ANATOMICAL CONSIDERATIONS

The anal canal is a short tubular structure, approximately 3–4 cm in length, extending from the anorectal junction to the anal verge. It is divided into the anatomical anal canal, which extends from the dentate line to the anal verge, and the surgical anal canal, which spans from the anorectal junction to the anal verge.

Fig 1: Anatomy of the anal canal. Modified image from the original. The pink line shows the anorectal junction. The small curly brace in blue shows the anatomical anal canal. The large curly brace in purple shows the surgical anal canal. Congedo A, Mallardi D, Danti G, De Muzio F, Granata V, Miele V. An updated review on imaging and staging of anal cancer—Not just rectal cancer. Tomography. 2023;9(5):1694–1710.

Fig 2: Schematic representation of the anal canal. Modified image from the original. The pink line shows the anorectal junction. Puborectalis muscle in purple. Abdool Z, Sultan AH, Thakar R. Ultrasound imaging of the anal sphincter complex: a review. Br J Radiol. 2012;85(1015):865–75.

On high-resolution T2-weighted (T2W) MRI, the anal canal displays a characteristic layered appearance, crucial for accurate assessment. The internal anal sphincter (IAS) is a continuation of the rectal circular smooth muscle and appears as a circumferential structure of intermediate-to-low signal intensity. The intersphincteric space, which consists mainly of fat and loose connective tissue, is hyperintense, providing a clear distinction between the IAS and the external anal sphincter (EAS). The EAS, puborectalis (P), and levator ani (LA), composed of striated muscle, appear hypointense due to their higher collagen content and compact muscle fibers [1,3].

Fig 3: Anatomy of the anal canal. (A) T2-weighted axial MR image: 1 = internal sphincter (intermediate to low signal), 2 = fatty intersphincteric space (high signal), 3 = external sphincter (low signal, white arrow); (B) T2-weighted coronal MR image: 1 = levator ani muscle, 2 = puborectalis muscle, 3 = external sphincter, 4 = internal sphincter. Congedo A, Mallardi D, Danti G, De Muzio F, Granata V, Miele V. An updated review on imaging and staging of anal cancer—Not just rectal cancer. Tomography. 2023;9(5):1694–1710.

Fig 4: T2-weighted parasagittal images show distinct components of EAS. (D: deep; S: superficial and Sc: subcutaneous). PR: puborectalis muscle. LA: levator ani muscle. Erden A. Abdom Radiol. 2018;43:1334–1352.

After gadolinium administration, the IAS enhances due to its vascular smooth muscle composition, while the EAS, P and LA remain non-enhancing because they are composed of striated muscle with lower vascularity [3].

Fig 5: Contrast-enhanced, fat-suppressed T1-weighted oblique axial (left) and oblique coronal (right) images show greater enhancement of the IAS compared to the EAS (white arrows). Erden A. Abdom Radiol. 2018;43:1334–1352.

MRI IMAGING PROTOCOL

• High-resolution T2-weighted imaging in three planes (axial oblique, coronal oblique, and sagittal) is the main imaging sequence, with a slice thickness of ≤3 mm.
• Additional sequences: T1-weighted imaging (T1WI), short tau inversion recovery (STIR), diffusion-weighted imaging, and T1 fat-suppressed post-contrast with gadolinium.
• DWI is essential in restaging and follow-up, improving sensitivity for detecting residual tumor within fibrotic tissue.
• T1WI with fat suppression, before and after gadolinium administration, help detect subtle tumor enhancement, fistulas, and nodal involvement [1-4].

Fig 6: It is important to acquire images aligned with the anal canal. Sagittal (A), coronal (B), and axial (C) T2 sequences are shown. The blue box shows the coronal orientation to the anal canal, while the purple box indicates the axial orientation, perpendicular to it for optimal layer visualization. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 7: Recommended protocol for MRI evaluation of the anal canal. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

TNM STAGING SYSTEM FOR ANAL CANCER

Table 1: Anal cancer TNM staging AJCC 8th edition. Lymph nodes in the obturator region are part of the internal iliac group and, therefore, classified as N1a. Ceylan C, Eröz E, Saçlı A, Doğan S. Perianal region malignancies. 2023:221–33.

T-stage

Unlike rectal cancer, where T-stage is based on invasion depth, anal cancer staging depends on the tumor's maximum diameter, except for T4, which indicates adjacent organ invasion. Accurate measurement in all planes is key to prevent understaging.

Fig 8: Axial (left) and sagittal (right) T2 sequences. It is important to measure the tumor in all planes to ensure the longest axis of the lesion and avoid underestimating the T stage. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

T4 tumors invade other organs or structures such as the prostate and seminal vesicles, penis, vagina, cervix/uterus, ovaries, ureters and urethra, bone, nerves and vessels, and striated muscles (e.g., pelvic sidewall). The T4 stage classification for anal canal carcinoma is not affected by invasion of the external and/or internal anal sphincter, puborectalis muscle, levator ani muscle, rectal wall or perianal skin [3-5].

The primary tumor appears intermediate to hyperintense on T2/STIR and intermediate to hypointense on T1 sequences.

Main aspects to consider when describing tumor involvement in the anal canal and surrounding structures include [5-7]:

• Indicate the craniocaudal extent (e.g., upper half, lower half, or full length) and circumferential involvement (e.g., from … till … o’clock) of sphincter involvement.
• State whether the anal margin (perianal skin) or anal canal is affected.
• Specify the layers of the anal sphincter and pelvic floor involved.
• Clarify if the tumor extends above the anorectal junction into the rectum.

Fig 9: Nodular lesion smaller than 2 cm in size in the external anal margin (yellow arrow), with intermediate signal on T2 sequences (A sagittal, B coronal, C coronal, and D axial), with diffusion restriction (E axial), intermediate signal on T1 (F axial), and uptake after intravenous gadolinium (G T1postGad fat-sat axial). Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 10: Sagittal T2 (A) and coronal T2 (B and C). Nodular lesion between 2-5 cm located in the lower third of the anal canal (yellow arrow). The integrity of the sphincter apparatus is observed (white arrow). Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 11: Sagittal T2 (A), coronal T2 (B and C), and axial T2 (D, E, and F) sequences show an ulcerated mass (with the presence of air inside) in the lower third of the anal canal, extending through the intergluteal fold. Total involvement of the sphincter apparatus. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 12: Sagittal (A), coronal (B), and axial (C) T2 sequences show a hyperintense nodular lesion on T2 in the external anal margin, extending towards the anal canal, with an intact sphincter apparatus. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

N-stage

Nodal involvement occurs in about 25-45% of patients with anal cancer. Unlike in rectal cancer where the N-stage is based on the number of suspicious nodes, N-staging in anal cancer is based on the location of N+ nodes.

Lymph nodes in the obturator region are part of the internal iliac group and, therefore, classified as N1a 

Nodes along the common iliac vessels and aorta are considered non-regional (M1) and typically entail incurable disease, that is managed with palliative chemotherapy.

Tumors above the dentate line often spread to the mesorectal, internal iliac and obturator lymph nodes, while tumors below this line typically spread to the inguinal and external iliac nodes. Tumors crossing the dentate line can involve both regions

There are no widely accepted MRI criteria for characterizing anal cancer lymph nodes, and some authors propose using rectal cancer criteria for staging. Reported criteria include[1-4]:

• Short axis > 1 cm for mesorectal nodes
• Short axis > 1.5 cm for other nodes
• Short axis > 0.8 cm for parailiac nodes
• Heterogeneity, necrosis, border irregularity
• Strong enhancement

These criteria have limited evidence and may lead to over- or understaging. MRI has limited accuracy for N-staging, requiring FDG-PET/CT or ultrasound with fine needle aspiration (FNA) for a more accurate assessment of lymph nodes. 

Fig 13: Sagittal T2 (A), axial T2 (B), and coronal T2 (C and D) sequences. Mass smaller than 5 cm occupying the entire anal canal with extension to the lower rectum. Complete involvement of the sphincteric apparatus (white arrow). Pathological mesorectal lymphadenopathies (yellow arrows). Therefore, T3N1aM0. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 14: Sagittal T2 (A), coronal T2 (B), axial T2 (C), DWI (D) with ADC map (E), and post-contrast T1-weighted image after intravenous gadolinium (F). A mass smaller than 5 cm is observed on the anterior aspect of the anal canal, showing diffusion restriction on DWI (D) and signal drop on the ADC map, with enhancement after intravenous gadolinium administration. The mass is displaced anteriorly, involving the rectovaginal septum and infiltrating the vagina, with no identifiable cleavage planes between the anal canal and the vagina (yellow arrows). See figure 15 for further details on lymph node involvement in the same patient. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 15: Lymph node involvement in the same patient as shown in figure 14. Bilateral necrotic inguinal lymphadenopathies (blue arrows) on post-contrast T1 fat-sat (A), T2 (B), and DWI (C), showing diffusion restriction and ADC signal drop, similar to the tumor lesion in the anal canal (white arrow). Coronal T1 (D) shows lymphadenopathies in the internal iliac chains (green arrows). Axial T1 (E) and axial T2 (F) depict a pathological right perirectal lymph node (E) and a pathological right inguinal lymph node (F) (yellow arrows). Therefore, T4N1aM0. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

M-stage

CT with intravenous contrast is the preferred method for detecting distant metastases, but FDG PET/CT has greater sensitivity, identifying additional metastases in 3–5% of cases. Around 6% of anal cancer patients present with distant metastases at diagnosis, leading to a poor prognosis with a 10-20% five-year survival. Common metastatic sites include para-aortic and common iliac lymph nodes, liver, and lungs [1-6].

RESTAGING AND FOLLOW-UP AFTER CRT

Post-treatment imaging findings

MRI is the preferred imaging modality for post-treatment evaluation. Typical tumor response on T2-weighted sequences includes tumor size reduction, hypointense fibrosis affecting the anal canal wall, and post-radiotherapy edema with rectal mucosal thickening, which can mimic a pseudotumor [1-3,7].

Evaluation of treatment response and residual disease after CRT

MRI should evaluate fibrotic transformation, the likelihood of residual or recurrent tumor, and lymph node size changes. CRT achieves complete remission in 80-90% of cases, with peak response at ~6 months, when final restaging is recommended. Around 10% of patients may have residual tumor requiring surgical resection. Diffusion-weighted imaging (DWI) is useful for detecting residual disease, while absence of intermediate/hyperintense signal on T2W and no diffusion restriction on DWI indicate a complete response [1-3,8].

Fig 16: Nodular lesion at the external anal margin, measuring 2 to 5 cm in greatest diameter, treated with CRT. The post-treatment MRI shows no evidence of the tumor, with residual hypodense fibrotic tracts. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 17: Nodular lesion at the right external anal margin with an intraluminal nodule, measuring 2 to 5 cm in greatest diameter, treated with CRT. The post-treatment MRI shows no evidence of tumor lesion, with linear hypointense tracts suggestive of residual fibrotic changes. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 18: Hyperintense nodular lesion on T2 at the external anal margin with extension into the anal canal. After CRT treatment, no tumor lesion is evident, with residual fibrotic tracts. Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Timing Considerations

Performing imaging before 6 months may lead to misinterpretation, as tumor response can still be evolving. Some centers perform early MRI (6-10 weeks post-CRT) as an interim assessment. Digital rectal examination (DRE) and clinical inspection remain primary assessment tools, with MRI reserved for inconclusive cases [1,5-8].

Local Recurrence

Local recurrence is defined as biopsy confirmed reappearance of tumor or locoregional lymph nodes after an initial complete response, at least 6 months after the last radiation fraction. Around 30% of patients experience local failure (residual tumor after CRT or local recurrence during follow up), with higher rates in those with basaloid subtypes, advanced tumors, and HIV-positive status. About half of the recurrences occur in the first 2 years after completion of CRT.

MRI findings include a new intermediate signal mass on T2W with restricted diffusion on DWI or nodes showing growth during follow up. FDG-PET-CT is useful for detecting both locoregional recurrence and distant metastases. Early identification improves the success rate of salvage surgery, often requiring abdominoperineal resection [1-4,8].

Fig 19: In the images on the left, a tumor larger than 5 cm (T3) with a perirectal lymph node (N1a) is observed. After treatment (CRT), the images on the right show persistent tumor with adjacent venous vascular infiltration (yellow arrow), now classifying it as T4, along with persistent perirectal lymphadenopathy and new pathological right inguinal lymph nodes (white arrows). Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.

Fig 20: Left: 3 cm nodular lesion at the upper right internal margin of the anal canal (green arrow). Right: 2.5 years post-treatment. Tumor recurrence. A very significant increase in tumor size (yellow arrow), invasion of neighboring organs (infiltration of the seminal vesicle, not shown), bilateral internal iliac lymphadenopathies (red arrows), and metastases: tumor implant in the left gluteal muscles (white arrows) and left femoral bone metastasis (blue arrow). Department of Radiology, Hospital Universitario Cruces, Baracaldo, Spain 2025.