PS-24-025 Subvisual nuclear characteristics are different between keratoacanthoma and keratoacanthoma-like squamous cell carcinoma K. Metze * , M. Tabai, R. Adam, I. Watanabe, A. de Moraes, M. Cintra * University of Campinas, Pathology, Brazil Objective: The differential diagnosis between keratoacanthorna and keratoacantoma-like squamous cell carcinoma may be extremely diffcult. Our objective was to study whether computerized image analysis could be helpful. Method: In 34 patients biopsies of keratoacantoma-like lesions were taken at admission. One month later surgical excision was performed in growing lesions, whereas regressing lesions were left untreated. A final diagnosis was established combining clinical and histological evaluation, Digitalized images from KI-67 immunostained and hematoxylin counterstained sections of first biopsies were obtained. Tumor nuclei were marked interactively. An inhouse computer program analyzed the geometric relations between the nuclei. Nuclear gray values and their histogram entropy were calculated. Results: 27 keratoacnthomas and 7 keratoacantomalike squamous cell carcinomas entered this investigation. Basic variables of the geometric analysis did not differ between the two entities regardless whether all nuclei or only the Ki67 positive ones had been examined. Chromatin gray values were significantly lower and their histogram entropy higher in the keratoacantoma-like squamous cell carcinomas. Conclusion: Basic geometric variables do not seem to be different beteen both lesions, but a shift in the gray value histogram to lower values with increased entropy in carcinomas indicated important differences of the chromatin structure, Supported by FAPESP and CNPq.