Breast enlargement in prepubertal boys is a rare condition. This case report describes an otherwise healthy 3-year old non-obese boy who developed a large unilateral cystic breast mass measuring approximately 9 × 6 × 4 cm. The mass was initially treated as a lymphatic malformation, and sclerotherapy with Picibanil (OK-432) was attempted without any detectable effect on size. The mass was later excised. The pathological examination revealed mammary gland tissue suggestive of idiopathic gynecomastia. FISH revealed 47, XXY mosaicism in the abnormal breast epithelial cells, but not in peripheral blood lymphocytes.
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