Differential diagnosis of keratoacanthoma and squamous cell carcinoma of the epidermis by MIB-1 immunohistometry.

Objective: For the analysis of cellular proliferative activity, the MIB-1 immunopositivity of keratoacanthoma (KA, n=49), squamous cell carcinoma (SCC, n-48) and each of four cases diagnosed probably as KA or probably as SCC were analyzed by means of immunohistometry. Study Design: Immunohistochemical reactions were performed on 3-μm sections from routinely formalin-fixed and paraffin-embedded surgical specimens, using an indirect peroxidase method. The rate of immunostained cells was determined using a TV-image analysis system CM-2 (Hund, Germany). Twenty viewing fields (0.97 mm 2 ) were measured with 20:1 objective magnification. An average of 1688 cells were assessed in each case. Results: The mean MIB-1 immunopositivity (MIB-1 mean ) was higher in SCC (42.3%±19.1%) than in KA (26.8%±9.8%). The distribution of the single values differed significantly (p=0.0002). To test the suitability of MIB-1 immunohistometry for the differential diagnosis between KA and SCC, various thresholds were investigated. Using a threshold of 30%, SCC can be detected with a sensitivity of 70.8% (34 out of 48) and a specificity of 67.3% (33 out of 49). if a specificity of ≥85% is required (42 out of 49, 85. 7%), the sensitivity of the test decreases to 56.3% (27 out of 48) based on a threshold of 37.5%. Using the MIB-1 value of the most positive focus of the lesion (MIB-1 max ), the results were of minor significance; at a specificity level of ≥85% (42 out of 49, 85.7%) a sensitivity rate of only 43.8% (21 out of 48) could be obtained (threshold: 75%). Conclusion: As some overlap of the single values has to be considered, MIB-1 immunohistometry, although presenting new insights into the proliferative potential of KA and SCC, is of only limited value for the differential diagnosis of the two lesions in routine surgical pathology.