Uterine Malignant Mixed Müllerian Tumors Following Treatment with Selective Estrogen Receptor Modulators in Patients with Breast Cancer: A Report of 13 Cases and Their Clinicopathologic Characteristics

Background Breast cancer treatment with selective estrogen receptor modulators (SERMs) increases the incidence of uterine malignant mixed Müllerian tumors (uMMMTs). We examine clinicopathologic characteristics and prognosis of SERM-associated uMMMTs (S-uMMMTs) and discuss possible pathogenetic mechanisms. Methods Among 28,104 patients with breast cancer, clinicopathologic features and incidence of uMMMT were compared between patients who underwent SERM treatment and those who did not. Of 92 uMMMT cases that occurred during the same period, incidence, dose, and duration of SERM treatment, as well as overall survival rate, were compared for patients with breast cancer who underwent SERM treatment and those who did not (S-uMMMT vs NS-uMMMT) and for patients without breast cancer (de novo-uMMMT). Histopathological findings and immunophenotypes for myogenin, desmin, p53, WT-1, estrogen receptor (ER) α, ERβ, progesterone receptor, and GATA-3 were compared between S-uMMMT and de novo-uMMMT. Results The incidence of S-uMMMT was significantly higher than that of NS-uMMMT (6.35-fold). All patients with SERM were postmenopausal and received daily 20–40 mg SERM. Cumulative SERM dose ranged from 21.9 to 73.0 g (mean, 46.0) over 39–192 months (mean, 107). Clinicopathologic features, such as International Federation of Gynecology and Obstetrics stage and overall survival, were not significantly different between patients with S-uMMMT and NS-uMMMT or between patients with S-uMMMT and de novo-uMMMT. All 11 S-uMMMT cases available for immunostaining exhibited strong overexpression/null expression of p53 protein and significantly increased ERβ expression in carcinomatous and sarcomatous components. Conclusions SERM therapy seemingly increases risk of S-uMMMT development; however, clinicopathologic features were similar in all uMMMTs from different backgrounds. p53 mutation and increased ERβ expression might be involved in the etiology of S-uMMMT.