Phenotypic Drift as a Cause for Intratumoral Morphological Heterogeneity of Invasive Ductal Breast Carcinoma Not Otherwise Specified

Invasive ductal carcinoma (IDC) not otherwise specified (NOS), the most common type of breast cancer, demonstrates great intratumoral morphological heterogeneity, which encompasses the presence of different types of morphological structures—tubular, trabecular, solid, and alveolar structures and discrete groups of tumor cells, the origins of which remain unclear at present. In this study of 162 IDC NOS patients, we investigated whether the distribution of different types of morphological structures is related to the basic clinicopathological parameters of IDC NOS. Our results showed that in patients with only one type of tumor structure, the presence of any one of the five types was equally probable; however, cases with two types of structures were more likely to contain trabecular structures than the other four types. The development of intratumoral morphological heterogeneity was not associated with menopausal status, tumor size, histological grade, hematogenic metastasis, or recurrence. However, the number of different types of morphological structures was significantly higher in luminal tumors than in triple-negative tumors. An increase in the frequency of lymph node metastasis correlated with the increased number of different types of structures in breast tumors; however, in contrast to premenopausal patients, this association was explained by the presence of alveolar structures in postmenopausal women. In addition, we showed a significant decrease in the numbers of positive lymph nodes in tumors with high numbers of morphological variants. The frequency of lymph node metastases and the number of positive nodes were generally independent features and formed by different mechanisms. Based on the evidence, the term “phenotypic drift” has been designated as the basis for the development of intratumoral morphological heterogeneity of IDC NOS.