Background: We examined if cancer patients' health-related quality of life (HRQoL) scores on the European Organisation for Research and Treatment of Cancer (EORTC) QLQ-C30 are affected by the specific time point, before or during treatment, at which the questionnaire is completed, and whether this could bias the overall treatment comparison analyses.
Patients and methods: A 'completion-time window' variable was created on three closed EORTC randomised control trials in lung (non-small cell lung cancer, NSCLC) and colorectal cancer (CRC) to indicate when the QLQ-30 was completed relative to chemotherapy cycle dates, defined as 'before', 'on' and 'after'. HRQoL mean scores were calculated using a linear mixed model.
Results: Statistically significant differences (P<0.05) were observed on 6 and 5 scales for 'on' and 'after' comparisons in the NSCLC and two-group CRC trial, respectively. As for the three-group CRC trial, several statistical differences were observed in the 'before' to 'on' and the 'on' to 'after' comparisons. For all three trials, including the 'completion-time window' variable in the model resulted in a better fit, but no substantial changes in the treatment effects were noted.
Conclusions: We showed that considering the exact timing of completion within specified windows resulted in statistical and potentially clinically significant differences, but it did not alter the conclusions of treatment comparison in these studies.