Giant fibrous hamartoma of infancy: pitfall of CD34 positive dermal mesenchymal tumor.

Fibrous hamartoma of infancy (FHI) is a rare benign soft tissue tumor with a triphasic organoid histologic appearance. The authors present a case of a 21-month-old healthy girl with a slowly growing flesh-colored subcutaneous plaque 12cm in size on the lower back, with overlying hypertrichosis. A punch biopsy revealed a proliferation of spindle cells infiltrating the dermis and hypodermis organized in a dense storiform pattern with a strong diffuse positivity for CD34. The diagnosis of congenital dermatofibrosarcoma protuberans (DFSP) was considered and an excision was performed. Histopathologic analysis showed an extensive poorly demarcated mass infiltrating the dermis and hypodermis, composed of different components: a monomorphous fibroblastic/myofibroblastic component, a mature adipose component, and an immature mesenchymal basophilic component. The clinical aspects with the histologic and immunohistochemical features led to the diagnosis of giant fibrous hamartoma of infancy. In our case the strong diffuse CD34 positivity was a diagnostic pitfall leading to an incorrect hypothesis of congenital DFSP. The characteristic triphasic histology of FHI was missed owing to the small size of the punch biopsy. This article highlights the importance of being aware of the CD34+ dermal mesenchymal tumor differential diagnosis and the necessity of appropriate size biopsies to avoid sampling error.