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Fig. 7.21 Secondary Paget disease of the anus. A The underlying adenocarcinoma is present beneath the squamous epithelium. High molecular weight keratin immunostain is largely restricted to normal squamous epithelium. B Low molecular weight keratins 8 and 13 immunostaining of tumour cells.

Malignant melanoma

Anal melanoma is rare. It is a disease of adults with a wide age range; most patients are white {339, 182}. Presentation is usually with mass and rectal bleeding, but tenesmus, pain and change in bowel habit also occur {339}. Macroscopy. Lesions may be sessile or polypoid. Pigmentation of the lesion is often appreciated. Satellite nodules may occur.

Histopathology. The features resemble those of cutaneous melanomas. The majority shows a junctional component adjacent to the invasive tumour, and this finding is evidence that the lesion is primary rather than metastatic. The tumour cells express S-100 and HMB-45. Prognosis. Anal melanomas spread by lymphatics to regional nodes, and haematogenously to the liver and thence to other organs. Metastases are frequent at time of presentation, and the prognosis is poor; the 5-year survival is less than 10% {339, 157}. The chances of long-term survival are increased if the lesion is small.

Mesenchymal and neurogenic tumours

These are all rare and the exact point of origin may be difficult to establish. Recent reports on tumours in the anorec-tal and perianal area include haeman-gioma, lymphangioma {372}, hemangiopericytoma {478}, leiomyoma, malignant fibrous histiocytoma and leiomyosarcoma {1110}, rhabdomyoma in a newborn {1014}, and rhabdomyosarcoma in childhood {1560} and adulthood

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