Solitary Fibrous Tumor With a 12-Year Recurrence Interval in a Female Patient: A Case Report.

Solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm characterized by unpredictable biological behavior, with the potential for late recurrence even in tumors initially classified as low risk. We report the case of an 83-year-old woman with a history of complete resection of a right thoracic SFT 12 years earlier, who was found on surveillance imaging to have a new anterior left mediastinal mass. Positron emission tomography-computed tomography (PET-CT) demonstrated low metabolic activity (maximum standardized uptake value (SUVmax) 2.32). Surgical exploration revealed a 6.5 × 5 cm lesion adherent to the pericardial pleura, which was completely resected. Histopathologic examination showed a spindle cell neoplasm with a patternless architecture, without hypercellularity, necrosis, or pleomorphism. Immunohistochemistry demonstrated strong nuclear positivity for signal transducer and activator of transcription 6 (STAT6) and diffuse CD34 expression, with negative S100 protein and muscle markers. Risk stratification using the Demicco model categorized the tumor as low risk. Review of the original tumor resected in 2013, which measured 19.3 × 16.2 cm, revealed similar histologic features and an intermediate-risk score. The long disease-free interval raises the diagnostic dilemma of late recurrence versus a metachronous SFT, a distinction that remains challenging due to shared morphologic and molecular characteristics. This case underscores the limitations of metabolic imaging in indolent SFTs and highlights the importance of complete surgical resection and long-term, potentially lifelong, surveillance.