Papillary apocrine metaplasia and columnar cell lesion with atypia: is there a shared common pathway?

Papillary apocrine metaplasia (PAM) and columnar cell lesion with atypia (CCL) are considered as candidates of early premalignant breast lesions. In this study, we investigated their relationship at the morphological level, as we noticed their histologic coexistence and proximity in daily routine. We selected 93 neoplastic and nonneoplastic cases with both PAM and CCL among 477 breast specimens and reevaluated sections by measuring the distance between these lesions. Ninety-three (19.4%) of 477 breast specimens contained both PAM and CCL; in 73.1% of the cases, the 2 lesions were in continuity with or adjacent to each other. Lesions less distant than 1 mm are grouped as "adjoining lesions"; and the rest, as "distant lesions." A significant difference (P = .006) was found between adjoining ("zero" + "<1 mm") and distant ("≥ 1 mm") lesions; that is, PAM and CCL were much closer in the neoplastic group, especially tumors with low grade, high estrogen and progesterone receptor expression, and less lymph node metastasis. During the detailed examination of adjoining lesions, some "hybrid cells" showing properties of both lesions were encountered. Papillary apocrine metaplasia and CCL tend to appear in close contact, especially in neoplastic breasts with favorable features. This result implies that PAM and CCL may be included in the same pathogenetic pathway despite their distinct immunophenotypical properties.