A total of 514 hits were retrieved. After deleting duplicates (n = 213), and irrelevant papers (n = 272), there were 29 studies eligible with a total of 362,766 patients with HNC to be included, as shown in Fig. 1 [9, 14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41].

The studies showed high-quality assessment scores (Table 2). Ten (34.5%) of them had been carried out in Europe [14, 16, 19, 22,23,24, 26, 28, 32, 36], 9 (31.0%) in the United States [17, 18, 21, 27, 29, 30, 33,34,35], 8 (27.6%) in Asia [15, 20, 25, 31, 38,39,40,41], and 2 (6.9%) were international multicenter studies [9, 37]. From the included studies, 13 (44.8%) reported that the patients included in their studies received surgery, radiotherapy, chemoradiotherapy, and surgery + radiotherapy and/or chemoradiotherapy [9, 16, 20, 21, 24, 25, 28, 29, 32, 33, 36, 37, 40], 4 (13.8%) studies reported that the included patients received either radiotherapy or chemoradiotherapy [15, 22, 31, 39], 1 (3.4%) highlighted that the patients received either surgery alone or radiotherapy or chemotherapy, 1 (3.4%) study reported that the patient received only radiotherapy [14], 2 (6.9%) studies mentioned that the included patients received surgery only [17, 18], 3 (10.3%) studies highlighted that the included patients in their studies received chemoradiotherapy alone [26, 27, 34], and 5 (17.2%) studies mentioned the treatment modalities of the patients [19, 30, 35, 38, 41]. Several assessment scales were used to measure the psychological factors as predictors of treatment outcomes. These included Distress Thermometer, European Organization for Research and Treatment of Cancer Quality-of-Life-Questionnaire-C30, European Organisation for Research and Treatment of Cancer Quality-of-Life Questionnaire-Head and Neck 35, Mental Adjustment to Cancer Scale, University of Washington-QoL, MD Anderson Symptom Inventory-Head and Neck Module, Patient Concerns Inventory, Financial Index of Toxicity, National Health Interview survey, Hospital Anxiety and Depression Scale, Eating Assessment Tool, Generalized Anxiety Disorder–7, Patient Health Questionnaire–8, and Suffering Inventory.

The diagnosis of patients with HNC have been found to have various psychological consequences such as distress (Section "Psychological Consequences of HNC Diagnosis") [9, 20, 26, 32, 33, 36, 39, 41]. These psychological factors have been found to correlate with poorer treatment outcomes such as the patients’ well-being [16, 18], quality of life [9, 19, 20, 23, 24, 26, 28, 31, 32, 35, 36, 38, 40, 41], and health-related quality of life [14, 15,