Low-grade serous carcinoma metastasizing to the chest wall and incidental Müllerian inclusion in a retrosternal lymph node 16 years after the initial ovarian serous borderline tumor: a case report.

Ovarian serous borderline tumors (SBTs) generally exhibit indolent behavior; however, a small subset of SBTs progresses to low-grade serous carcinoma (LGSC), in some cases after prolonged follow-up. Notably, very late progression with distant metastasis has rarely been reported. A 50-year-old woman with a history of bilateral ovarian SBTs presented with a chest wall mass 16 years after initial diagnosis. At 34 years of age, the initial left ovarian tumor was treated with fertility-sparing surgery and pathologically diagnosed as an SBT pT1c3 because of intraoperative capsule rupture and positive ascitic cytology. After 5 years, the contralateral side lesion was resected, and the patient was diagnosed with recurrent SBT. Sixteen years after the initial diagnosis, imaging revealed a calcified mass at the right sixth costochondral junction. Surgical resection revealed papillary to micropapillary carcinoma with destructive invasion consistent with metastatic LGSC. The tumor cells were positive for estrogen receptor, PAX8, and WT-1, with wild-type p53 expression. A nearby lymph node showed micro-metastasis consisting of a few tumor cells. In addition, an incidentally resected retrosternal lymph node was identified as a nodal Müllerian inclusion. Recurrent SBT, LGSC, and a nodal inclusion commonly harbored KRAS hotspot mutations. This case highlights the potential for the very late malignant progression of ovarian SBTs and the need for long-term surveillance. In addition, the unique location of a nodal inclusion and its clonal relationship with LGSC provide further insights into the pathogenesis of ectopic Müllerian tissues.