*2.1. Study Population*

This was a retrospective study. Formalin-fixed paraffin-embedded (FFPE) tissue specimens stored at room temperature were obtained from 425 patients with pathologic stage I–IIIA NSCLC who had undergone anatomical lung resection with mediastinal lymph node dissection between November 1994 and April 2004 at Samsung Medical Center in Seoul, Korea. Patients with incomplete resection of lung tissue (e.g., positive malignant cell in resection margin) or history of neoadjuvant therapy were excluded from this study. Postoperative follow-up was performed according to a previously described protocol [25]. Information including recurrence, death, and platinum-based adjuvant chemotherapy was obtained from our hospital's electronic medical records (EMRs) and outside medical records as of 31 July 2018. Thirty-two (7.5%) patients received postoperative adjuvant chemotherapy comprising cisplatin combined with vinorelbine, vinblastine, etoposide, fluorouracil, gemcitabine, pemetrexed, or docetaxel. The chemotherapy regimens were selected by medical oncologists responsible for

treatment decisions. This study was approved by the Institutional Review Board of the Samsung Medical Center (2018-04-153), and pre-operative informed consent for the use of samples was obtained from all patients. Pathologic stage was determined according to the guideline of the 7th edition of the tumor-node-metastasis (TNM) staging system maintained by the American Joint Committee on Cancer [26]. Supporting data for this study are available from the corresponding author upon request.
