Terminal respiratory unit type lung adenocarcinoma is associated with distinctive EGFR immunoreactivity and EGFR mutations

Approximately 10% to 20% of nonsmall cell lung cancer patients respond to epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors, such as gefitinib. Responders are mostly nonsmokers and women with tumors displaying bronchioloalveolar features. Mutations of the tyrosine kinase domain of the EGFR gene have been associated with a clinical response to gefitinib. A recent study reported that the terminal respiratory unit (TRU)-type adenocarcinoma shares the clinical profile and EGFR mutations of gefitinib responders. EGFR immunoreactivity in this context has not been reported in the literature. We performed a detailed immunohistochemical analysis of EGFR expression on 124 consecutive lung resection specimens for malignancy, to survey the EGFR immunoreactivity in lung cancers in general and to correlate EGFR immunoreactivity with EGFR mutations and TRU-type histology. EGFR positivity was seen most frequently in squamous cell carcinomas (77%), followed by TRU-type adenocarcinomas (63%), large cell carcinomas (23%), and non-TRU-type adenocarcinomas (12%). A distinctive basally oriented cytoplasmic positivity was observed exclusively in TRU-type adenocarcinomas. EGFR mutation was identified in 6 of 54 cases studied and all 6 cases were TRU-type adenocarcinomas. Five of six cases with EGFR mutation were positive for EGFR immunostain with the basal cytoplasmic localization. In conclusion, EGFR immunoreactivity with basal cytoplasmic pattern was exclusively seen in TRU-type adenocarcinoma and a subset of these cases was seen with EGFR mutations in the responders to EGFR inhibitor therapy.