Bilateral Subareolar Epidermal Inclusion Cysts of the Breast After Prior Surgery for Mastitis: Differing Sonographic Appearances of Ruptured and Unruptured Cysts

Background and Clinical Significance: An epidermal inclusion cyst (EIC) of the breast is an uncommon benign lesion, and bilateral subareolar involvement is exceptionally rare. Because a ruptured EIC can mimic malignancy or a primary infection, recognition of its imaging spectrum is clinically important. Case Presentation: A woman in her 40s presented with bilateral palpable subareolar masses and bloody, purulent nipple discharge, which had developed gradually over several months, ten years after bilateral surgery for mastitis. Mammography showed a right subareolar focal asymmetry with nipple–areolar thickening and a left circumscribed hyperdense mass. Sonography demonstrated a right mass with a preserved lamellated “onion-ring” pattern that proved unruptured at excision, and a left heterogeneously hypoechoic mass with small cystic changes that proved ruptured, with microabscess and hemorrhage; both masses were avascular on color Doppler, and the left mass showed focally high stiffness (peak 214.7 kPa) on shear-wave elastography. Surgical excision confirmed bilateral EIC. Conclusions: The coexistence of ruptured and unruptured cysts in one patient illustrates their differing sonographic appearances. Recognizing the onion-ring sign and integrating multiparametric ultrasound help distinguish benign EICs from malignancy or infection, while underscoring the need for histologic confirmation.