Posttraumatic lobular squamous metaplasia of breast. An unusual pseudocarcinomatous metaplasia resembling squamous (necrotizing) sialometaplasia of the salivary gland.

Squamous metaplasia arising in nonneoplastic breast parenchyma is reportedly rare. We present a unique case which occurred following severe blunt trauma to the right breast of a 59-yr-old woman. The lesion contained sheets of squamous cells with a lobular growth pattern, bland cytology with few mitoses, and keratin and keratohyalin granules. It bore a striking resemblance to squamous (necrotizing) sialometaplasia of the salivary gland in that it exhibited lobular, pseudocarcinomatous growth. The patient has remained free of disease 49 mo after segmental resection of the lesion. Four previous cases of squamous metaplasia of the female breast have been reported, though none presented with a history of trauma or previous surgical manipulation. It is important to differentiate this entity from pure squamous cell carcinoma, and metaplastic change in ductal breast carcinoma, fibroadenoma, and other lesions. Breast aspiration biopsies revealing squamous cells cannot exclude carcinoma; thus, caution must be exercised.