A Rare Case of Ovarian Serous Borderline Tumor Recurrence with Muscle Metastasis.

BACKGROUND Ovarian serous borderline tumors (SBTs) generally have a favorable prognosis, with a very low recurrence rate. However, in rare cases, they can recur as invasive low-grade serous carcinoma (LGSC) after a prolonged follow-up period. Here, we report a case of LGSC originating from SBT that recurred 23 years after the initial surgery, with metastasis to the quadratus lumborum muscle - an exceptionally rare site of metastasis. CASE REPORT A 50-year-old woman, initially diagnosed with stage IIIC SBT and treated with complete tumor resection 23 years prior, presented with an asymptomatic recurrence detected by an elevated serum cancer antigen 125 (CA125) level. Contrast-enhanced computed tomography (CT) revealed multiple nodules suspected of peritoneal dissemination and a tumor infiltrating the quadratus lumborum muscle, suggesting recurrent SBT. A CT-guided needle biopsy confirmed that the tumor within the quadratus lumborum was a recurrence of SBT. Complete cytoreductive surgery was performed with the assistance of an orthopedic surgeon. Histopathological examination revealed progression to LGSC with cytoplasmic expression of the BRAF proto-oncogene (BRAF) V600E, indicating the presence of the BRAF V600E mutation, which is a characteristic feature of both SBT and LGSC. A retrospective review of CT images taken 10 years prior to the recurrence diagnosis showed a peritoneal tumor with calcification attached to the ileocecum, suggesting that the patient had remained asymptomatic for more than a decade after the actual onset of recurrence. CONCLUSIONS This case illustrates a rare instance of recurrent SBT with metastasis to the quadratus lumborum muscle. Given the exceptionally slow progression of recurrent SBT, long-term follow-up with CT imaging and serum CA125 monitoring is crucial for timely intervention and appropriate management upon recurrence.