Uterine Spindle Cell Smooth Muscle Tumor of Uncertain Malignant Potential With Mitotic Figures Greater Than or Equal to 15/10 High-Power Fields: A Clinicopathologic Study of 20 Patients With a Rare Tumor

Background. This study aimed to investigate the prognostic factors associated with uterine spindle cell smooth muscle tumors of uncertain malignant potential (STUMPs) with mitotic figures ≥15/10 high-power fields (HPFs). Methods. A total of 20 patients between January of 2009 and December of 2020 at the Department of Pathology of the Obstetrics and Gynecology Hospital of Fudan University were enrolled. Pathological features and immunohistochemical staining (including estrogen receptor (ER), progesterone receptor (PR), Ki67, p53, and phosphorylated histone H3 (PHH3)) were assessed, and clinical follow-up data were collected. Results. Among the 16 patients with Stage I, 3 recurrence, mitotic figures and PHH3 were associated with recurrence ( P = 0.014 and P = 0.001, respectively). Among all 20 patients, stage, mitotic figures, PHH3 and Ki67 were associated with recurrence ( P = 0.007, P = 0.003, P = 0.000, and P = 0.028, respectively). The areas under the receiver operating characteristic curve were 0.890 and 0.934 for mitotic figures and PHH3, with a cutoff value of mitotic figures 21/10 HPFs. Three patients (43%) did not experience recurrence without chemotherapy, and 6 patients (86%) did not experience recurrence after bilateral ovariectomy. Conclusions: For STUMPs with mitotic figures ≥15/10 HPFs, the higher the number of mitotic figures, the more likely it is for the tumor be extrauterine and recurrent. Recurrence could be assessed by using the diagnostic cutoff value of mitotic figures >21/10 HPFs or PHH3 >21/10 HPFs. We postulate that ovariectomy or antiestrogen therapy may be beneficial in preventing recurrence.