https://doi.org/10.4081/ejtm.2025.14121
Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G: reliability, construct validity, and agreement in cancer survivors
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.
Published: 3 September 2025
Health-related quality of life (HRQoL) is a critical outcome measure in oncology research and clinical care. This study aimed to translate, culturally adapt, and psychometrically validate the Bulgarian versions of the European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire–Core 30 (EORTC QLQ-C30) and the Functional Assessment of Cancer Therapy–General (FACT-G), focusing on Internal Consistency (IC), Test–Retest Reliability (TRR), and Construct Validity (CV) in Cancer Survivors (CS) diagnosed with breast, lung, or colorectal cancer. A total of 235 Bulgarian CS (mean age 62.3 ± 11.2 years; 118 males, 117 females) were recruited from five tertiary care hospitals in a cross-sectional study design. The EORTC QLQ-C30 and FACT-G were translated and culturally adapted according to the official EORTC and FACIT guidelines. Psychometric evaluation included data quality (floor and ceiling effects, missing responses), IC, TRR, and CV assessed via convergent and Discriminant Validity (DV). A subsample of 78 study participants (33.2%) completed both questionnaires twice over a 14-day interval for TRR assessment. CV was supported by strong item-scale and cross-instrument correlations (r = 0.62–0.81). IC was excellent across all domains (Cronbach’s α = 0.88–0.95), and TRR demonstrated high stability (ICC = 0.75–0.89). Bland–Altman analysis showed acceptable agreement, with a mean difference of 23.20 (95% CI: 21.87–24.53) and limits of agreement ranging from +3.77 to +42.63. Minimal Floor Effects (FE) were observed, while Ceiling Effects (CE) were noted in specific domains (e.g., Social Functioning (SF) and Global Health (GH) in the EORTC QLQ-C30; Social Well-Being (SWB) and Emotional Well-Being (EWB) in the FACT-G). The findings support that the Bulgarian versions of the EORTC QLQ-C30 and FACT-G are valid, reliable, and culturally appropriate instruments for evaluating HRQoL in Bulgarian CS. Their use is recommended in both clinical practice and research contexts.
Downloads
1. Ferlay J, Colombet M, Soerjomataram I, et al. Cancer incidence and mortality patterns in Europe: Estimates for 40 countries and 25 major cancers in 2018. Eur J Cancer 2018;103:356-87. DOI: https://doi.org/10.1016/j.ejca.2018.07.005
2. Eurostat. Available from: https://ec.europa.eu/eurostat/statistics-explained/index.php?title=Cancer_statistics_-_specific_cancers. Accessed May 27, 2025.
3. ECIR. European Cancer Inequalities Registry. Available from: https://cancer-inequalities.jrc.ec.europa.eu. Accessed May 27, 2025.
4. Miller KD, Nogueira L, Devasia T, et al. Cancer treatment and survivorship statistics, 2022. CA Cancer J Clin 2022;72:409-36. DOI: https://doi.org/10.3322/caac.21731
5. Olson JL, Conroy DE, Mama SK, Schmitz KH. Lifestyle behaviors and health-related quality of life in cancer survivors: a latent class analysis. Health Educ Behav 2024;51:341-51. DOI: https://doi.org/10.1177/10901981231203978
6. Vetter TR. Health-related quality of life in pain medicine: a review of theory and practice. In: Preedy VR, Watson RR, eds. Handbook of disease burdens and quality of life measures. New York, NY: Springer; 2010. pp.2305-2318. DOI: https://doi.org/10.1007/978-0-387-78665-0_228
7. EORTC. European Organization for Research and Treatment of Cancer. Available from: https://www.eortc.org/.Accessed May 27, 2025.
8. FACT-G. Functional Assessment of Cancer Therapy – General. Available from: https://www.facit.org/measures/fact-g. Accessed May 27, 2025.
9. Aaronson NK, Ahmedzai S, Bergman B, et al. The European Organization for Research and Treatment of Cancer QLQ-C30: a quality-of-life instrument for use in international clinical trials in oncology. J Natl Cancer Inst 1993;85:365-76. DOI: https://doi.org/10.1093/jnci/85.5.365
10. Cella DF, Tulsky DS, Gray G, et al. The Functional Assessment of Cancer Therapy scale: development and validation of the general measure J Clin Oncol 1993;11:570-9. DOI: https://doi.org/10.1200/JCO.1993.11.3.570
11. Di Maio M, Basch E, Denis F, et al. The role of patient-reported outcome measures in the continuum of cancer clinical care: ESMO Clinical Practice Guideline. Ann Oncol 2022;33:878-92. DOI: https://doi.org/10.1016/j.annonc.2022.04.007
12. Giesinger JM, Loth FLC, Aaronson NK, et al; EORTC Quality of Life Group. Thresholds for clinical importance were established to improve interpretation of the EORTC QLQ-C30 in clinical practice and research. J Clin Epidemiol 2020:118:1-8. DOI: https://doi.org/10.1016/j.jclinepi.2019.10.003
13. Cocks K, Wells JR, Johnson C, et al. Content validity of the EORTC quality of life questionnaire QLQ-C30 for use in cancer. Eur J Cancer 2023;178:128-38. DOI: https://doi.org/10.1016/j.ejca.2022.10.026
14. Sánchez R, Ballesteros M, Arnold BJ. Validation of the FACT-G scale for evaluating quality of life in cancer patients in Colombia. Qual Life Res 2011;20:19-29. DOI: https://doi.org/10.1007/s11136-010-9727-0
15. Koller M, Aaronson NK, Blazeby J, et al; EORTC Quality of Life Group. Translation procedures for standardised quality of life questionnaires: The European Organisation for Research and Treatment of Cancer (EORTC) approach. Eur J Cancer 2007;43:1810-20. DOI: https://doi.org/10.1016/j.ejca.2007.05.029
16. Webster K, Cella D, Yost K. The Functional Assessment of Chronic Illness Therapy (FACIT) Measurement System: properties, applications, and interpretation. Health Qual Life Outcomes 2003:1:79. DOI: https://doi.org/10.1037/t77273-000
17. Tavakol M, Dennick R. Making sense of Cronbach's alpha. Int J Med Educ 2011;27:2:53-5. DOI: https://doi.org/10.5116/ijme.4dfb.8dfd
18. Luckett T, King MT, Butow PN, et al. Choosing between the EORTC QLQ-C30 and FACT-G for measuring health-related quality of life in cancer clinical research: issues, evidence and recommendations. Ann Oncol 2011;22:2179-90. DOI: https://doi.org/10.1093/annonc/mdq721
19. Holzner B, Bode RK, Hahn EA, et al. Equating EORTC QLQ-C30 and FACT-G scores and its use in oncological research. Eur J Cancer 2006;42:3169-77. DOI: https://doi.org/10.1016/j.ejca.2006.08.016
20. Pilz MJ, Rothmund M, Lidington E, et al; EORTC Quality of Life Group. Content comparison of the EORTC CAT Core, SF-36, FACT-G, and PROMIS role and social functioning measures based on the International Classification of Functioning, Disability and Health. Psychooncology 2023 32:1372-84. DOI: https://doi.org/10.1002/pon.6188
21. Koo TK, Li MY. A Guideline of Selecting and Reporting Intraclass Correlation Coefficients for Reliability Research. J Chiropr Med 2016;15:155-63. DOI: https://doi.org/10.1016/j.jcm.2016.02.012
22. Euser AM, Dekker FW, le Cessie S. A practical approach to Bland-Altman plots and variation coefficients for log transformed variables. J Clin Epidemiol 2008;61:978-82. DOI: https://doi.org/10.1016/j.jclinepi.2007.11.003
23. Schurr T, Loth F, Lidington E, et al; European Organisation for Research, Treatment of Cancer Quality of Life Group (EORTC QLG). Patient-reported outcome measures for physical function in cancer patients: content comparison of the EORTC CAT Core, EORTC QLQ-C30, SF-36, FACT-G, and PROMIS measures using the International Classification of Functioning, Disability and Health. BMC Med Res Methodol 2023;23:21. DOI: https://doi.org/10.1186/s12874-022-01826-z
24. Oerlemans S, Lodder P, van der Baan F, Giesinger JM, Ezendam NPM. Development and validation of crosswalks between the EORTC QLQ-C30 physical, role, social and emotional functioning, fatigue and global health status/quality of life scales and their corresponding PROMIS scales. J Clin Epidemiol 2025:184:111853. DOI: https://doi.org/10.1016/j.jclinepi.2025.111853
25. Yun YH, Park YS, Lee ES, et al. Validation of the Korean version of the EORTC QLQ-C30. Qual Life Res 2004;13:863-8. DOI: https://doi.org/10.1023/B:QURE.0000021692.81214.70
26. Revicki D, Hays RD, Cella D, Sloan J. Recommended methods for determining responsiveness and minimally important differences for patient-reported outcomes. J Clin Epidemiol 2008;61:102-9. DOI: https://doi.org/10.1016/j.jclinepi.2007.03.012
27. El Alami Y, Essangri H, Majbar MA, et al. Psychometric validation of the Moroccan version of the EORTC QLQ-C30 in colorectal Cancer patients: cross-sectional study and systematic literature review. BMC Cancer 2021;21:99. DOI: https://doi.org/10.1186/s12885-021-07793-w
28. Hoopman R, Muller MJ, Terwee CB, Aaronson NK. Translation and validation of the EORTC QLQ-C30 for use among Turkish and Moroccan ethnic minority cancer patients in the Netherlands. Eur J Cancer 2006;42:1839-47. DOI: https://doi.org/10.1016/j.ejca.2005.08.047
29. Tomaszewski KA, Püsküllüoğlu M, Biesiada K, et al. Validation of the polish version of the eortc QLQ-C30 and the QLQ-OG25 for the assessment of health-related quality of life in patients with esophagi-gastric cancer. J Psychosoc Oncol 2013;31:191-203. DOI: https://doi.org/10.1080/07347332.2012.761323
30. ESMO. European Society for Medical Oncology. Available from: https://www.esmo.org/. Accessed May 27, 2025.
How to Cite
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.
PAGEPress has chosen to apply the Creative Commons Attribution NonCommercial 4.0 International License (CC BY-NC 4.0) to all manuscripts to be published.