CD140b (PDGFRB) Expression in Atypical Fibroxanthoma/Pleomorphic Dermal Sarcoma and its Cutaneous Neoplastic Mimics

Atypical fibroxanthoma (AFX) and pleomorphic dermal sarcoma (PDS) are closely related tumors, believed to represent a continuum of the same neoplastic process, sharing numerous clinical features as well as morphologic, immunohistochemical, and genetic characteristics. Recently, whole-exome sequencing analysis revealed high-level PDGFRA/B gene expression and suggested PDGFRB immunohistochemistry as a "lineage specific" marker for PDS. To evaluate the potential diagnostic utility of PDGFRB status, we examined CD140b (PDGFRB) protein expression in a well-characterized cohort of AFX, PDS, and morphologic mimics. Formalin-fixed, paraffin-embedded sections from 18 AFX, 8 PDS, 26 squamous cell carcinomas (9 sarcomatoid, 11 poorly differentiated, and 6 moderately differentiated), 39 melanomas (including 10 desmoplastic and 6 spindle cell), 7 cutaneous leiomyosarcomas, and 2 cutaneous pleomorphic rhabdomyosarcomas were retrieved. CD140b expression was scored by extent (0 to 3+) and intensity (0 to 3) of staining. All AFX and PDS were diffusely positive (26/26). CD140b showed a variable extent of staining in 5/26 squamous cell carcinoma (5/9 sarcomatoid and 0/17 poorly/moderately differentiated SCC) and 16 of 39 melanomas (10/10 desmoplastic, 1/6 spindle cell, and 5/22 nondesmoplastic/spindled). Partial positivity was observed in 2/7 cutaneous leiomyosarcoma, while diffuse staining was present in both (2/2) cases of cutaneous pleomorphic rhabdomyosarcoma. In summary, CD140b is a sensitive but imperfectly specific marker for AFX and PDS. Several targeted drugs have shown efficacy in PDGFR-expressing tumors, and our findings further delineate therapeutic strategies for a selected group of poorly differentiated cutaneous neoplasms.