36 Another focus of attention is the survey follow-up time. Twenty of the 27 studies had patients complete the questionnaire only once, making it impossible to evaluate changes in chewing capacity.23,26,28,30,32-40,42,45-49,53 Three of these studies had an assessment range of one year or less.23,28,33 Patients were asked to complete the questionnaire at baseline in five studies.24,27,29,39,41 However, one study failed to report the data at baseline39 and another study did not report follow-up data in a table.27 Rogers et al. included 230 patients, however, only 111 of them filled in the questionnaires 2 and 10 years after treatment.31 As stated in the method section, our literature search for this review was limited by publication language and eligibility criteria, and therefore selection bias might have occurred. Moreover, reporting bias may have occurred as statistically significant studies in general have a higher likelihood of publication. Future research There is a need for standardized methodology across studies, enabling comparison of data. For review and comparison purposes, criteria should be narrowed down and limited to a specific type of (oral) cancer. Ideally, HR-QoL questionnaires should be an integrated part of cancer treatment, because they are a non-invasive way to obtain information about the effect of treatment on patients’ HR-QoL. In addition, a baseline assessment followed by multiple assessments over time is favored to avoid misinterpretation of HR-QoL by a single outcome measure, and to be able to identify changes in HR-QoL over time. Finally, we recommend a combination of (a) HR-QoL questionnaire(s) with an objective measurement of chewing function in patients treated for oral cancer in future research. This can contribute to a better understanding of differences between objective findings and the patients’ subjective perception. Conclusion The results of this review provide insight in the available literature regarding HR-QoL in patients treated for oral cancer with an emphasis on masticatory ability after primary curative treatment. Currently, there is a lack of comparable HR-QoL studies regarding mastication in oral cancer patients. This prevents identifying possible relations between oral cancer treatment, masticatory ability and QoL. Our findings underline the flaws in the available literature and highlight the necessity for improvement in future HR-QoL research.
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