Warranting investigation of primary lung adenocarcinoma in patients with an extrapulmonary malignancy and lung nodules due to high frequency

Objectives: To distinguish primary lung adenocarcinoma (PLA) from metastatic adenocarcinoma/malignancy to optimize therapy. Methods: We investigated the utility of thyroid transcription factor 1 (TTF-1) and napsin A in distinguishing PLA from metastatic adenocarcinoma/malignancy and assessed the frequency of PLA in patients with extrapulmonary malignancy/adenocarcinoma (PLA-EPM/EPA). Results: TTF-1 and napsin A identified PLA in PLA-EPM/ EPA with a sensitivity of 89.4% and 93.3% and a specificity of 93.9% and 94.7%, respectively. PLA was confirmed in 47.4% of PLA-EPM and 40.2% of PLA-EPA. Overall, 38.5% of patients with PLA had EPM. The common organs for PLA-EPA were breast (35.8%), colon (13.2%), and others, whereas the common organs resulting in pulmonary metastasis were the colon (32.8%), breast (28.1%), and others. A patient with a smoking history and without EPM had a higher chance of having PLA. Multiple nodules are not a reliable indication of metastatic adenocarcinoma. Conclusions: Our results firmly support the role of TTF-1 and napsin A in identifying PLA-EPM/EPA. We reason that all new lung nodules in patients with a history of EPM should be screened using these techniques due to the high frequency of PLA-EPM, which will affect treatment and prognosis of patients with EPM/EPA.