Solitary Fibrous Tumor of the Abdominal Wall With Long-Term Progressive Enlargement and Malignant Features.

Solitary fibrous tumor (SFT) is a rare mesenchymal neoplasm that usually arises from the pleura, while extrapleural cases are uncommon. Abdominal wall SFTs are exceedingly rare, and their biological behavior remains unpredictable, with limited long-term follow-up data. An 83-year-old man was incidentally found to have an 18 mm mass on the liver surface in 2008 but declined further evaluation. Sixteen years later, he presented with abdominal pain, and imaging demonstrated enlargement of the lesion to 71 mm. Contrast-enhanced computed tomography (CT) and magnetic resonance imaging (MRI) showed a well-circumscribed subperitoneal mass contiguous with the abdominal wall and supplied by the superior epigastric artery. Laboratory tests, including tumor markers, were within normal limits. Elective laparotomy revealed a smooth, well-demarcated mass continuous with the abdominal wall, and en bloc resection was performed. Histopathology showed spindle cell proliferation with necrosis and a mitotic count of two per 10 high-power fields. Immunohistochemistry was positive for cluster of differentiation 34 (CD34), CD99, B-cell lymphoma 2 (BCL2), and signal transducer and activator of transcription 6 (STAT6), with a Ki-67 index of 6%, confirming malignant SFT. This case illustrates that indolent-appearing SFTs may exhibit progressive enlargement and demonstrate malignant potential over time, even after a prolonged period of clinical quiescence. To our knowledge, this is the first reported case of an abdominal wall SFT with malignant transformation documented after extended follow-up. Given the potential for recurrence or metastasis, vigilant long-term surveillance is warranted.