The many faces of solitary fibrous tumor; diversity of histological features, differential diagnosis and role of molecular studies and surrogate markers in avoiding misdiagnosis and predicting the behavior

Abstract

Background: Solitary Fibrous Tumor (SFT) is a distinct soft tissue neoplasm associated with NAB2-STAT6 gene fusion. It can involve a number of anatomic sites and exhibits a wide spectrum of histological features.

Main body: Apart from diversity in morphological features seen even in conventional SFT, two histologic variants (fat-forming and giant cell-rich) are also recognized. In addition, a malignant form and dedifferentiation are well recognized. Owing to diverse histological features and involvement of diverse anatomic locations, SFT can mimic other soft tissue neoplasms of different lineages including schwannoma, spindle cell lipoma, dermatofibrosarcoma protuberans, liposarcoma, gastrointestinal stromal tumor (GIST), malignant peripheral nerve sheath tumor (MPNST), and synovial sarcoma. SFT is classified as an intermediate (rarely metastasizing) tumor according to World Health Organization Classification of Tumors of Soft tissue and Bone, 5th edition. The management and prognosis of SFT differs from its malignant mimics and correct diagnosis is therefore important. Although SFT expresses a distinct immunohistochemical (IHC) profile, the classic histomorphological and IHC profile is not seen in all cases and diagnosis can be challenging. NAB2-STAT6 gene fusion has recently emerged as a sensitive and specific molecular marker and its IHC surrogate marker signal transducer and activator of transcription 6 (STAT6) has also shown significant sensitivity and specificity. However, few recent studies have reported STAT6 expression in other soft tissue neoplasms.

Conclusion: This review will focus on describing the diversity of histological features of SFT, differential diagnoses and discussing the features helpful in distinguishing SFT from its histological mimics.

Keywords: CD34; Hemangiopericytoma; Immunohistochemistry; NAB2-STAT6; STAT-6; Solitary fibrous tumor; Staghorn, fusion transcript.

Fig. 1

Gross appearance of SFT: a. Benign SFT appearing as a well-circumscribed, white and firm tumor. b. Malignant SFT of retroperitoneum exhibiting ill-defined borders, variegated cut surface and cystic degeneration

Fig. 2

a. SFT exhibiting hypo and hypercellular areas of spindle cells against collagenous background stroma along with, b. HPC-like vessels, c. stromal and perivascular fibrosis and, d. myxoid change in stroma

Fig. 3

a. Classic SFT exhibiting cracking artifact and, b. abundant keloid-type collagen, c&d. Cellular SFT exhibiting increased cellularity, gaping blood vessels and more darkly stained nuclei

Fig. 4

a. Markedly cellular tumor showing sheets of small sized cells with hyperchromasia, b. Tumor cells exhibiting round cell morphology, c. Lipomatous SFT composed of mature adipocyte intermixed with tumor cells, d. Giant cell SFT exhibiting multinucleated giant cells focally lining pseudovascular spaces

Fig. 5

Dedifferentiated SFT; a. Abrupt transition of conventional SFT areas with high grade sarcomatous areas with, b. marked nuclear pleomorphism and increased mitoses

Fig. 6

Tumor cells showing positive staining for, a. CD34, b. CD99, c. BCL2 and, d. nuclear staining for STAT6 IHC stains