The concept of bronchioloalveolar cell adenocarcinoma: redefinition, a critique of the 1999 WHO classification, and an ultrastructural analysis of 155 cases.

Bronchioloalveolar cell adenocarcinoma (BACA) is bronchioloalveolar because (1) it arises in bronchioles and alveoli and (2) differentiates into bronchiolar and alveolar cells. Every entity possesses unique characteristics that separate it from other entities. The unique characteristic of BACA is its cell type. Lepidic growth is a clue to the cell type and, even though present in the vast majority, is not unique or absolutely essential. Because of the algebraic nature of concepts, the degree of differentiation, the extent of lepidic growth, and the degree of stromal desmoplasia cannot be used as definitional requirements. Likewise, in malignant tumors, absence of stromal invasion cannot be required. An epistemologically valid definition of BACA is proposed and a study of 155 cases defined this way and examined ultrastructurally is presented.