Hidradenoma masquerading digital ganglion cyst: A rare phenomenon

• We describe the rare phenomenon of an atypical cystic hidradenoma mimicking in presentation a mucous ganglion cyst on the DIPJ of the finger.
• We put this rarity into context by performing a literature review of reported presentations of such tumours.
• We emphasise the importance of a thorough and systematic assessment in patients presenting with such lesions.
• Also highlighted is the importance of considering these tumours in the differential diagnosis of lesions of the DIPJ.
• Thorough immunohistochemistry of resulting biopsies is invaluable in aiding diagnosis in these scenarios of ambiguous soft tissue swellings.

IntroductionMucous 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 reportA 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.DiscussionThis 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.