Anal transition zone carcinoma

The mucosa of the anal transition zone has both squamous and transitional features. Carcinomas arising in this zone

have, in the past, been referred to as “cloacogenic”, a term of doubtful validity. They vary from well-differentiated

squamous cell carcinomas with evident keratinization to basaloid carcinomas (as seen here) composed of

small undifferentiated cells with peripheral palisading. In some tumors, the pattern is mixed squamous-basaloid.

Basaloid tumors must be differentiated from a basal cell carcinoma extending from perianal skin.