Clinicopathologic and molecular study of superficial CD34‐positive fibroblastic tumours mimicking atypical fibrous histiocytoma (dermatofibroma)

Aims Superficial CD34‐positive fibroblastic tumour (SCD34FT) is an uncommon but distinctive low‐grade neoplasm of the skin and subcutis that shows frequent CADM3 expression by immunohistochemistry (IHC). In this study, prompted by an index case resembling ‘atypical fibrous histiocytoma (FH)’ that was positive for CADM3 IHC, we systematically examined a cohort of tumours previously diagnosed as ‘atypical FH’ by applying CADM3 and fluorescence in situ hybridization (FISH) for PRDM10 rearrangement, to investigate the overlap between these tumour types. Methods and Results Forty cases of atypical FH were retrieved, including CD34‐positive tumours ( n = 20) and CD34‐negative tumours ( n = 20). All tumours were stained for CADM3. All CADM3‐positive tumours were evaluated by FISH to assess for PRDM10 rearrangement. Eleven CD34‐positive tumours (11/20, 55%) coexpressed CADM3 and were reclassified as SCD34FT. None (0/20) of the CD34‐negative atypical FH were CADM3‐positive. Reclassified SCD34FT (10/11) arose on the lower extremity, with frequent involvement of the thigh ( n = 8). Features suggestive of atypical FH were observed in many reclassified cases including variable cellularity, spindled morphology, infiltrative tumour margins, collagen entrapment, epidermal hyperpigmentation, and acanthosis. Variably prominent multinucleate giant cells, including Touton‐like forms, were also present. An informative FISH result was obtained in 10/11 reclassified tumours, with 60% (6/10) demonstrating PRDM10 rearrangement. Conclusion A significant subset of tumours that histologically resemble atypical FH, and are positive for CD34, coexpress CADM3 and harbour PRDM10 rearrangement, supporting their reclassification as SCD34FT. Awareness of this morphologic overlap and the application of CADM3 IHC can aid the distinction between SCD34FT and atypical FH.