Phosphohistone-H3 and Ki-67 immunostaining in cutaneous pilar leiomyoma and leiomyosarcoma (atypical intradermal smooth muscle neoplasm)

Background The mitotic index is important in the assessment of tumors such as leiomyoma (LM) and leiomyosarcoma (LMS), which may exhibit a range of cytological atypia. The mitotic marker phosphohistone-H3 (PHH3) was shown to improve interobserver and intraobserver variability in many tumors. Materials and methods We evaluated the mitotic index in 20 pilar LM and cutaneous LMS using PHH3 and hematoxylin and eosin (H&E)-stained sections. Ki-67 staining characteristics of the tumors were also assessed. Results Mitotic figures were more easily identified within PHH3 sections. The mitotic index per 10 high power fields (HPF) on the PHH3 stain was slightly higher than H&E both in the LM (mean 0.1, range 0–1 vs. mean 0) and LMS groups (mean 8.6 vs. 8.0 with range of 1–24 for both stains). The difference in mitotic index between the two stains was not statistically significant in either group (p = 0.7). The Ki-67 proliferative index showed a statistically significant correlation with a diagnosis of LMS. Conclusion PHH3 immunostain can simplify counting of mitotic figures in cutaneous smooth muscle neoplasms, especially those with many pyknotic nuclei, and may help to reduce interobserver variability. Ki-67 staining may also be of help in establishing a diagnosis of LMS.