Minimally important differences for interpreting European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire Core 30 scores in patients with head and neck cancer.

Background: We aimed to estimate minimally important difference (MID) for interpreting group-level change over time for European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire core 30 (EORTC QLQ-C30) scores in head and neck cancer.
Methods: Data were derived retrospectively from two published EORTC trials. Clinical anchors were selected using correlation strength and clinical plausibility of the given anchor/QLQ-C30 scale pair. MIDs for within-group and between-group change were estimated via the mean change method and linear regression, respectively. Distribution-based MIDs were also examined. MIDs for two of the scales, dyspnea and nausea/vomiting, are more uncertain considering their low correlations with the anchors.
Results: Anchor-based MIDs could be determined for deterioration in 7 of the 14 QLQ-C30 scales assessed, and in 3 scales for improvement. MIDs varied by scale, direction of change, and anchor. Absolute MID values ranged from 5 to 15 points for within-group change and 4 to 12 for between-group change. Most MIDs were within 4 to 10 points.
Conclusions: Our findings, if confirmed, will aid interpreting changes in selected QLQ-C30 scale scores over time and inform sample size calculations in future clinical trials in head and neck cancer.
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