Pulmonary Adenocarcinoma Mimicking Desquamative Interstitial Pneumonia: Report of 2 Cases With Genetic Analysis.

We report 2 cases of pulmonary adenocarcinoma mimicking desquamative interstitial pneumonia (DIP) with genetic analysis occurring in a 74-year-old woman and a 76-year-old woman. In both cases, the tumor was mainly composed of discohesive tumor cells, which filled and floated in the alveolar space in a DIP-like pattern. The tumor cells had abnormally large round to oval nuclei with fine chromatin and relatively abundant eosinophilic cytoplasm lacking pigmentation. Immunohistochemically, tumor cells in both cases were positive for CK7, TTF-1, napsin A, E-cadherin, β-catenin, and PD-L1 (one case had high expression and the other had low expression), and negative for CK5/6, CK20, p40, and ALK. However, the positive pattern of E-cadherin and β-catenin was incomplete on the circumference of the cell membrane in both cases and in one case, respectively. On genetic analysis, EGFR alteration (exon 21, L858R mutation) was observed in one case and ALK translocation was not observed in either. To the best of our knowledge, this is the first report of pulmonary adenocarcinoma mimicking DIP with genetic analysis.