CD34-negative Solitary Fibrous Tumor

Abstract

CD34-negative solitary fibrous tumors (SFTs) are rare and have not been comprehensively studied. We retrospectively reviewed all cases of SFT confirmed with STAT6 immunohistochemistry and/or STAT6 gene fusion between 2013 and 2020 and collected pertinent clinicopathologic parameters. Of a total of 244 cases, 25 (10%) lacked CD34 expression by immunohistochemistry. Compared with CD34-positive SFT, CD34-negative SFT are more likely to arise in the head and neck area (32% vs. 24%, P=0.02) and present as metastatic disease (28% vs. 1%, P<0.0001). A significantly higher percentage of CD34-negative SFT exhibit high-grade cytologic atypia (hypercellularity, round cell or anaplastic morphology, nuclear pleomorphism, etc.) (48% vs. 22%, P=0.0073). There are no significant differences in the distributions of age, sex, tumor size, mitotic count, tumor necrosis, or risk stratification between CD34-negative and CD34-positive SFT. In addition, only 56% of CD34-negative SFT display a typical hemangiopericytoma-like vascular pattern. Special histologic features among CD34-negative SFT include prominent alternating hypercellular or fibrous and hypocellular myxoid areas with curvilinear vessels mimicking low-grade fibromyxoid sarcoma, pulmonary edema-like microcystic changes, and prominent amianthoid collagen fibers. In conclusion, compared with their CD34-positive counterparts, CD34-negative SFT is more likely to present as metastatic disease, show high-grade nuclear atypia, and lack the characteristic hemangiopericytoma-like vasculature, posing a unique diagnostic challenge. The use of STAT6 immunohistochemistry and/or molecular studies may be prudent in soft tissue tumors that appear CD34 negative and lack conventional SFT histopathologic characteristics.

Primary NTRK-rearranged Spindle Cell Neoplasm of the Lung

“…Similar to NTRK -rearranged mesenchymal tumors, SFT is also characterized by uniform spindle cells showing haphazard arrangement or “patternless” pattern with variable stromal and perivascular hyalinization. Besides CD34, SFT is consistently positive for STAT6 including those CD34-negative tumors 19. SFT rarely shows immunoreactivity for S100 protein and pan-TRK.…”

Wolves in Sheep's Clothing and Vice Versa: Fine-Needle Aspiration of Low-Grade Spindle Cell Lesions

“…These CD34-negative SFTs were more likely to arise in the head and neck area, and only 56% showed the typical staghorn vascular pattern. 16 Solitary fibrous tumors are characterized by a paracentric inversion of chromosome 12q, which results in a NAB2-STAT6 translocation. Strong nuclear expression of immunohistochemistry for the carboxyterminal portion of STAT6 has been found to reliably distinguish SFT from histologically similar tumors.…”

Cytomorphological spectrum of solitary fibrous tumour: revisited

“…The underlying genetic pathology is a paracentric inversion in chromosome 12q, leading to NAB2‐STAT6 gene fusion, which further activates several growth factor pathways 1,2,4 . Morphologically, these tumours are composed of spindle‐shaped cells along with prominent, dilated and branching blood vessels, described as “staghorn vasculature” 5 . SFT and hemangiopericytoma have been now combined into a single entity 6–8 .…”

“…The characteristic metachromatically staining collagen may show cells embedded within it 11,12 . These tumours are positive for CD34, CD99, Bcl‐2, and EMA immunostains and are negative for S100, SOX10, cytokeratin, CD31 and desmin 5,12 . The immunological marker STAT6 is increasingly being utilised to distinguish SFT from other possible differentials 5,13–15 .…”

“…These tumours are positive for CD34, CD99, Bcl‐2, and EMA immunostains and are negative for S100, SOX10, cytokeratin, CD31 and desmin 5,12 . The immunological marker STAT6 is increasingly being utilised to distinguish SFT from other possible differentials 5,13–15 . The diverse cytomorphological features of SFTS frequently overlap with other soft tissue entities, thus precluding an accurate cytodiagnosis.…”