2.1. Patient’s Data Source

This study was performed in 12 institutes associated with Kyoto University and its Affiliated Hospitals—Head and Neck Oncology Group (Kyoto–HNOG) in Japan. Clinical data for patients with oral cancer treated between March 2010 and February 2017 were retrospectively extracted from medical charts. The patients’ data were anonymized and utilized for another previous research study examining the impact of lingual lymph node metastases on patients’ survival [3]. In this data source, one thousand and fifty-five patients were enrolled, and the location of the primary tumor was categorized into 7 subsites: tongue, floor of mouth, hard palate, upper gingiva and lower gingiva, oral lip, and buccal mucosa. Five cases that were not categorized using TNM classification 8th version were excluded from the study. Subsequently, patients with buccal primary tumors were separated, and the data were dichotomized into 2 groups: 92 buccal cancers (8.8%) and 958 other oral cancers (91.2%) (Figure 1). This study was approved by the institutional review board of each participating institution in collecting patients’ data and was led by the Kobe City Medical Center General Hospital Review Board (ethics code: Zn191105). Informed consent was waved owing to the retrospective nature of this study.

A flow chart of the inclusion and exclusion process of the patients.