Hypersecretory Thyroid-like Adenosis of the Breast: A Distinct Benign Entity Devoid of Myoepithelial Cells

Abstract

Most infiltrative epithelial proliferations of the breast lacking a myoepithelial cell layer are malignant. The notable exception is Microglandular Adenosis (MGA). Lack of myoepithelial cells has also been noted occasionally in metaplastic apocrine glands. Here, we present a previously undescribed breast lesion that we have termed “Hypersecretory Thyroid-Like Adenosis” (HTLA). HTLA consists of multiple dilated glands of varying sizes preserving a lobulated architecture. The glands are lined by a single layer of bland cuboidal or columnar cells. Some nuclei have cleared chromatin, some exhibit grooves and others contain pseudo-inclusions reminiscent of papillary carcinoma of thyroid. Dense eosinophilic secretions, positive for PAS-Diastase and resembling thyroidal colloid, are present in many of the lumens. A thick basement membrane is present around each gland. There are no Myoepithelial (ME) cells present on H&E examination, immunohistochemistry or ultrastructural study. The immunohistochemical stain for S-100 protein shows weak focal nuclear and cytoplasmic staining of the minority of the luminal cells (<20% of cells). The lesion is ER negative. HTLA has some features similar to, but is overall distinctly different from MGA. In particular, the preserved lobular architecture, macroacinar structures, distinctive nuclear features, and lack of diffusely strong immunostaining for S-100 protein are features unlike MGA. This lesion was an incidental finding in a partial mastectomy performed for invasive ductal carcinoma but showed no spatial relationship, morphological or immunohistochemical similarities to the breast carcinoma.