Alveolar solitary fibrous tumor: an uncommon morphological form.

Solitary fibrous tumor (SFT) is a fibroblastic tumor characterized by a prominent staghorn vasculature and collagen deposition. However, little is known about SFTs with alveolar structures. Herein, we present a case of an alveolar pattern SFT in a 55-year-old woman. The tumor was present in the lumbosacral spinal canal and showed an alveolar architecture composed of ovoid to spindle-shaped cells. Immunohistochemical examination showed that the tumor cells were positive for STAT6 (nuclear expression), CD34, CD99, and Bcl-2, but negative for cytokeratins (CK-pan and AE1/AE3), EMA, GFAP, CD31, progesterone receptor, S-100 protein, and smooth muscle actin. Furthermore, NAB2::STAT6 fusion was detected using DNA-based next-generation sequencing, which established the diagnosis of SFT at a molecular level. The present case expands the morphological categories of SFT.