This study aimed to examine the validity and reliability of the EORTC QLQ-BR45 questionnaire among breast cancer patients in Ethiopia. This study included 248 breast cancer patients who completed the QLQ-BR45 and QLQ-C30 questionnaires. The internal reliability, test-retest reliability, and the content, concurrent, convergent, divergent, and clinical validity of the tool were examined. The statistical analyses included Cronbach's α coefficient, Pearson's correlation coefficient, standardised root mean square residual (SRMR), comparative fit index (CFI), t-test, and root mean square error of approximation (RMSEA). All items were marked as relevant, and item-level content validity index (I-CVI) scores ranged from 0.83 to 1. The S-CVI/Ave was calculated by dividing the sum of I-CVI values by the total number of items, which was found to be 0.94. The average CVR value was 0.76. The Cronbach's α coefficient was 0.80 for all domains. All subscales met the minimal standards of reliability except the arm symptom scale (0.66). The test-retest reliability coefficient was 0.77 for all domains. Seven out of the 12 hypothesised scales showed positive correlations (r > 0.40) between the QLQ-BR45 and QLQ-C30 scales. Multitrait scaling analysis showed that the item-scale correlations exceeded the 0.40 criterion for item-convergent validity for 11 of the 12 hypothesised scales. The correlation coefficients between an item and its own subscale were significantly higher than with other subscales. The EORTC QLQ-BR45 had good reliability and validity, and it can be used to measure the quality of life of breast cancer patients in Ethiopia.
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