pHH3 immunostaining improves interobserver agreement of mitotic index in thin melanomas

According to the seventh edition of the American Joint Committee on Cancer guidelines, the TNM staging category in thin cutaneous melanomas depends on the mitotic rate (MR). In this study, we analyze the interobserver agreement of the MR in a series of 92 thin cutaneous melanomas. Serial sections of the tumors were either stained with hematoxylin and eosin or immunohistochemically stained with pHH3, an antibody for phosphohistone H3, and analyzed by 4 observers. Determination of MR with pHH3 immunostaining resulted in higher sensitivity in counting mitosis for all observers. Moreover, interobserver agreement was higher with pHH3. Immunostaining with pHH3 is a sensitive method to detect mitosis in thin cutaneous melanomas, with good reproducibility of MR between independent observers. Further studies are needed to find out if higher sensitivity in the detection of mitosis by pHH3 immunostaining has additional prognostic relevance.