157 Elastin in pulmonary pathology Pre-existing disease Adenocarcinoma may arise as a secondary disease on top of other pre-existing diseases, such as emphysema, so there may be an already modified tissue architecture. When the pattern of remodeling in the adenocarcinoma component is similar to that in the adjacent non-malignant lung with pre-existing disease, remodeling is probably due to the pre-existing disease and not to adenocarcinoma. Adenocarcinoma, including mimickers of papillary adenocarcinoma Iatrogenic collapse in a lung with non-mucinous AIS may mimic a papillary and/or acinar pattern as defined according to the WHO classification4. In this differential diagnosis, the presence of elastin shows that these structures represent pre-existing collapsed alveolar walls, and should be interpreted as part of the pre-existing peripheral pulmonary framework90, supporting the diagnosis of AIS or, if invasion is present elsewhere, a lepidic pattern of adenocarcinoma. At the center of the controversy is the fact that true papillary cores include endothelial and fibroblastic cells with extracellular matrix, but elastic fibers are not structural components in papillary adenocarcinoma, unless there is pre-existing elastosis (Table 1). An example of a true papillary carcinoma of the lung is shown in Figure 4. The papillary carcinoma with partly intrabronchial location was chosen, so that it could not be confused with alveolar collapse. AIS is influenced by iatrogenic collapse, and has a simple pattern with an epithelial monolayer and, frequently, thin alveolar stromal cores (Figure 5). 12
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