Abstract
BACKGROUND: The EORTC Quality of Life Group recently developed the new version of its breast-cancer-specific quality of life questionnaire, the EORTC QLQ-BR42. This questionnaire updates EORTC QLQ-BR23, which was published in 1996. Since then, significant changes have occurred in breast cancer treatment. The aims of this study were to evaluate the psychometric properties of the QLQ-BR42 in Spanish breast cancer patients and compare the results with those of the EORTC international validation study. METHODS: A retrospective analysis of QLQ-BR42 data from three Spanish breast cancer patient cohorts was conducted to evaluate the questionnaire’s psychometric properties, including scale structure, internal consistency, test-retest reliability, validity (convergent, discriminant, known-groups), and responsiveness to change. The three cohorts comprised, respectively, patients in stages I-III undergoing endocrine treatment, patients with metastatic disease, and patients in stages I-III diagnosed with COVID-19. The metastatic and COVID-19 cohorts completed the EORTC QLQ-BR42 and the EORTC QLQ-C30 once each, while the endocrine treatment cohort completed the questionnaires three times. RESULTS: A total of 516 patients were included in the study (N = 159 in the endocrine treatment cohort, N = 98 in the metastatic patients cohort and N = 259 in the COVID-19 cohort). Confirmatory factor analyses generally supported the structure of all scales included (RMSEA = 0.051). Factor loadings were above 0.60, except for the systemic chemotherapy side effects (SCSE) scale, which includes three items with factor loadings ranging from 0.29 to 0.41. Correlations with (un)related QLQ-C30 scales supported convergent validity (r > 0.40) and discriminant validity (r < 0.10). The QLQ-BR42 scales distinguished between stage I-III patients who received endocrine treatment and those with metastatic disease, as well as between surgical approaches. Substantial changes over time were observed in patients whose global QOL (measured with QLQ-C30 item #30) had improved or worsened. CONCLUSIONS: The EORTC QLQ-BR42 is a reliable and valid tool when used with a sample of Spanish breast cancer patients, although the SCSE requires further research on its internal consistency. These findings generally match those of the EORTC international validation study. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s12885-026-15831-8.