Micropapillary histological subtype in lung adenocarcinoma of 2 cm or less: impact on recurrence and clinical predictors

Objective: This study examined the clinical and radiological characteristics of adenocarcinoma having the micropapillary histological subtype. Methods: We included 233 patients who were operated from 2001 to 2012 for lung adenocarcinoma of 2 cm or less. The pathology was reviewed according to the 2015 WHO classification. We defined adenocarcinoma with a micropapillary component as adenocarcinoma in which the area of the micropapillary histological subtype exceeded 5% of the tumor. The difference in cumulative incidence of recurrence (CIR) in the presence of death as a competing risk between two groups was assessed using the methods of Gray. Results: Twenty-one cases (9.0%) had a micropapillary component. The micropapillary component was associated with a higher frequency of lymphatic invasion (28.6 vs. 7.5% in adenocarcinoma without a micropapillary component; P = 0.008) and vascular invasion (38.1 vs. 15.1%, P = 0.014) and lymph node metastasis (31.3 vs. 5.2%, P = 0.003). The median follow-up period was 6.5 years. CIR at 5 years was 23.8% [95% confidence interval (CI), 8.3–43.7%] for adenocarcinoma with a micropapillary component, and 11.4% (95% CI, 7.4–16.2%) for adenocarcinoma without a micropapillary component (P = 0.033). Adenocarcinoma with a micropapillary component was more frequent in solid nodules (17.8%, 16/90) on high-resolution computed tomography (HRCT) than in either ground-glass nodules (1.5%, 1/67) or part-solid nodules (5.3%, 4/76) (P = 0.001). The HRCT finding was the only preoperative factor that was associated with a micropapillary component in the multivariate analysis. Conclusions: The micropapillary component in adenocarcinoma should be regarded as indicative of a high-grade malignancy and was associated with the HRCT finding.