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General
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• Very rare tumor of perianal skin, representing 0.2% of anorectal tumors (Am J Dermatopathol 1996;18:371) and less than 0.5% of all basal cell carcinomas (Rev Gastroenterol Mex 2013;78:52)
• Nodular subtype most common in this location

Sites
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• Anus

Pathophysiology
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• Appears to behave similarly to basal cell carcinoma at other locations

Clinical features
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• Usually 1-2 cm, but can be 10 cm or larger; recurrence uncommon (Br J Surg 1981;68:856)
• Patients may also have basal cell carcinomas elsewhere (Dis Colon Rectum 1999;42:1200)

Diagnosis
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• Biopsy

Case reports
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• 69 year old man with perianal basal cell carcinoma (Indian J Dermatol 2010;55:178)
• 88 year old woman with polypoid basal cell carcinoma on the perianal region (J Dermatol 2004;31:51)

Treatment
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• Wide local excision with negative margins

Clinical images
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Perianal basal cell carcinoma

Lesion after biopsy

Gross description
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• Indurated, ulcerated lesion with irregular borders

Micro description
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• Resembles basal cell carcinoma at other sites (see Skin nonmelanocytic chapter)
• Retraction artifact helps distinguish from basaloid squamous cell carcinoma

Micro images
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Cell clusters

Arising from basal layer of epidermis

Positive stains
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• Ber-EP4, bcl-2 (Mod Pathol 2013;26:1382)

Negative stains
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• p16, SOX2

Flow cytometry description
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• Lower S-phase fraction than anal basaloid squamous cell carcinomas (Am J Dermatopathol 1996;18:371)

Differential diagnosis
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• Squamous cell carcinoma of anal canal, basaloid type: much more common

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