What is spread through air space?
Recently, aerogenous tumor spread was recognized as a prognostic factor in the pathologic diagnosis of lung adenocarcinoma. Onozato et al. (1) showed that a tumor island, which is defined as an isolated, large collection of tumor cells within alveolar spaces that lack a well-demarcated micropapillary configuration, increased the risk of recurrence in patients with stage I lung adenocarcinoma. Interestingly, the connection of a tumor island to the main tumor was demonstrated by three-dimensional reconstruction analysis. Kadota et al. (2) reported that tumor spread through air spaces (STAS), defined as the presence of tumor cells that goes beyond the edge of the main tumor to within at least the first alveolar layer in the lung parenchyma, was a significant risk factor for recurrence of small (≤2 cm) lung adenocarcinoma in patients who underwent limited resection. The 2015 World Health Organization classification (3) introduced STAS as a new route of invasion and included it in the exclusion criteria for minimally invasive adenocarcinoma. A study by Warth et al. (4) defined STAS as detachment of small solid cell nests (at least 5 tumor cells) away from the main tumor mass and classified it according to the distance from the main tumor mass as limited type (<3 alveolar layers) or extensive type (>3 alveolar layers). That study showed that the presence of STAS, regardless of extent, significantly reduced recurrence-free survival and overall survival of patients with resected adenocarcinoma of any stage.