A case of posterior mediastinal Müllerian cyst characterized by morphology of serous papillary cystadenoma.

Background: Müllerian cysts of the posterior mediastinum are rare, benign lesions typically discovered incidentally via routine medical exams.

Case Presentation: We present a distinctive case of a 49-year-old asymptomatic woman, illustrating a rare Müllerian cyst located in the posterior mediastinum with serous papillary cystadenoma-like features, a novel finding in the medical literature. Identified during a routine health screening in December 2020, a 20 mm cystic lesion adjacent to the T4-5 vertebral body was detected through chest CT and MRI, initially suggesting a neurogenic tumor. Histopathological analysis revealed a Müllerian cyst characterized by an epithelium resembling serous papillary cystadenoma, with positive immunohistochemical markers for Paired Box 8 (PAX8), Estrogen Receptor (ER), and Progesterone Receptor (PR), indicative of Müllerian differentiation. The papillary growth pattern initially raised concerns about malignancy, yet the benign nature was confirmed by minimal cellular atypia, well-defined borders, absence of invasion, and a low proliferation index.

Conclusion: This case underscores the significance of detailed pathological analysis and immunohistochemical profiling in managing atypical Müllerian cysts, adding to the limited knowledge on Müllerian cysts. Future research should explore the mechanisms leading to Müllerian cysts with serous papillary cystadenoma-like features in the posterior mediastinum.