Validation of Korean Version of the COmprehensive Score for Financial Toxicity (COST) among Breast Cancer Survivors

Purpose Little is known about the impact of financial toxicity in disease-free breast cancer survivors. We aim to validate the COmprehensive Score for financial Toxicity in Korean (COST-K) and evaluate financial toxicity among disease-free breast cancer survivors. Materials and Methods We conducted linguistic validation following a standardized methodology recommended by Functional Assessment of Chronic Illness Therapy multilingual translation (FACITtrans). For psychometric validation, we conducted a cross-sectional survey with 4,297 disease-free breast cancer survivors at a tertiary hospital in Seoul, Korea between November 2018 and April 2019. Survivors were asked to complete the COST-K and European Organization for Research and Treatment of Cancer Quality of Life Core 30 (EORTC QLQ-C30) questionnaires. The test-retest reliability, internal consistency, and validity of the COST-K were assessed using standard scale construction techniques. Results The COST-K demonstrated good internal consistency, with a Cronbach's a of 0.81. The test-retest analysis revealed an intraclass correlation coefficient of 0.78. The COST-K had moderate correlation (r=-0.60) with the financial difficulty item of the EORTC QLQ-C30 and week correlation with the items on acute and chronic symptom burdens (nausea/vomiting, -0.18; constipation, -0.14; diarrhea, -0.14), showing good convergent and divergent validity. The median COST-K was 27 (range, 0 to 44; mean +/- standard deivation [SD], 27.1 +/- 7.5) and about 30% and 5% of cancer survivors experienced mild and severe financial toxicity, respectively. Younger age, lower education, lower household income was associated with higher financial toxicity. Conclusion The COST-K is a valid and reliable instrument for measuring financial toxicity in disease-free breast cancer survivors. Considering its impact on the health-related quality of life, more studies need to be conducted to evaluate financial toxicity in cancer survivors and design interventions.