26 author (CS) was consulted in case of doubt. The ideal score for non-comparative studies is 16 and 24 for comparative studies. Synthesis of results and additional analyses The ability of MINORS to differentiate between poor or excellent quality studies has not been validated.22 Thus, rating the methodological quality as ‘poor’ or ‘excellent’ based on MINOR scores cannot be done. However, the scores can be displayed as a fraction of the ideal score and corresponding percentage. Results The search of PubMed (n=336), Embase (n=474) and Cochrane (n=53) provided a total of 863 records. After duplicate deletion, 575 unique records remained. After title and abstract were screened and consensus was reached between authors on all records, 464 studies were discarded. Of the 111 records that were read full text, a total of 27 studies were identified for inclusion in the review. No meta-analysis was performed in this study. An overview of the study selection process is shown in Figure 1. Study characteristics The studies selected for this review were 20 cross-sectional studies and 7 longitudinal studies, of which 11 were prospective, 10 were retrospective and 6 did not report prospective or retrospective data collection. The included studies involved a total of 1849 oral cancer patients of which at least 1308 were male, as one study did not report sex of patients.23 Eleven studies only reported mean or median age with SD and did not mention range.24-34 One study did not mention age of patients,23 and 3 studies categorized patients in age groups without further details about mean and SD.31,34,35 Studies were mainly conducted in China,28,33-44 followed by India,24,45-47 the United Kingdom,25,27,31,48 Brazil,26,32 the United States,30,49 Germany,29 and Pakistan.23 Tumor sites included: tongue, buccal mucosa, gingiva, floor of mouth (FOM), palate, retromolar region, lip, alveolar process, and gum. Tumor stage was reported in all studies, except one.48 All studies included patients with curative intended treatment, as mentioned in the inclusion criteria. However, the type of primary treatment differed: besides primary surgery (tumor resection), a (selective/functional) neck dissection was performed in some studies.24,25,32,33,35-37,39,42,45,48,49 Most surgical treatments included reconstruction.26-31,33-35,3740,42-48 Only two studies did not apply or mention adjuvant therapy.42,49 One study did not report any treatment details.23