MRI of rectal cancer

Author: Rodrigo Horstmann Castilhos

MR image acquisition

Hardware

MRI (first choice)

• Mandatory for both primary staging and restaging of rectal cancer.
• Should use an external surface coil
• 1.5T or 3.0T

Endorectal ultrasound (EUS)

• Staging for early tumours considered for local excision
• Superior diagnostic performance for differentiating T1 from T2 tumors

Patient preparation

• Use of an enema is not routinely recommended
• (Use of spasmolytics may be useful to reduce bowel movement artefacts (no consensus: 57 % recommended/mandatory))
• (Use of endorectal filling is not routinely advised (no consensus: 71 % not recommended))

Sequences and sequence angulation

• A routine protocol should (at least) include 2D T2-weighted sequences in 3 planes and a diffusion-weighted sequence (including at least a high b-value of ≥ 800)
• Diffusion-weighted images (including Apparent Diffusien Coefficient maps) should mainly be assessed visually; quantitative ADC measurements are not routinely advised
• Diffusion-weighted imaging is recommended for restaging of the yT-stage.
• Fatsuppressed, T1-weighted (non-enhanced and contrast-enhanced) and dynamic contrast enhanced (DCE) sequences are not routinely recommended
• Slice thickness ≤3 mm (axial and coronal T2W)

Sequence angulation

• Transverse sequences: perpedicular to the rectal tumour axis
• Coronal sequences: parallel to the rectal tumour axis
• Coronal sequence parallel to the anal canal: should be included in distal tumours to assess the relation between tumour and anal sphincter

Patient preparation

Spasmolytics (optional)

• Can be benefical for upper rectal tumors and when imaging is performed at 3.0T (bowel movement artifacts are most prevalent)

Endorectal filling (optional) (~60 ml of gel)

• When used, some experts suggest using a volume of only ~60 ml of gel, since higher volumes will compress perirectal tissues significantly
• Useful to reduce susceptibility artefacts related to luminal gas during diffusion-weighted MRI.
• Its use is not reccomended routinely: rectal wall distension may interfere with interpretation of the distance between the tumour and the mesorectal fascia, and high T2 signal of the gel may cause T2 shine through effects on DWI.

Structured reporting

Structured reporting is recommended and should include the items described in the report template of ESGAR.

Primary staging

Local tumour status

• Morphology
• Distance from the anorectal junction to the lower pole of the tumour
• Tumour length
• T-stage
• Sphincter invasion

T-stage

• MRI doesn't differentiate T1 from T2
• T1-T2: limited to intestinal wall
  • Good prognosis
  • Rectal wall has an intact black line (outer muscle) surrounding the tumor
• T3: extramural growth
  • T3a or T3b: ≤5 mm extramural growth
  • T3c or T3d: >5 mm extramural growth
• T4

Observations:

• Stranding into the mesorectal fat is an equivocal sign that may indicate either a T2 or T3 tumour
• The mesorectal fascia (MRF) is 'involved' if the distance between MRF and tumour is ≤1 mm
• When a tumour shows stranding into the MRF, the MRF should be considered involved
• A tumour that involves the MRF should be considered a T3 (and not a T4) tumour
• Tumour invasion above the level of the peritoneal reflection (at the anterior side) should be considered at risk for peritoneal rather than MRF invasion
• A tumour that invades the pelvic floor or pelvic side wall muscles should be considered a T4 tumour
• A tumour that grows into the internal anal sphincter muscle should be considered a T3 (and not a T4) tumour

Sphincter invasion

This ifformation is relevant to surgical approach For low tumours with sphincter invasion, describe:

Depth of invasion

• invades only the internal sphincter muscle
• also involves the intersphincteric plane
• also involves the external sphincter

Height of invasion

• involves only the proximal 1/3 of the complex/anal canal
• also involves the middle 1/3 of the complex/anal canal
• also involves the lower 1/3 of the complex/anal canal
• involves pelvic floor (levator)

Restaging after neoadjuvant treatment

• Structured reporting is recommended
• When considering organ preservation (watchful waiting) after CRT, MRI findings should be correlated with clinical examination (endoscopy / digital rectal examination)

Local tumour status

• On T2-weighted MRI, a normalised, two-layered wall after CRT is suggestive of a complete response
• On T2-weighted MRI, a completely hypointense (fibrotic) residue without an isointense mass indicates a complete or near-complete response

Mesorectal fascia (and peritoneal) involvment

• If a fatpad re-appears between the tumour and MRF after CRT, the MRF should be considered uninvolved/cleared.
• Persistent stranding of tumour into the MRF should be considered an equivocal sign that may or may not indicate persistent MRF involvement

Lymph nodes and tumour deposits

• For nodal restaging the criteria described in Table 4 are recommended

N staging

• Important risk factor for local recurrence

Morphologically suspicious characteristics

• Round shape
• Irregular border
• Heterogeneous signal

Malignant node criteria

• Short axis diameter ≥9 mm
• Short axis diameter 5-8 mm + ≥ 2 morphologically suspicious characteristics
• Short axis diameter <5 mm + 3 morphologically suspicious characteristics
• Mucinous lymph node (of any size)

Downloads

• [1] Magnetic resonance imaging for clinical management of rectal cancer: Updated recommendations from the 2016 European Society of Gastrointestinal and Abdominal Radiology (ESGAR) consensus meeting

References

Beets-Tan RGH, Lambregts DMJ, Maas M, et al. Magnetic resonance imaging for clinical management of rectal cancer: Updated recommendations from the 2016 European Society of Gastrointestinal and Abdominal Radiology (ESGAR) consensus meeting [published correction appears in Eur Radiol. 2018 Jan 10;:]. Eur Radiol. 2018;28(4):1465–1475. doi:10.1007/s00330-017-5026-2