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Clear cell acanthoma is a unique clinical and histological entity of unknown aetiology which was first described by Degos et al. in 1962 .
Clear cell acanthoma is defined as a benign, slow-growing tumour, presenting as nodular or plaque-like hyperplasia of the epidermis. It is characterised clinically by a predilection for the lower limbs of middle-aged and elderly subjects and histologically, by well demarcated intra-epidermal proliferation of glycogen-rich keratinocytes.
The pathogenesis of the clear cell acanthoma is still unknown. Some authors report a deficiency in the metabolism of an enzyme that plays a significant role in keratinocyte synthesis  while others consider the lesion to be a benign epithelial neoplasm. Another view is that the lesions result from inflammatory reactive dermatosis .
The clinical appearance of clear cell acanthoma is variable. It often presents a firm, reddish, crusty or scaly nodule or plaque, well demarcated by a scaly collarette at the periphery and usually ranging in size from 5 to 20 mm . The lesion is often found on the lower limbs of subjects between the ages of 50 and 70 years and it occurs equally in both genders . However, it has been reported in younger subjects, and rarely at sites other than the legs (abdomen). The lesion is generally solitary, but there have been reports of patients with multiple lesions . The clinical differential diagnosis includes benign and malignant lesions, i.e. seborrheic keratosis, eczematiform dermatitis, pyogenic granuloma, hemangioma, psoriatic plaques, basal and squamous cell carcinoma. Clear cell acanthoma may also occur together with other skin diseases including xerosis, ichtyosis and seborrheic keratosis . It is slow-growing and spontaneous regression is possible. There have been no reports of malignant transformation. The condition may be diagnosed by dermoscopy, but it is indispensable to confirm the diagnosis histologically.
Histologically, clear cell acanthoma is characterised by sharply demarcated epidermal hyperplasia made up of slightly enlarged, pale, glycogen-rich keratinocytes positive to periodic-acid-Schiffe (PAS) stain . The intra-epidermal adnexal structures are spared from the clear cell changes. Mild spongiosis and parakeratotic scales are often present. The epidermis is often infiltrated by numerous polymorphonuclear neutrophils (PMN) that may form small micro-abscesses in the corneal layer. Cytonuclear atypia and a high mitotic index are rarely encountered in clear cell acanthoma. Characteristic changes may also be observed in the dermis including edematous dermal papillae, vascular ectasia and a mixed inflammatory infiltrate made up of lymphocytes, plasma cells and PMN [4, 8].
Treatment is based on electrocoagulation, curettage and simple surgical excision . Patients with multiple lesions may be treated with cryotherapy or 5-fluorouracil . Lesions may also be successfully treated with carbon dioxide laser . Histological confirmation of at least 2 lesions is indispensible prior to commencing treatment.
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