Patient selection tree. Among the 1540 patients with lung adenocarcinoma who underwent surgical resection from 1995 to 2009, intraoperative frozen section was performed on tumors from 821 patients. Among these patients, 692 had stage I disease with tumor size ≤3 cm. The original frozen section slides were available for 379 patients. Among these patients, we excluded 17 cases of invasive mucinous adenocarcinoma and 1 case of colloid adenocarcinoma. The remaining 361 cases constituted the study cohort. FS, frozen section; MSKCC, Memorial Sloan-Kettering Cancer Center.