Objective: The aim of this study was to apply the classical test theory (CTT) and the modern test theory (MTT) to evaluate the psychometric characteristics of the European Organization for Research and Treatment of Cancer 30-item core quality of life questionnaire (EORTC QLQ C-30) and QLQ-prostate specific 25-item (EORTC QLQ-PR25) module in prostate cancer patients.
Materials and Methods: There were 91 prostate cancer patients included in this study. We analyzed the characteristics of each item of the EORTC QLQ-C30 and EORTC QLQ-PR25 by CTT and IRT (item response theory). The infit statistics of Rasch model was used to examine the unidimensionality of each domain, and also evaluate the item difficulty, the range of item threshold, item targeting and the reliability of the two questionnaires. Furthermore, patients were measured before therapy and at 1, 3 and 6 months after the start of therapy and then compared each health-related quality of life (HRQoL) score of the post-therapy groups with the baseline scores. Finally, the item psychometric properties were examined for different groups stratified by age and disease severity.
Results: Most of the items within each domain fitted the assumption of unidimensionality. The range of item thresholds for EORTC QLQ-C30 and EORTC QLQ-PR25 could not cover the range of the patients’ ability. The mean difficulties of functional domain of EORTC QLQ-PR25 were more difficult and the other domains were too simple for the prostate cancer patients. The HRQoL scores of EORTC QLQ-C30 and EORTC QLQ-PR25 could not discriminate patients from age and disease severity. The chronic pain in patients markedly reduced the global health-related quality of life. After radical prostatectomy for prostate cancer, prostate-specific antigen (PSA) of patients would return to the baseline but their HRQoL scores were rapidly reduced in most of the domains of EORTC QLQ-C30 and EORTC QLQ-PR25. Urinary and sexual functioning would not return to the baseline until 6 months after the therapy. While radiotherapy was used, patients were affected only in their sexual functioning domain and treatment-related symptom domain and it took just 3-6 months to return to baseline HRQoL.
Conclusions: In general, both questionnaires showed a lower reliability in each domain and the mean difficulties were too simple for the prostate cancer patients. EORTC QLQ-PR25 has better discriminant ability on clinical indices than EORTC QLQ-C30. The symptom domain showed better discriminant validity. The distributions of item difficulty of EORTC QLQ-C30 and EORTC QLQ-PR25 and person ability can be used as the bases of developing short-form computer-adapted testing. The results can also be used by clinicians in daily clinical practice to determine items that can predict the impairment of quality of life related to function and symptom for prostate cancer patients.
