Clinicopathological and genetic heterogeneity of the head and neck solitary fibrous tumours: a comparative histological, immunohistochemical and molecular study of 36 cases.

Abstract

Solitary fibrous tumour (SFT) harbours recurrent inv12(q13q13)-derived NAB2-STAT6 fusions, resulting in STAT6 nuclear expression. SFTs affecting the head and neck are rare, for which we reported their clinicopathological, immunohistochemical, and genetic features. With 19 cases assessable for NAB2-STAT6 fusions, 36 head and neck SFTs (18 males; 18 females) diagnosed between 13 and 79years (median, 47years) of age were analysed for clinicopathological features and STAT6 immunoexpression. These SFTs, ranging from 5 to 80mm (median, 25mm), affected the oral cavity/pharynx (12), orbit (11), sinonasal structures (seven), and somatic soft tissues or skull (six). Histologically, 20 SFTs were conventional, six were giant-cell angiofibroma-like, one was fat-forming, four were cellular/atypical, and five were malignant (two developing metastases). STAT6 distinctively decorated the tumoral nuclei in 35 (97.2%) SFTs, but not in 29 site-relevant histological mimics categorized into 12 entities. Sixteen (84.2%) SFTs showed NAB2-STAT6 fusions with highly heterogeneous exon compositions, including NAB2ex6-STAT6ex17 in four cases, NAB2ex4-STAT6ex2 in three cases, NAB2ex2-STAT6ex2, NAB2ex4-STAT6ex4, NAB2ex6-STAT6ex16 and NAB2ex6-STAT6ex18 in two cases each, and NAB2ex3-STAT6ex19 in one case. Nuclear STAT6 immunoexpression is sensitive and specific for distinguishing SFT from mimics. However, considerable heterogeneity exists in the head and neck SFTs regarding the locations, histological patterns, and NAB2-STAT6 fusion variants.