Usefulness of SynCAM3 and cyclin D1 immunohistochemistry in distinguishing superficial CD34-positive fibroblastic tumor from its histological mimics

Expanding the Range of Epithelial Marker and SMA Staining of Superficial CD34 + Fibroblastic Tumors: A Case Report and Review of the Literature

ABSTRACT

Superficial CD34 + fibroblastic tumor (SCD34FT) is a relatively recently described borderline mesenchymal neoplasm. Owing to a relative lack of specificity in clinical presentation, radiopathologic findings, and immunohistochemical staining, the diagnoses of SCD34FT can be challenging. In this study, we present a case of a 55-year-old woman with an indolent painless nodule on the right shin. Histopathologic evaluation of the resected specimen showed a moderately cellular, subcutaneous lesion composed of spindled cells with mild pleomorphism arranged in sheets and fascicles. Immunohistochemical staining demonstrated diffuse positivity for CD34. Surprisingly, the tumor also showed diffuse expression of smooth muscle actin (SMA) and more than focal (ranging from subset to diffuse) expression of AE1/AE3 and CAM5.2. DNA and RNA next-generation sequencing revealed a t(X; 11)(q13; q24) MED12::PRDM10 fusion, confirming the diagnosis of SCD34FT. To the best of our knowledge, this is the first reported case highlighting SCD34FT with more extensive immunoreactivity to epithelial markers (AE1/AE3 and CAM5.2) and SMA compared with the focal staining reported in the existing literature. We hope that adding this case to the existing literature will raise awareness among pathologists to recognize the more variable staining pattern of epithelial markers and SMA when considering the diagnosis of SCD34FT.

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.

Superficial CD34-Positive Fibroblastic Tumor

Superficial CD34-positive fibroblastic tumor is a mesenchymal neoplasm of "intermediate malignancy" recently included in the fifth edition of the World Health Organization classification of soft tissue and bone tumors. In this review, we summarize the current knowledge on this rare entity with a special focus on its clinicopathological features, morphologic spectrum, and differential diagnosis. We also provide data regarding recent discoveries on its molecular profile and discuss its prognosis and management.