A Proposal for Definition of Minimally Invasive Adenocarcinoma of the Lung Regardless of Tumor Size

Background Based on the current World Health Organization classification, adenocarcinoma with a predominantly lepidic pattern larger than 3.0 cm in diameter is considered to be invasive adenocarcinoma, specifically lepidic adenocarcinoma, even if it shows pathologic invasion of 0.5 cm or less. However, we sometimes encounter lepidic adenocarcinomas larger than 3.0 cm with a good prognosis as well as minimally invasive adenocarcinoma. Methods Of 4,606 resected primary lung cancers at our institute between 1998 and 2011, this study focused on 18 lepidic adenocarcinomas with invasion of 0.5 cm or less. Clinicopathologic characteristics, including epidermal growth factor receptor mutation and anaplastic lymphoma kinase rearrangement status, and prognosis were evaluated. Results The median maximum pathologic tumor diameter was 3.4 cm (range, 3.1 to 4.7 cm) and the median maximum diameter of the invasive area within the tumor was 0.4 cm (range, 0 to 0.5 cm). No lymph node metastasis, vascular invasion, lymphatic permeation, or pleural invasion was pathologically confirmed in any of the 18 cases. Although epidermal growth factor receptor mutation was detected in 13 cases (72%), none of the patients showed anaplastic lymphoma kinase rearrangement. Recurrence was not observed in any of the 18 patients. The 5-year overall and cancer-specific survival rates were 94.4% and 100%, respectively. Conclusions Lepidic adenocarcinoma with invasion of 0.5 cm or less had a good prognosis. Adenocarcinoma with a predominantly lepidic pattern and invasion of 0.5 cm or less can be regarded as minimally invasive adenocarcinoma even if the tumor is larger than 3.0 cm in diameter.