Anal cancer

Anal cancer is a relatively uncommon malignancy. It accounts for less than 2% of large bowel malignancies and 1-6% of anorectal tumours (~1.5% of all gastro-intestinal tract malignancies in the Unites States ¹⁴). 

Epidemiology

There may be a slight male predilection where its incidence has been reported to be approximately 0.5 per 100 000 in men and 1.0 per 100 000 in women ¹. Its incidence is thought to be rising over the years ⁵.

Clinical presentation

Approximately 45% of patients may present with bleeding per rectum. Around 30% of patients may have pain and/or a sensation of a mass.

Pathology

Anal carcinoma typically originates between the anorectal junction above and the anal verge below. The vast majority of anal canal cancers are squamous cell cancers. See WHO classification of anal canal tumours.

Risk factors

Both male and female ¹⁵:

• HPV / HIV infection
• immunosuppression 
• number of lifetime sexual partners, and receptive anal intercourse
• smoking

In females: previous in situ or invasive cervical, vulval or vaginal cancer ¹⁵. 

Lymphatic spread

• tumour above dentate line: to pararectal and paravertebral nodes ¹³
• tumour below dentate line: to inguinal and femoral nodes ¹³

Radiographic features

Imaging performed prior to treatment provides assessment of the local disease extent and nodal involvement. Accurate delineation of the disease in relation to the rest of the perineal anatomy is of paramount importance in initial imaging assessment. The size of the tumour is also considered a very important prognostic factor (see staging) ¹³.

MRI

MRI is the modality of choice in the assessment of locoregional disease. This generally requires a dedicated protocol: see MRI protocol for assessment of anal cancer.

Reported usual signal characteristics include ²

• T1: primary and recurrent tumours are usually of low to intermediate signal intensity relative to skeletal muscle
• T2: primary and recurrent tumours are usually of high signal intensity relative to skeletal muscle

Nodal metastases are of similar signal intensity to the primary tumour.

PET-CT

Recent research suggests PET/CT being useful that it alters the initial staging sufficiently frequently that it should be used routinely in anal cancer, where it is available ⁴. At the time of initial writing (2012), the role of PET/CT in the follow-up of anal cancer is not as clear ⁴.

Endoanal ultrasound

Sometimes can be used in the locoregional staging ⁶. Some authors suggest that endoanal ultrasound can accurately determine the depth of penetration of the carcinoma into the sphincter complex and can be used to gauge accurately the response of these tumors to chemoradiation therapy ⁸.

Staging

See: staging of anal cancer

Treatment and prognosis

Treatment is often with a using a combination of chemotherapy and radiotherapy (often given concurrently) and is considered to be usually curative. Approximately 50-60% are thought to present with T1 to T2 lesions carrying a 5 year survival of 80-90% ³. Some authors suggest a benefit of a salvage abdominoperineal resection (APR) for those patient with failed chemoradiation ^10,12.