Abstract
Quality of life (QoL) is an important outcome measure in clinical studies, but interpretation is hindered by incompleteness of data. We addressed this issue in a population-based cohort study of 146 patients with newly diagnosed rectal cancer. QoL was assessed by means of European Organization for the Research and Treatment of Cancer questionnaires at discharge from hospital after primary treatment and then every 3 months for 2 years. In parallel, objective clinical data were documented. Analyses were conducted in three steps: participants versus non-participants with QoL-assessment; poor compliers who filled in only one or two questionnaires (n=20) versus good compliers who filled in all or nearly all questionnaires (n=18); and the proportion of missing forms and critical (very poor) QoL scores in risk patients versus non-risk patients over the course of 2 years. Non-participants and poor compliers were older, were more likely to receive palliative (rather than curative) treatment, and had worse scores for physical status. Tumour progression and therapeutic interventions were more frequent in poor compliers than in good-compliers. Patients with risk factors (age 475 years, poor physical status, palliative treatment) were more likely to have missing questionnaires and critical QoL scores in respect of physical functioning and global quality of life over the course of 2 years. Missing values for QoL have clinical as well as methodological implications, because QoL scores can enhance a clinician's insight. Unwillingness to fill in a questionnaire is an indicator of serious illness. Studies that report sample statistics without specifying compliance rates and the characteristics of non-compliers will give a misleadingly positive picture.