Other Pathology

Carcinoma of the anal canal (F:M 3:2) is commoner (3:1) than carcinoma of the anal margin (M :F 4:1).

Human papilloma virus infection is a common aetiological agent associated with a spectrum of anal viral lesions, preneoplasia (AIN) and carcinoma. HPV subtypes 16 and 18 are particularly neoplasia progressive. Infection with HIV and other sexually transmitted viruses also contributes.

Condyloma accuminatum, giant condyloma of Buschke-Lowenstein, and Bowen's disease of anal skin are associated with perianal margin/skin squamous carcinoma and its variants. Some authors equate giant condyloma to verrucous carcinoma (indolent growth, exophytic, deep bulbous processes with bland cytology, more aggressive after radiotherapy). Bowenoid papulosis (perineal brown patches, histology of Bowen's disease) has no significant malignant potential.

Concurrent cervical intraepithelial neoplasia (CIN) and AIN grades I, II, III are associated with anal canal carcinoma, with AIN being present in up to 55% of cases. A premalignant phase or model of progression in AIN is not as well established as in CIN, although cancer risk appears to be greatest for high-grade (III) AIN.

The majority of anal canal carcinomas arise in the vicinity of the dentate line from the transitional/cloacal zone and spread preferentially upwards in the submucosal plane, thereby presenting as ulcerating tumour of the lower rectum. Due to the differential options of primary adjuvant therapy vs. primary resection, anal canal carcinoma must be distinguished by biopsy from both rectal adenocarcinoma superiorly and basal cell carcinoma or squamous cell carcinoma of the perianal margin/skin inferiorly.

Anal Paget's disease must be distinguished from Bowen's disease and pagetoid spread of malignant melanoma. Mucin stains and immunohis-tochemistry are necessary (mucicarmine, PAS ± diastase, cytokeratins, melanoma markers: pigment, S100, HMB-45, melan-A). It may be associated with concurrent or subsequent anal, or, low rectal adenocarcinoma with the Paget's cells showing intestinal-type gland formation and cyto-keratin 20 positivity. More often it is a primary anal epithelial lesion lacking intestinal glandular differentiation and cytokeratin 20 positivity but cytokeratin 7/GCDFP-15 positive and which may progress to sub-mucosal invasion. A majority remain as intraepithelial malignancy. A further differential diagnosis is pagetoid spread from a primary anorec-tal signet ring cell carcinoma. Immunohistochemistry is also important in the differential diagnosis of anal basaloid carcinoma (cytokeratins, EMA, CEA positive), malignant melanoma, lymphoma (CD45 positive), spindle cell carcinoma (cytokeratin positive) and leiomyosarcoma (desmin, h-caldesmon, smooth muscle actin positive). Distinction between anal canal basaloid carcinoma and basal cell carcinoma of the anal margin is by the anatomical location as well as histological characteristics.

Radiotherapy necrosis.

Leukoplakia with or without AIN is occasionally seen and needs biopsy to establish the presence of dysplasia.

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