Hidradenoma masquerading digital ganglion cyst: A rare phenomenon

Introduction: Mucous cyst is the commonest soft tissue tumor in the dorsum of the distal interphalangeal joint (DIPJ) of the finger. We report the first case of a recurring eccrine tumor (nodular hidradenoma), mimicking a mucous/ganglion cyst, on the dorsum of the DIPJ. Case report: A 54 year old man presented with painless, hemispherical, colored swelling on the dorsum of his right middle finger (dominant hand), which appeared to have recurred from a previous surgery. The lesion was excised and operative findings from the medical notes showed the gross appearance to be a soft, white, glistening, smooth-surfaced, myxoid nodule resembling a "ganglion cyst". Immunohistochemistry showed the tumour to be positive for S100, smooth muscle actin and cytokeratin 7. Ductal differentiation was confirmed by staining for epithelial membrane antigen and carcinoembryonic antigen. The histological features were that of atypical and solid cystic hidradenoma. Discussion: This is the first reported case of this rare tumour presenting as mucous cyst. We conduct a review of the literature of nodular hidradenomas, illustrating the immunohistologic findings in this tumour to emphasise the atypical features. We emphasise the importance of considering hidradenoma in the differential diagnosis of such lesions of the finger, in view of its high recurrence rate and the possibility of malignant transformation. (C) 2016 The Author(s). Published by Elsevier Ltd on behalf of IJS Publishing Group Ltd.