Rare keratinocytic tumours include Merkel cell carcinoma (MCC), cutaneous Paget’s disease and atypical fibroxanthoma.
Pathology of Merkel cell carcinoma
Merkel cell carcinoma is a high-grade primary neuroendocrine carcinoma of the skin. It primarily effects the elderly and the immunosuppressed. Both chronic ultraviolet (UV) light exposure and integration of the Merkel cell polyomavirus are associated with MCC.
Histologically MCC is composed of small blue round cells with stippled chromatin and an inconspicuous nucleolus. It stains positively for Cytokeratin 20 (CK20) and for the neuroendocrine markers chromogranin A and synaptophysin.
Merkel cell carcinoma is locally aggressive and also shows a tendency to metastasise to local lymph nodes. It can also be more widely metastatic.
Sentinel lymph node biopsy is often indicated and specialist referral is recommended when this diagnosis is made.
Pathology of mammary and extramammary Paget’s disease
Mammary Paget’s disease (also called Paget’s disease of the nipple) is a condition that involves the nipple and areolar complex. It has an eczematous appearance. Paget’s disease is often associated with underlying carcinoma of the breast.
Histologically there is intraepidermal spread of carcinoma cells in a single cell scattering pattern (so called Pagetoid spread).
Extramammary Paget’s disease is most common in the anogenital region. It may be primary to the skin or may present as complicated colorectal carcinoma, urethral carcinoma or carcinoma of the female genital tract.
Specialist referral is recommended when this diagnosis is made.
Pathology of atypical fibroxanthoma
Atypical fibroxanthoma is a dermally based tumour of uncertain histogenesis which is characterised by its pleomorphic appearance but generally low-grade clinical behaviour. It occurs in the older age group, usually in the setting of marked solar elastosis on the head and neck. Atypical fibroxanthoma presents as a solitary nodule, which is often ulcerated and has a short clinical time course.
Histologically, the tumour is often pleomorphic and shows frequent and abnormal mitotic figures. There is a spindle cell variant.
Immunoperoxidase stains are very important in the diagnosis of atypical fibroxanthoma, which is essentially a diagnosis of exclusion. The differential diagnosis includes squamous cell carcinoma, melanoma, leiomyosarcoma and angiosarcoma. Atypical fibroxanthoma usually stains for CD10 and is negative for the markers of cutaneous squamous cell carcinoma, melanoma, angiosarcoma and leiomyosarcoma. Frequently only CD10 is positive. Most atypical fibroxanthomas are benign, provided that strict criteria are used for diagnosis. A low percentage of cases recur and metastasis are rarely reported. If the tumour involves the subcutis it is better classified as a 'pleomorphic dermal sarcoma not otherwise specified' (PDS-NOS). Pleomorphic dermal sarcoma is a deeper form of AFX. It infiltrates into the subcutis and shows greater risk of recurrence and metastasis (although metastasis is rare).
PP 5.4.1 When a diagnosis is made on histopathology in the following conditions referral to a specialistMedical practitioners who through training, experience and peer opinion specialise in the management of keratinocyte cancers for assessment and treatment should be undertaken:
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