[Keratoacanthoma of the left forearm].

We here report the case of a 54-year old woman presenting with a swelling in the left forearm occurred eight weeks before and rapidly increasing in volume. Clinical examination showed ulcero-budding painless purplish skin lesion measuring 2 cm along its longer axis (A). The patient underwent simple biopsy. Histological examination showed dyskeratosic, disorganized, hyperplastic epithelium with cytonuclear abnormalities, suggesting malignant transformation. Given the absence of infiltration in the chorion and the presence of hyperkeratosis, the diagnosis of keratoacanthoma was made. Resection of the tumor was indicated to confirm or deny this diagnosis. Histological examination revealed a protruding epithelial tumor-like lesion circumscribed by two species of lateral "bird beaks" delineating a crater filled with many layers of keratin. The crater was bordered by hyperplastic epithelium. The crater base was characterized by irregular papillomatous projections as well as by few cellular strands which seemed to shred in the underlying dermis. These were basophilic made up of cells displaying a certain degree of cytonuclear abnormalites arranged in one or two peripheral areas and, in their center, some eosinophilic, keratin, homogeneous cells, few mitoses as well as many horny globes, most often completely keratinized (B). The diagnosis of keratoacanthoma was retained. Keratoacanthoma is a well defined anatomo-clinical entity which can be very difficult to distinguish from squamous cell carcinoma, whose incidence is three times higher. A distinction between these two lesions is necessary due to their different management.