An epidermal cyst is the most common type of cyst to occur in subcutaneous tissue. When its size is greater than 5 cm, it is recognised as a giant epidermal cyst. A subcutaneous giant epidermal cyst with intramuscular extension is extremely rare. The authors report a case of a 74-year-old man who presented with a painless, slow-growing left gluteal mass of 6-month duration. Examination revealed a large left gluteal mass that was fixed to the underlying structures. A small epidermal cyst with visible punctum was noted at the medial aspect of the mass. MRI demonstrated a large, lobulated left gluteal lesion measuring 20 cm×16 cm×10 cm. The lesion was partly within the gluteal maximus muscle and partly within the subcutaneous tissue. MRI and ultrasound features of the lesion were consistent with a giant epidermal cyst with intramuscular extension. The lesion was excised and histology confirmed the diagnosis.
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