Anal carcinoma: MRI diagnosis, staging and follow-up

Clinical History

An elderly lady complained of dull, spontaneous pain in the anal region exacerbated by defecation, with episodes of blood-stained stools. Her complex past medical history included diabetes mellitus, ischaemic - hypertensive cardiomyopathy, chronic obstructive lung disease, and bilateral hip prostheses.

Imaging Findings

Digital rectal examination and proctoscopy disclosed hard-consistency anal tumour. Histology of biopsy specimen diagnosed squamocellular carcinoma with deep mural infiltration.
MRI requested for tumour staging showed circumferential wall thickening with abnormal signal intensity and inhomogeneous contrast enhancement involving the entire anal canal (6-cm longitudinal diameter), without detectable abnormalities in the ischioanal and mesorectal spaces nor signs of invasion of adjacent organs.
A centimetric inguinal lymphnode showed abnormal signal and enhancement analogous to the primary tumour, consistent with metastasis. Neoplastic stage was assessed as T3N2 according to the TNM system.
Chemo-radiotherapy was interrupted shortly after initiation, because of patient’s cardio-respiratory status and worsening of renal function.
Follow-up MRI, obtained 4 months after diagnosis, disclosed moderate local progression of anal cancer with exophytic tissue protruding from the external orifice plus marked size increase of the left inguinal lymphnode. Both the anal tumour and the metastatic node showed decreased central contrast enhancement consistent with partial necrosis.

Discussion

Anal cancer is an uncommon malignancy, accounting for less than 5% of anorectal tumors and most often diagnosed in the seventh decade of life with a significant female predominance. Histologically, the vast majority are squamous cell carcinomas. Predisposing conditions include pelvic irradiation, Crohn’s perianal disease, HIV and Papillomavirus infections.
Anal tumours are usually indolent and locally invasive, with possible formation of metastases to the inguinal, perirectal or internal iliac lymph nodes. Currently, successful treatment with combined chemo-radiotherapy yields an overall 5-years survival rate of 70-80%, whereas surgery is reserved for persistent or recurrent tumours needing extensive abdomino-perineal resection.
Because of its location, anal cancer is detected clinically. Both endoanal ultrasound (EAUS) and MRI with external phased-array probes have been recently recommended for locoregional disease staging. Good agreement is reported between these two imaging modalities: EAUS is superior in detecting small, superficial lesions and can assess depth of mural invasion, whereas MRI has a larger field-of-view allowing visualization of internal iliac and inguinal lymphnodes.
MRI findings provide accurate staging according to the AJCC TNM classification: T1 tumours have a measured longest diameter up to 2 cm, whereas size over 5 cm represents T3 stage. Invasion of the adjacent organs, particularly in the anterior urogenital triangle, is assessed as T4. Nodal staging includes assessment of the perirectal (N1), unilateral iliac and/or inguinal lymphnodes (N2), and perirectal plus inguinal and/or bilateral nodes (N2).
At MRI, neoplastic tissue in the anal canal has low- to intermediate T1 signal intensity and higher T2 signal intensity relative to skeletal muscles of the external sphincter and gluteus, with some heterogeneity in larger lesions. Positive contrast enhancement is observed in neoplastic lesions, but, according to most authors, post-gadolinium T1-weighted acquisition does not offer additional information to the intrinsic soft tissue contrast of high-resolution multiplanar T2 images.
As demonstrated by this case, specificity for lymph node metastatic involvement is increased over size criteria by consideration of nodal signal intensity features. Furthermore, MRI allows differentiation from other causes of local pain and masses arising in the perineal region.
After chemo-radiotherapy, follow-up MRI allows assessment of therapeutic response, documenting size reduction and diminished T2 signal intensity in effectively treated lesions: stability of any residual abnormality in the site of the treated lesion after 1 year is strongly associated with a good outcome, whereas locally recurrent tumours often show aggressive behaviour.

Differential Diagnosis List

Final Diagnosis

Squamous cell carcinoma of the anal canal

URL:	https://www.eurorad.org/case/9271
DOI:	10.1594/EURORAD/CASE.9271
ISSN:	1563-4086