Background: Health-related quality of life (HRQoL) has been positioned as one of the major endpoints in oncology. Thus, there is a need to validate cancer-site specific survey instruments. This study aimed to perform a transcultural adaptation of the 50-item Expanded Prostate cancer Index Composite (EPIC) questionnaire for HRQoL in prostate cancer patients and to validate the psychometric properties of the French-language version.
Methods: The EPIC questionnaire measures urinary, bowel, sexual and hormonal domains. The first step, corresponding to transcultural adaptation of the original English version of the EPIC was performed according to the back translation technique. The second step, comprising the validation of the psychometric properties of the EPIC questionnaire, was performed in patients under treatment for localized prostate cancer (treatment group) and in patients cured of prostate cancer (cured group). The EORTC QLQ-C30 and QLQ-PR25 prostate cancer module were also completed by patients to assess criterion validity. Two assessments were performed, i.e., before and at the end of treatment for the Treatment group, to assess sensitivity to change; and at 2 weeks' interval in the Cured group to assess test-retest reliability. Psychometric properties were explored according to classical test theory.
Results: The first step showed overall good acceptability and understanding of the questionnaire. In the second step, 215 patients were included from January 2012 to June 2014: 125 in the Treatment group, and 90 in the Cured group. All domains exhibited good internal consistency, except the bowel domain (Cronbach's α = 0.61). No floor effect was observed. Test-retest reliability assessed in the cured group was acceptable, expect for bowel function (intraclass coefficient = 0.68). Criterion validity was good for each domain and subscale. Construct validity was not demonstrated for the hormonal and bowel domains. Sensitivity to change was exhibited for 5/8 subscales and 2/4 summary scores for patients who experienced toxicities during treatment.
Conclusions: The French EPIC questionnaire seems to have adequate psychometric properties, comparable to those exhibited by the original English-language version, except for the construct validity, which was not available in original version.
Keywords: Classical test theory; Cross-cultural adaptation; EPIC questionnaire; Health-related quality of life; Prostate cancer; Psychometric properties; Validation.