COMT and major depressive disorder: Simple regression analysis was performed between pain intensity numerical rating scale (NRS) (day 8) as the dependent variable, expectation of pain decrease NRS (day 1), tumor types, and the following covariates as independent variables: patients’ characteristics such as age, gender, PS (day 1), genotype of catechol-O-methyltransferase, total scores of Hospital Anxiety and Depression Scale (day 1), and pain intensity NRS (day 1).