Ovarian microcystic stromal tumor is exceptionally rare. Since it was first delineated as a distinct entity, only a limited number of cases have been documented in the medical literature worldwide. The tumor belongs to the broad family of ovarian stromal neoplasms, which arise from the supportive tissue of the ovary rather than from the egg cells themselves. What distinguishes MCST is its striking microscopic appearance: the tumor is composed of sheets and nests of relatively uniform cells arranged around a network of tiny, fluid-filled microcysts that give the lesion its name. Under the microscope, this architecture can resemble other ovarian tumors, including yolk sac tumor or even certain metastatic cancers, which historically made diagnosis challenging.

The key to recognizing MCST lies in its immunohistochemical fingerprint. Tumor cells in these lesions characteristically show nuclear expression of β-catenin, a protein that serves as a hallmark feature and helps separate MCST from its many microscopic mimics. The tumors also typically express Cyclin D1, CD10, vimentin, WT1, FOXL2, and SF1, a constellation of markers that pathologists use to confirm the diagnosis and to exclude other possibilities. Despite this growing list of identifying features, the biological behavior and molecular underpinnings of MCST remain incompletely characterized, largely because so few cases exist to study. Most reported tumors have behaved in a benign fashion, and conservative surgical management has generally proven sufficient.

The newly reported case involved a 38-year-old woman who had never been pregnant and who carried a history of irregular menstruation. Her ovarian mass was discovered incidentally, meaning it was found without the patient having shown symptoms directly attributable to the tumor. An initial intraoperative frozen-section examination pointed to a benign sex cord-stromal tumor, allowing the surgical team to proceed with a laparoscopic tumor enucleation, a fertility-sparing approach in which the tumor is shelled out while the ovary is preserved. This minimally invasive strategy reflects the growing recognition that, in carefully selected patients with apparently benign ovarian lesions, organ-preserving surgery can offer excellent outcomes without the need for more radical procedures.

Definitive histopathological examination confirmed the diagnosis of microcystic stromal tumor. The tumor displayed the characteristic morphology and the expected immunoprofile, with positivity for β-catenin, Cyclin D1, CD10, vimentin, WT1, FOXL2, and SF1. But two findings set this case apart. First, the tumor cells showed partial positivity for S100, a calcium-binding protein commonly expressed in neural, melanocytic, and certain other tissues, and one that is not typically associated with MCST. Second, and perhaps more strikingly, the tumor exhibited what the authors describe as a distinctive hormone receptor signature: androgen receptor was strongly expressed in approximately 90 percent of tumor cell nuclei, while progesterone receptor was weak, present in only about 15 percent of cells.

Androgen receptor expression in ovarian tumors carries particular interest because the androgen receptor participates in signaling pathways relevant to ovarian physiology and ovarian carcinogenesis. Its strong expression in this MCST raises questions about whether hormone signaling might play a role in the growth or regulation of these tumors, or whether the finding represents an incidental epiphenomenon without clinical consequence. Similarly, S100 expression introduces a diagnostic wrinkle: S100 positivity can suggest alternative diagnoses, including neural tumors or melanocytic lesions, and its partial presence in an otherwise classic MCST underscores the importance of interpreting immunohistochemical panels in context rather than relying on any single marker. Although androgen receptor expression has been reported in MCST before, and focal S100 positivity has been documented in at least one prior case, the co-existence of both features in a single tumor has only rarely been recorded.

The clinical course of the patient adds a reassuring dimension to the report. Following her laparoscopic enucleation, she recovered well, and follow-up at 28 months revealed no evidence of recurrence. This outcome is consistent with the prevailing understanding of MCST as a typically indolent neoplasm, and it supports the feasibility of conservative surgery combined with careful long-term surveillance in affected patients. For a young woman whose tumor was detected before any malignant transformation or complication, the preservation of ovarian tissue represents a meaningful quality-of-life benefit, and the extended recurrence-free follow-up provides additional confidence in the benign trajectory of this tumor type.

The authors of the report paired their case with a review of the existing literature, aiming to consolidate what is known about the clinicopathological features of MCST and its unusual immunophenotypic variants. Because the entity is so rare, individual case reports effectively serve as the primary source of knowledge, each one contributing data points that slowly refine the diagnostic criteria and the understanding of biological behavior. The review situates the present case within this accumulating record, noting both the consistency of core features—microcystic architecture, nuclear β-catenin, and benign behavior—and the variability in secondary findings such as hormone receptor status and unexpected marker expression.

The report also highlights the practical implications for diagnostic pathology. Pathologists confronting an ovarian microcystic lesion must distinguish MCST from a differential diagnosis that includes yolk sac tumor, juvenile granulosa cell tumor, and clear cell carcinoma, among others. Unusual immunostaining results, such as partial S100 positivity, could easily send the diagnostic workup down an erroneous path if not weighed against the full morphological and immunophenotypic picture. Cases like this one, in which atypical marker expression arises within an otherwise typical tumor, help calibrate the diagnostic thresholds that practicing pathologists apply, reducing the risk of both overdiagnosis and missed diagnosis.

For now, the biological significance of the combined androgen receptor and S100 expression observed in this tumor remains unresolved. The authors emphasize that larger case series will be needed to determine whether such immunophenotypic variation carries any prognostic or therapeutic relevance, or whether it simply reflects the natural heterogeneity of a rare neoplasm. In the meantime, the case stands as a reminder of how much remains to be learned about even the quietest corners of tumor biology—and of how a single carefully documented patient can meaningfully advance the field. The 28-month disease-free outcome offers comfort to patients facing similar diagnoses, while the unusual molecular profile offers scientists a fresh line of inquiry into the mechanisms that drive this enigmatic ovarian tumor.

Subject of Research: A rare benign ovarian stromal neoplasm exhibiting an atypical immunophenotype with androgen receptor and partial S100 expression

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

Article References: Liu, M., Sun, Y., Yin, J., Liu, K., & Jiang, Z. (2026). Ovarian microcystic stromal tumor with unusual expression of AR and partial S100: a case report and literature review. Journal of Ovarian Research. https://doi.org/10.1186/s13048-026-02276-7

Image Credits: AI Generated

DOI: 10.1186/s13048-026-02276-7

Keywords: ovarian tumor, microcystic stromal tumor, MCST, androgen receptor, S100, β-catenin, immunohistochemistry, sex cord-stromal tumor, case report, ovarian pathology, laparoscopic surgery, Journal of Ovarian Research