Ovarian microcystic stromal tumor with unusual expression of AR and partial S100: a case report and literature review

The ovarian microcystic stromal tumor (MCST) is a rare, typically benign stromal neoplasm characterized by a distinctive microcystic architecture and nuclear β-catenin immunoreactivity. This report describes a case that exhibits unusual AR positivity and partial S100 positivity. The aim of this report is to describe this case and review the literature, with a view to exploring the clinicopathological features of this variant. A 38-year-old nulliparous woman with a history of irregular menstruation presented with an incidental left ovarian mass. Laparoscopic tumor enucleation was performed after an intraoperative frozen-section diagnosis of a benign sex cord-stromal tumor. Definitive histopathology confirmed MCST, revealing characteristic morphology and immunoprofile (β-catenin, Cyclin D1, CD10, vimentin, WT1, FOXL2, and SF1 positive). Notably, the tumor cells exhibited partial S100 protein positivity. A distinctive “AR-strong/PR-weak” hormone receptor signature (AR 90%, PR 15%) was also observed. The patient recovered well with no evidence of recurrence at 28-month follow-up. We report a case of ovarian microcystic stromal tumor (MCST) with an unusual immunophenotype, characterized by AR positivity and partial S100 expression. Although AR expression has been reported in MCST, and focal S100 positivity has been documented in at least one prior case, their co-existence in a single tumor has been only rarely documented. The biological significance of this immunophenotypic combination remains unclear and warrants further investigation in larger case series.