1. Rectal Anatomy on MRI

The rectum is divided according to its distance from the anorectal junction into low (0–5 cm), mid (5–10 cm), and high (10–15 cm).

MRI depicts the rectal wall layers, the surrounding tissues and the sphincter complex, all of which are critical landmarks for staging and surgical planning.

Fig 1: Illustration depicts sagittal view of the rectum and axial view of its division with its layered composition and anatomical relationships.

Fig 2: Illustration and T2w oblique coronal MR image of the rectum visualizing the internal and external sphincter complex, as well as the intersphincteric space. Important anatomical structures in the staging and surgical management of these patients.

2. MRI Tumor Staging

T Stage

Differences in T2w signal intensity of the rectal wall layers allow accurate assessment of mural invasion and regional spread. The mucosa and muscularis propia appears hypointense while the submucosa is hyperintense.

• T1-T2: Preservation of the hypointense muscularis propia indicates tumor confined to the wall. Differentiation of T1 or T2 is not possible without submucosal edema.

Fig 4: Axial T2w MR image with 9-11h thickening of the rectal wall and preservation of the muscularis propia with its lineal hypointense signal.

• T3: Tumor extension into the mesorectal fat, beyond muscularis propia. T3 can be subclassified according to the extramural invasion depth: 
  • T3a: <1 mm
  • T3b: 1-5 mm
  • T3c: 5-15 mm
  • T3d: >15 mmm.

Fig 5: Axial T2w MR image with 6-11h thickening of the rectal wall and affectation of mesorectal fat <5 mm.

• Involvement of the mesorectal fascia (MRF+) is a associated with higher risk of local recurrence.

Fig 6: Axial T2w MR image with 4-12h thickening of the rectal wall and affectation of the mesorectal fascia (arrow).

• T4: can be classified in: 
  • T4a: invasión of adjacent organs

Fig 7: Axial T2w MR imagen with 9-7h thickening of the rectal wall and affectation of peritoneal reflection.

• • T4b: invasion or pelvic organs including: pelvic organs, vascular structures outside the mesorectum, fat outside the mesorectum, small or large bowel in the pelvis, excretory system (ureters and urethra), muscle (external anal sphincter, puborectalis, levator ani, obturator, piriformis and ischioccygeus), sciatic or sacral nerves, ligaments (sacrosìnous /sacrotuberous), bone.

Fig 8: Coronal T2w MR image of a low rectal cancer with an extramural component invading mesorectal fat, directly involving the right sphincter complex.

N stage

Regional lymph nodes include mesorectal, internal iliac, obturator, superior rectal, and inferior mesenteric nodes.

Fig 9: N-stage regional lymph nodes in rectal MRI.

Non-regional nodes are considered metastasis and include external iliac, common iliac, inguinal, and retroperitoneal lymph nodes.

Fig 10: Non-regional stage lymph nodes in rectal MRI.

Malignant nodal features are round morphology, irregular margins, heterogeneous signal intensity, and size criteria. Mucinous lymph nodes are always suspicious. 

Table 1: Imaging evaluation criteria for lymph nodes.

Prognostic factors with higher risk of recurrence and metastasis:

• Extramural vascular invasion (EMVI): tumor sign into vascular structures.
• Anal sphincter and pelvic floor involvement: invasion of the external sphincter, puborectalis and elevator is considered a T4b whereas invasion of the internal sphincter does not change the staging.
• Tumor deposits within the mesorectal: irregular nodules situated within vessels. There is no criteria to discriminate them with lymph nodes.

Fig 11: Axial T2w MR image demonstrating extramural vascular invasion with intermediate signal into the vascular structure (arrow).

Fig 12: Axial T2w MR image of a tumor deposit in the mesorrectal fat with irregular margins (arrow).

3. Rectal cancer tumor staging

The managment of rectal cancer is guided by de TNM staging system which helps categorize patients into different risk groups and determine the most appropiate management for each group:

• Local or low risk disease (Stage 0 and I): Tumor confined to the rectal wall (T1 and T2) without lymph node involvement (N0) and without distant metastasis (M0).  
• Locally advanced or intermediate risk disease (Stage II and III): Includes T3-T4 or any T stage with regional lymph node involvement (N1-N2). These patients typically needs neoadyuvant therapy to reduce tumor size and decrease local recurrence. 
• Metastatic or high risk disease (Stage IV): Defined by the presence of distant metastasis. 

Therapeutic pathways by stage

• T1N0: local excision. radical surgery is recommended in the presence of high risk features includig:
  • Poorly differentiates histology
  • Large tumor size (>5 cm) or involvemente of more than one-third of the rectal circumference
  • Deep submucosal invasion
  • Extensive tumor budding, perineural invasion or lymphovascular invasion
  • Mucinous or signet ring cell carcinoma
• T2N0: Major surgical resection with lymphadenectomy, most commonly low anterior resection with total mesorectal excision or abdominoperineal resection depending on the rectal localization of the tumor.
• T3-T4 and/ or N+ disease: Neoadjuvant therapy followed by imaging postneoadyuvant evaluation:
  • Complete response: watch-and-wait organ-preservation strategy
  • Incomplete response: surgical resection
• M+: Treatment consist of systemic palliative chemotherapy.

Fig 13: Management Algorithm for Rectal Cancer based on TNM Staging.

Reevaluation of patients with neoadyuvant therapy

Multidisciplinary re-evaluation determines if the patient is candidate for organ-preserving strategies. This assessment is performed at scheduled intervals (starting at 12 weeks) using a combination of MRI, endoscopy, and clinical examination with digital rectal exam.

• Complete response: Watch and wait strategy, avoiding the morbidity of radical surgery.
• Adequate / partial response: May requiere a secondary assesment 4 weeks later to determine if the patient can avoid radical surgery.
• Poor response: Total Mesorectal Excision (TME)

Fig 14: Post-neoadjuvant treatment re-evaluation algorithm.

Watch and Wait strategy

Patients with a complete radiological response can be individually evaluated to implement an organ-sparing strategy (watch and wait) if there is also a complete response in endoscopic and digital rectal exams.

This strategy requires close follow-up which includes:

• Clinical examination with digital rectal exam
• Endoscopy
• High-resolution pelvic MRI
• Analysis with markers (CEA)

Table 3: Follow up of patients in Watch and Wait strategy.

4. MRI protocol

• Scout in all 3 planes
• TSE T2 sagittal FOV pelvis; 3/0.3mm thickness. Include sacral promontory. Used as reference to angle the rest of the sequences
• TSE T2 axial pelvis (including promontory); 3/0.3 mm slice thickness
• TSE T2 axial oblique to the tumor; 3/0.3 mm slice thickness. Oriented with the short axis of the tumor/affected rectal segment.
• TSE T2 coronal oblique to the tumor; 3/0.3 mm slice thickness. Oriented with the long axis of the tumor/affected rectal segment (perpendicular to the previous)
• DWI axial oblique to the tumor. Oriented with the short axis of the tumor/affected rectal segment
• T2 coronal oblique to the anal canal; 3/0.3 mm slice thickness

5. Neoadjuvant Treatment and MRI Response Assessment

Patients with locally advanced rectal cancer typically undergo neoadjuvant chemoradiotherapy followed by surgical resection. In selected patients with complete clinical and radiological response, watch-and-wait strategy may be considered.

MRI reassessment is usually performed 6–12 weeks after neoadyuvant treatment. Expected post-treatment changes of the primary tumor includes:

• Decrease in tumor volume and extension,
• Development of low-signal fibrosis on T2-weighted images,
• Normalization of layered structure of the wall.

The tumor response is classified by MR in complete response, adequate response and poor response.

Table 2: Types of tumor response after treatment and its imaging and histological characteristics.

Fig 15: Example of poor response in a T2N1bMx rectal cancer after neoadjuvant treatment, showing minimal tumor size decrease (<20%), persistent intermediate signal on T2-weighted images, and diffusion restriction on restaging MRI.

Fig 16: Example of adequate response in a T4N2bMx rectal cancer after neoadjuvant treatment, showing moderate decrease in tumor size (<70%), predominance of fibrosis wall signal with an intermediate signal nodular foci on T2w of 8 mm with DWI restriction and persistence of pathologic mesorectal and hypogastric lymph nodes.

Fig 17: Example of adequate response in a T3cN1cMx rectal cancer after neoadjuvant treatment, showing moderate decrease in tumor size (<70%) and diminution of the mesorectal fat invasion, heterogeneous wall signal with predominance of fibrosis, decrease in size and signal intensity of the mesorectal lymph node with no pathological features on restaging MRI (yellow circle) and low signal implant intensity and diminution of diffusion restriction suggesting the presence of fibrosis (yellow arrow).

Fig 18: Example of complete response in a T3bN0Mx rectal cancer after neoadjuvant treatment, showing homogeneous low signal on T2w MR image on the previous tumoral area (scar) and absence of restriction in DWI.

Fig 19: Example of complete response in a T1-2N1bMx rectal cancer after neoadjuvant treatment, showing homogeneous low signal on T2w MR image on the previous tumoral area (yellow arrow), persistence of intermediate signal T21 intensity from 1 to 5 hours with absence of restriction in DWI (yellow arrow).

The imaging criteria for recurrence after complete response include:

• Increase in fibrotic scar depth angle >21°.
• Thickening of the fibrotic scar >10 mm (first sign of suspicion)
• New nodular areas with heterogeneous or intermediate T2 signal or diffusion restriction.

Fig 20: Axial oblique T2-weighted rectal MRI after neoadjuvant treatment showing a complete response with a dense fibrotic scar, from 1 to 4 hours with recurrence in the follow-up imaging demonstrating nodular thickening of the fibrosis with intermediate signal intensity and diffusion restriction.

6. Common pitfalls in MRI restaging

• Persistent high T2 signal in mucinous tumors, with or without tumoral cells
  

  Fig 21: Staging MR demonstrates a concentric mural thickening of the lower and middle rectum with a high T2 signal component, suggestive of a mucinous tumor. Post-neoadyuvance evaluation MR shows persistent high-signal thickening with no residual tumor in the histological analysis after surgery.
• Mucin pools development in nonmucinous adenocarcinoma because of colloid degeneration, with or without tumoral cells.
  

  Fig 22: Pitfall: Colloid degeneration of nonmucinous adenocarcinoma after treatment, with or without tumoral cells. Axial MR of the mid rectum which shows (A) intermediate T2 mural thickening and (B) New high signal intensity after neoadjuvant treatment due to mucin pool/colloid degeneration.
• Submucosal edema due to neoadyuvant treatment, often misinterpreted as residual tumor.
• Diffusion-weighted imaging artifacts (T2 shine-through and T2 dark-through)
  

  Fig 23: Illustration of the different types of diffusion weighted imaging (DWI) pitfalls.
• Signal alterations related to intraluminal contents. In these cases, tumor recurrence should be suspected if the shape of the signal increase is curvilinear or in the shape of a C or U, similar to a mass. If the morphology is star-shaped, it suggests content.

7. Proposed structured report

PROTOCOL: Overall study quality: Adequate/Suboptimal/Non-diagnostic: Reason.

PRIOR MRI:   Yes/No   Date and staging:

LOCALIZATION AND CHARACTERISTICS OF THE LESION:

• Low rectum (0-5 cm) / medium (5-10 cm) / high (10-15 cm).
• Lesion length: Distance from the most distal end of the tumor to the anal margin = cm.
• Distance from the most distal end of the tumor to the sphincter complex = cm.
• Tumor relationship with the peritoneal reflection.

RE-EVALUATION AFTER NEOADJUVANT CHEMORADIOTHERAPY:

• No response
• Inadequate poor response with regression reduction less than 25%. Small areas of fibrosis or mucin.
• Adequate incomplete/poorly moderate response: > 50% fibrosis or mucin and 50% of tumor foci visible.
• Adequate practically/possibly complete (assess new MRI in 4 weeks) Dense fibrosis, minimal tumor. Reduction >75% with fibrosis.
• Complete radiological response. No evidence of the previous presence of tumor. Only fibrosis (100%).
• yrmCRM: No change/probable negativization/positivization.
• yrmTRG IVEM (no change/negativization/positivization)
• Re-staging of categories T and N if applicable (yrmTNM):

CONCLUSION/EVALUATION OF RESPONSE:

• Almost complete-complete: normalization of rectal wall or only fibrosis.
• Good response/adequate: predominance of fibrosis, but with residual tumor component in pT2 or in DWI.
• Poor response/poor/inadequate: predominant tumor component

ADDITIONAL COMMENTS: