PAX8 Expression in Solitary Fibrous Tumor: A Potential Diagnostic Pitfall

Abstract

PAX8 is used as a diagnostic aid in classifying retroperitoneal (RP) spindle cell tumors. PAX8 positivity in a spindled RP tumor is typically associated with sarcomatoid renal cell carcinoma (SRCC). However, PAX8 expression in solitary fibrous tumor (SFT), a tumor not uncommon to the RP, has not been extensively studied. We investigated the expression of PAX8 in SFTs and other spindle cell RP tumors. We collected 30 SFT, 23 SRCC, 11 gastrointestinal stromal tumors, 2 synovial sarcomas, 6 dedifferentiated liposarcomas (DDLS), 4 well differentiated liposarcomas (WDLS), and select other tumors. We identified nuclear PAX8 expression in 13 of 30 (43%) SFT, 0 of 6 (0%) DDLS, and 1 of 4 (25%) WDLS. Twenty-eight of 30 (93%) SFT, 0 of 23 (0%) SRCC, 2 of 6 (33%) DDLS, and 1 of 4 (25%) WDLS showed nuclear STAT6 expression. All gastrointestinal stromal tumors were negative for both PAX8 and STAT6. Of the 13 SFT showing PAX8 expression, 8 showed diffuse expression and 5 expressed PAX8 focally. Extrapleural SFTs were more likely to express PAX8 compared with pleural SFTs (10/13; 77% vs. 3/17; 18%, respectively; P=0.00117). Twenty of 23 (87%) SRCC expressed PAX8; the sarcomatoid component of all 23 SRCC was negative for STAT6. Of the other spindle cell tumors studied, 1 of 2 synovial sarcomas and 1 of 2 histiocytic sarcomas showed PAX8 expression. Pathologists should be aware of the potential pitfall of the relatively frequent expression of PAX8 by SFT and STAT6 expression in liposarcoma. PAX8 expression by a spindle cell lesion of RP would not allow distinction between SFT, SRCC, or sclerosing liposarcoma by itself. A STAT6/PAX8 phenotype excludes SRCC.

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“…Moreover, an earlier study has investigated PAX8 expression in SFT. PAX8 is a member of the paired-box family of genes, which resulted positive in a small subset of extra-pleural SFT, especially in the retroperitoneum, while was observed in pleural SFT [51]. Accordingly, our case resulted not reactive for PAX8.…”

Bilateral renal involvement by solitary fibrous tumor – Report of a case in the post-WHO/2016 era

“…Retroperitoneal dedifferentiated liposarcomas should be in the differential of “fat containing” SFTs since they may show STAT6 positivity. 5 Sarcomatoid renal cell carcinoma should also be excluded, since most extrapleural SFTs express PAX8.…”

Retroperitoneal Malignant Solitary Fibrous Tumor: A Case Report and Review of the Literature

“…In our case, we found diffuse nuclear expression with PAX-8 (a member of the paired-box gene family expressed in renal, thyroid and Müllerian epithelial tumors) as an additional finding. Ulman, et al reported that SFTs often showed PAX-8 expression, and PAX-8 expression was observed more frequently at extrathoracal locations [23]. It has been suggested that this might cause some problems in the differential diagnosis of retroperitoneal malignant tumors with sarcomatoid differentiation and might lead to serious diagnostic complications in especially sarcomatous renal cell carcinomas [23,24].…”

“…Ulman, et al reported that SFTs often showed PAX-8 expression, and PAX-8 expression was observed more frequently at extrathoracal locations [23]. It has been suggested that this might cause some problems in the differential diagnosis of retroperitoneal malignant tumors with sarcomatoid differentiation and might lead to serious diagnostic complications in especially sarcomatous renal cell carcinomas [23,24]. Despite diffuse PAX-8 expression in our case, the negative results for cytokeratin, CD10 and RCC marker were excluded the possible diagnosis of sarcomatoid renal cell carcinoma.…”