Eosinophil infiltration in keratoacanthoma and squamous cell carcinoma of the skin.

The incidence of tissue eosinophilia in keratoacanthoma and in early and late cases of squamous cell carcinoma of the skin has been studied. Eosinophil infiltration of over 10 cells per high power field was found in 80 cases, and was not related to the size, site or aetiology of the lesions in which it was present. In cases where diagnostic difficulty arises between keratoacanthoma and well differentiated, keratinizing squamous cell carcinoma, if tissue eosinophilia is present the lesion is more likely to be malignant. Though in isolation the finding of an eosinophil infiltrate is not diagnostic it should be added to the list of criteria which help to distinguish these lesions. The pattern of tissue eosinophilia in late cases of squamous cell carcinoma was more extensive and there were two cases which showed massive tumour-associated tissue eosinophilia. This has previously been reported in squamous cell carcinoma at other sites, and may be related to the production of an eosinophilochemotactic factor.