Invasion of the inner and outer layers of the visceral pleura in pT1 size lung adenocarcinoma measuring ≤ 3 cm: correlation with malignant aggressiveness and prognosis.

Visceral pleural invasion (VPI) is defined as penetration by cancer cells of the elastic layer of the pleura. The purpose of this retrospective study was to compare the effect of invasion of the inner elastic layer of the pleura on survival to that of invasion of the outer elastic layer. One hundred twenty-four pT1 size lung adenocarcinomas were examined for visceral pleural invasion, which was classified into three types: no pleural invasion (NPI), invasion of the inner elastic layer only (IEL), and invasion of both inner and outer elastic layers (OEL). The relationship between the types of VPI and the prognosis was analyzed by univariate and multivariate analyses. Seventy-three (59 %) cancers showed NPI, while 51 cancers showed invasion of the pleura [(IEL) in 26 (21 %), OEL in 25 (20 %)]. The 5-year survival was 81, 60, and 37 % for patients with NPI, IEL, and OEL, respectively. Survival was poorest in patients with OEL (P < 0.01). Invasion of the outer elastic layer was also significantly associated with lymph node metastasis and frequent lymphatic involvement, micropapillary pattern, higher stromal invasion grade, and presence of small cluster invasion within tumors. Univariate analysis showed a significant relationship between invasion of the outer elastic layer and poor prognosis. However, multivariate analysis identified lymph node metastasis as the most significant predictor of poor prognosis. Analysis of invasion of the inner and outer visceral pleura is important; invasion of the outer elastic layer correlates with poor prognosis in pT1 size lung adenocarcinomas.