A Dedifferentiated Solitary Fibrous Tumor of Soft Tissue With Retiform and Papillary Features: Report of a Case and Review of the Literature

Abstract We report a case of a dedifferentiated solitary fibrous tumor (SFT) arising in the arm of a 74-year-old man, with the dedifferentiated component showing a unique retiform and papillary architecture. The patient presented with a tumor first noticed 12 years ago, which was diagnosed as a schwannoma. It remained clinically stable for about 10 years until he noticed growth of the mass 2 years ago. On comparative magnetic resonance imaging scans, while most of the tumor did not show major changes, a proximal lobule demonstrated increase in its size. After a new biopsy, which was interpreted as malignant, preoperative radiotherapy and surgical excision were performed. The surgical specimen had areas of typical SFT, with diffuse immunohistochemical expression of STAT6 and only focal CD34 and p16. The lobule that demonstrated growth showed a much more cellular, retiform/papillary proliferation, with plump atypical cells, necrosis, diffuse expression of STAT6 and p16, and complete lack of CD34 expression. With next-generation sequencing, the NAB2(3)-STAT6(18) fusion transcript was detected in both areas of the tumor. Only a few cases of SFT with papillary or retiform/papillary features were reported, most of them in the central nervous system, and none of them interpreted as a dedifferentiated SFT. To the best of our knowledge, these morphological features have not been reported in SFT of the soft tissue or in dedifferentiated SFT of any site. We think pathologists should be aware of this rare finding, which becomes particularly challenging in small biopsies or if classic areas of SFT are not found.