A case of giant cell‐rich solitary fibrous tumor in the external auditory canal

We present a rare case of giant cell‐rich solitary fibrous tumor (SFT) arising at the left external auditory canal in a 31‐year‐old woman. The tumor was well‐circumscribed and composed of spindle‐shaped cells with abundant collagenous bands. Scattered multinucleate giant cells were observed, some of which lined pseudovascular spaces. Although a focal mild‐hypercellular area was observed, mitoses were rare and necrosis was absent. Interstitial mast cells were scattered, especially in the hypercellular area. Immunohistochemically, CD34, vimentin, and Bcl‐2 presented diffuse positivity. Moreover, both mononuclear spindle cells and multinucleate cells showed nuclear STAT6 positivity, while NAB2‐STAT6 fusion gene could not be detected by reverse transcription polymerase chain reaction using formalin‐fixed specimen. These findings suggest the pathological diagnosis of giant cell‐rich SFT, previously known as giant cell angiofibroma, which is a rare variant of SFT with multinucleate giant cells and occurs predominantly in orbital region. Although giant cell‐rich SFTs of extra‐orbital sites have been reported, to our knowledge, this is the first case arising in the external auditory canal. Giant cell‐rich SFT should be considered as a differential diagnosis of spindle cell lesion with multinucleate giant cells, and STAT6 immunohistochemistry should be performed to distinguish this rare tumor from other mesenchymal neoplasms.