Solitary fibrous tumor of the pancreas in a patient with tumor duplicity: a case report.

This case report describes a 71-year-old male patient in whom a solitary fibrous tumor (SFT) of the pancreatic tail was incidentally discovered during staging of chronic lymphocytic leukemia/small lymphocytic lymphoma (B-CLL/SLL). The patient also had a history of excised malignant melanoma. SFT is a mesenchymal neoplasm characterized by NAB2::STAT6 gene fusion, STAT6 and CD34 immunohistochemical positivity, and unclear biological behavior. In this case, the tumor was a firm, well-circumscribed spindle cell lesion without cytologic atypia, necrosis, or significant mitotic activity, showing strong diffuse STAT6 and CD34 expression. According to WHO classification criteria, it was classified as a low-risk SFT with respect to metastatic potential. The diagnosis of SFT is based on characteristic morphology and nuclear expression of STAT6, which helps distinguish it from a broad spectrum of CD34-positive mesenchymal lesions. The article discusses relevant differential diagnoses and highlights the molecular basis of SFT, including the prognostic implications of different NAB2::STAT6 fusion variants and the association of TERT promoter mutations with more aggressive behavior. Although pancreatic SFT is rare, similar cases have been reported in the literature. From a clinical standpoint, accurate risk stratification for recurrence or metastasis is essential. Several scoring systems have been proposed and validated, including the one adopted in the WHO classification, which considers tumor size, mitotic rate, necrosis, and patient age. In this case, the tumor was completely resected, and the patient has remained disease-free with no signs of SFT recurrence or B-CLL/SLL progression more than six months after surgery.