Papillary Solitary Fibrous Tumor/Hemangiopericytoma: An Uncommon Morphological Form With NAB2-STAT6 Gene Fusion

Solitary fibrous tumor/hemangiopericytomas (SFT/HPCs) are mesenchymal tumors characterized by "staghorn" blood vessels and collagen deposition. Little is known about SFT/HPCs with papillary architecture. We summarized the clinicopathologic features of 12 patients with papillary SFT/HPCs (8 males and 4 females; median age: 59 years), including 8 previously reported cases. Tumors were present in the meninges (75%, 9/12), adrenal gland (8%, 1/12), orbit (8%, 1/12), or spinal canal (8%, 1/12). Six tumors (50%) had a true papillary architecture with fibrovascular cores and 6 tumors (50%) had a pseudopapillary architecture with vascular cores. Nuclear staining for STAT6 was present in all tested tumors (10/10). RT-PCR indicated NAB2 ex6-STAT6 ex17 fusion in 4 tumors (80%, 4/5) and NAB2 ex4-STAT6 ex2 fusion in 1 tumor (20%, 1/5). Five patients (42%, 5/12), all with tumors in the meninges, developed local recurrence at a median of 61 months after surgery (range: 56-165 months; mean: 88.6 months). These results indicated that the papillary architecture is a morphological form of SFT/HPCs. The recognition of this pattern, with appropriate immunohistochemical analysis and assessment of NAB2-STAT6 fusion, should facilitate the distinction of these rare neoplasms from morphologically similar tumors in the meninges, lung, pleura, and soft tissue.