mm0 mm muscularis propria preserved / less than 1mm1 mm microscopic invasion beyond muscularis propria (prognosis identical)
  • T3b: 1-5mm-5 mm invasion beyond muscularis propria (still carries good prognosis)

See also

• rectal cancer
• MRMRI anatomy for assessment of rectal cancer
• Preoperative post chemoradiationpreoperative postchemoradiation rectal cancer response assessment