Pathological findings of pulmonary papillary adenoma with EGFR mutation and literature review: two cases report.

Objective: Pulmonary papillary adenoma is an extremely rare benign tumor. It is derived from type II lung cells and club cells, suggesting that it may originate from stem cells with two-way differentiation. Only one case has been reported with FGFR2-IIIb overexpression.

Methods: Two cases of pulmonary papillary adenoma with available data on clinical features, histological morphology, immunophenotype and molecular characteristics were analyzed.

Results: Both tumors were well-circumscribed unencapsulated nodules composed of papillary structures with fibrovascular cores lined by a single layer of cuboidal or columnar epithelium without necrosis, nuclear atypia and mitoses, or invasion. But malignant transformation features include complex branching structures and significantly enlarged, irregular, and crowded malignant cells in one case. Immunohistochemistry showed that the tumor cells were strongly positive for TTF1, NapsinA, EMA and CK7 and negative for CEA and P63, with a low Ki-67 proliferation index. The EGFR somatic mutation exon19:c.2236_2256delinsATC (p.E746_S752delinsI) was found in one case by next-generation sequencing (NGS) technology.

Conclusion: Pulmonary papillary adenoma is very rare. Virtually all papillary adenomas are clinically silent and discovered incidentally. They are benign tumors, and resection is curative. An EGFR 19 exon deletion mutation in a patient with this tumor type was detected for the first time by NGS, and our results suggest that the malignant transformation of pulmonary papillary adenoma may be mediated by EGFR mutation.