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Tubular Adenoma of the Skin with Follicular and Sebaceous Differentiation: A Report of Two Cases

The main controversies regarding tubular apocrine adenoma and papillary eccrine adenoma are whether they are two distinct entities or are the very same tumor, and if so, which lineage of differentiation (apocrine versus eccrine) it pursues. We report two cases of tubular adenoma with follicular and, in one case, additionally, sebaceous differentiation. The features in both cases indicated apocrine differentiation of the tubular component. One patient was a 60-year-old woman with a 1-year history of a solitary nodule on the scalp. The other patient was a 48-year-old woman with a solitary nodule of unknown duration located on the back. In both patients, the tumors were surgically removed. The patients were alive and well 2.5 and 2 years after surgery, respectively. The histologic features that both cases had in common included the combination of a tubular adenoma, foci of follicular differentiation, and areas of immature squamous metaplasia. In case 1, follicular differentiation was seen in the form of strands of basaloid cells surrounded by a stroma resembling the embryonic perifollicular sheath. Some aggregates of basaloid cells were juxtaposed with small collections of plump fibroblasts, imparting a resemblance to rudimentary follicular germs associated with follicular papillae. Many minute lumens surrounded by more eosinophilic cells were seen within the strands. In case 2, follicular differentiation was seen as several infundibulocystic structures surrounded by isthmic epithelium housing scattered mature sebocytes. In addition, there were areas reminiscent of desmoplastic trichoepithelioma (columnar trichoblastoma). In both cases, the areas with immature squamous metaplasia were represented by solid nodules that were mostly devoid of lumens and a peripheral basal/myoepithelial cell layer. In conclusion, these two cases of cutaneous tubular adenoma with accompanying follicular and sebaceous differentiation give further support to the proposition that the majority of these neoplasms have apocrine differentiation. Rare cases occurring in the sites normally devoid of apocrine glands may represent the eccrine counterpart.