Objective
In paediatric oncology, little is known about trajectories of illness perceptions and their longitudinal associations with health-related quality of life (HRQoL). Therefore, the aim of this study was to investigate changes in illness perceptions in children and parents over a one-year-period and to investigate predictive value of child's and parent's illness perceptions during acute treatment for child's HRQoL 1 year later.

Methods
N = 65 child-parent-dyads participated in a longitudinal study (retention rate: 80.2%). Children were 4–18 years of age and underwent acute cancer treatment at baseline. Children and parents reported on their own illness perceptions (Illness-Perception-Questionnaire-Revised), as well as on the child's HRQoL (KINDL-R) at baseline and one-year-follow-up. Paired-samples t-tests were calculated to investigate changes over time. A hierarchical multiple regression analysis was performed to investigate predictive value of child's and parent's illness perceptions for child's HRQoL.

Results
Child's HRQoL t(63) = −6.73, p < 0.001, their perceptions of coherence (i.e. understanding; t(54) = −2.36, p = 0.022) and consequences of their illness (t(54) = 2.86, p = 0.006), and parent's perception of cyclical trajectory (t(61) = 2.06, p = 0.044) improved from baseline to 1-year-follow-up. All other illness perceptions remained stable. Exploratory post-hoc analyses showed differences in the pattern of change in age-, gender-, and diagnosis-specific subgroups. After controlling for baseline levels of HRQoL, child's perceptions of symptoms and consequences were independent predictors of their HRQoL 1 year later (R2 = 0.396, F(2,52) = 10.782, p < 0.001), whereas no parent's illness perceptions added predictive value.

Conclusion
In paediatrics, child's and parent's illness perceptions should be assessed. Our findings highlight the importance of illness perceptions as potential modifiable variables in interventions to improve child's HRQoL.