Objective[|]This study aimed to evaluate diagnostic parameters such as mitotic count and tumor size in; leiomyosarcomas (LMSs) and benign uterine smooth muscle tumors (USMTs) and to define the diagnostic value and prognostic significance of the Ki-67 proliferation index and p16, p53, and bcl-2 expressions by immunohistochemical (IHC) methods.[¤]Materials and Methods[|]In total, 44 cases diagnosed as LMS, atypical leiomyoma, or cellular leiomyoma at our pathology
department from January 2010 to December 2015 were included. IHC staining was performed for bcl-2, p16, p53, and Ki-67 using standard techniques.[¤]Results[|]Tumor size and mitotic index were significant prognostic factors (p=0.008 and p=0.001, respectively). The rate of diffuse p16 expression was significantly higher in the LMS group than in the other LM group (p=0.001). A Ki-67 positivity rate of >10% (increased proliferation) was statistically significantly higher in the LMS group than in the benign USMT group (p=0.0001). No statistically significant difference was found between the LMS and benign USMT groups with respect to bcl-2 expression (p=0.892). Mitotic count and high Ki-67 expression (>%10) were statistically high in cases with relapse/ metastasis (+) (p=0.0001 and p=0.0002, respectively).[¤]Conclusion[|]In addition to histopathological findings (tumor size and mean mitotic count), diffuse p16 expression and p53 overexpression can be used to distinguish between benign and malignant USMTs. A high mitotic index [≥10/10 (high-power field)] and high Ki-67 expression (>10%) can serve as useful indicators for diagnosing LMS, distinguishing benign tumors, and predicting an aggressive clinical course.[¤]