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Letter to the editor: Digital intraosseous epidermoid inclusion cyst of the distal phalanx

Dear Editor, Epidermal inclusion cyst is a benign cyst that is usually found in skin and occurs most frequently on face, scalp, neck, and trunk. Epidermoid cysts result from the proliferation of epidermal cells within a circumscribed space of the dermis. Intraosseous epidermoid inclusion cyst is an exceedingly rare entity. Herein we present a case of this rare pathological entity in the distal phalanx of the index finger. A 24-year-old male patient was referred to the cytology clinic of Postgraduate Institute of Medical Education and Research, Chandigarh, India with a firm painful swelling in the left index finger for last 6 weeks. Radiology of the lesion showed a well-defined lytic lesion in the terminal phalanx (Fig. 1) which was suggested as giant cell tumor of bone. Fine-needle aspiration cytology (FNAC) was performed using 21-gauge needle, pistol handle, and 20 mL syringe. We aspirated thick pultaceous material and four smears were made. Both May–Grunwald– Giemsa and hematoxylin and eosin (H & E) staining were done. Cytology smears showed dispersed population of anucleated squames (Figs. 2 and 3). No other epithelial cells or giant cells, or necrosis were seen in the smears. The case was diagnosed as epidermal inclusion cyst. Subsequently, we received the intralesional curetting sample from the distal phalanx of this patient and histopathology showed a cyst wall lined by stratified squamous lining with presence of laminated keratin that confirmed the FNAC diagnosis. Epidermal inclusion cyst (EIC) of the bone is a rare entity. It occurs most frequently in the distal phalanges of the hand and also in the skull. Though the etiology of intraosseous epidermal inclusion cysts remains unknown yet few hypotheses have been put forwarded. Congenital etiology with intraosseous inclusion of embryonal epithelial tissue has been proposed. A post-traumatic origin remains the most prevalent hypothesis by some literatures. Possibly the superficial epithelial cells are displaced into the underlying osseous compartment following digital trauma or surgery. EIC is most commonly seen in the distal phalanges and this phenomenon is secondary to the unique regional anatomy as the subungual bed adheres to the underlying periosteum. However in our case, the patient did not give any previous history of trauma. Rarely well differentiated squamous cell carcinoma is very difficult to differentiate from epidermal inclusion cyst on FNAC alone. A thorough histopathological