[Non-small cell lung carcinoma with co-expression of TTF1 and p40: a clinicopathological analysis of six cases].

<b>Objective:</b> To investigate the clinicopathological features, molecular pathology characteristics, and prognosis of non-small cell lung carcinoma (NSCLC) exhibiting co-expression of p40 and thyroid transcription factor1 (TTF1). <b>Methods:</b> Clinical and pathological data of six NSCLC cases with co-expression of p40 and TTF1 diagnosed at the First People's Hospital of Xiaoshan District, Hangzhou, China from January 2016 to December 2023 were collected. Relevant literature was also reviewed. <b>Results:</b> NSCLC with co-expression of p40 and TTF1 commonly occurred in male smokers and had been in stage Ⅲ-Ⅳ when diagnosis. Microscopic examination revealed that the tumor cells were arranged in solid nests and sheets with marked atypia and visible mitotic figures. There was no prominent evidence of keratinization or glandular formation. The tumor cells diffusely co-expressed p40 and TTF1, exhibiting a dual immunophenotype characteristic of both squamous cell carcinoma and adenocarcinoma. Molecular testing of four NSCLC co-expressing p40 and TTF1 revealed the presence of common EGFR mutations, as well as mutations of NRAS (mutation rate of 2.09%), EML4-ALK (mutation rate of 24.77%), and PIK3CA (exon 10 c.1658 G>C p.S553T, mutation rate of 4.32%). All six tumors were poorly differentiated, highly invasive, and associated with poor prognosis. Four of the six patients experienced widespread metastasis and died within 7 to 30 months after the diagnosis or initial treatment. <b>Conclusions:</b> NSCLC with co-expression of p40 and TTF1 exhibits distinct clinicopathological features, immunophenotypes, molecular alterations, and clinical outcomes, characterized by rapid progression and poor prognosis. Pathologists should be vigilant in recognizing this entity to avoid misdiagnosis and missed diagnosis.