Layout 1 Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 Abstract 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 col- orectal 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 cor- relations (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 rang- ing 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. Key Words: EORTC QLQ-C30; FACT-G; health-related quality of life; psychometric validation; Bulgaria. Eur J Transl Myol 35 (4) 14121, 2025 doi: 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 Gospodin Peykov,1 Hristo Bozov,2 Angel Uchikov,3 Petar Uchikov,3 Georgi Gelov,4 Desislava Stambolova,4 Yanna Kashilska,5 Maria Bojhkova,6 Nigyar Dzhafer,7 Kiril Panayotov,8 Jannis Papathanasiou9,10 1Department of Surgery, University Hospital “Sveta Marina”, Pleven, Bulgaria; 2State University of Burgas Prof. Dr. Asen Zlatarov, Burgas, Bulgaria; 3Department of Special Surgery, Faculty of Medicine, Medical University of Plovdiv, Plovdiv, Bulgaria; 4Surgical Department, Multiprofile Hospital for Active Treatment, Haskovo, Bulgaria; 5Department of Health Management and Health Economics, Faculty of Public Health, Medical University of Plovdiv, Bulgaria; Medical Diagnostic Consultative Center “St. Archbishop Luka”, Plovdiv, Bulgaria; 6Medical College, Medical University of Plovdiv, Bulgaria; 7Department of Health Policy and Management, Faculty of Public Health “Prof. Dr. Tzecomir Vodenicharov, DSc”, Medical University of Sofia, Bulgaria; 8Department of Medical and Clinical Diagnostic Activities, University of Rousse "Angel Kanchev", University Hospital “Medica Rousse”, Bulgaria; 9Department of Physical and Rehabilitation Medicine, Medical University of Plovdiv, Bulgaria; 10Department of Physical Medicine & Rehabilitation, Medical University of Sofia, Bulgaria. This article is distributed under the terms of the Creative Commons Attribution Noncommercial License (CC BY-NC 4.0) which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. - 64 - Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 The global population of Cancer Survivors (CS) continues to grow due to advances in diagnosis, surgical techniques, chemotherapy, and radiotherapy. Cancer remains a major public health challenge across EU member states, with Lung Cancer (LC) being the most commonly diagnosed cancer type in Europe, representing 12.2% of all new cancer diagnoses and approximately 20% of cancer-related deaths, followed by Colorectal Cancer (CC) (9.4%), and Breast Cancer (BC) (6.9%).1 Among European Union (EU) countries, the highest LC mortality rates in 2021 were reported in Denmark and the Netherlands (both 6.0%), followed by Ireland (5.5%). In contrast, Bulgaria reported comparatively lower proportions of cancer-related deaths due to LC (2.2%), as well as some of the lowest mortality rates for CC and BC, with rates of 1.8% for each.2 Both, LC and CC remain the most frequently diagnosed malignancies among Bulgarian men, with higher incidence rates compared to their counterparts in other EU countries. Approximately 5,600 new cases of LC and 4,000 new cases of CC are diagnosed in Bulgaria annually. LC is also the leading cause of cancer-related mortality among men, ac- counting for around 4.700 deaths annually, while CC con- tributes to over 2,000 cancer-related deaths per year. BC is the most common malignancy among Bulgarian women, with an estimated 4.100 new cases and approximately 1,400 deaths recorded annually.3 CSs represent a highly diverse population, with variations in age, sex, race and ethnicity, socio-economic, and em- ployment status. In addition, CSs exhibit a broad spectrum of health- and cancer-related characteristics, including cancer type, age at diagnosis, weight status, and co-mor- bidities. This heterogeneity underscores the importance of personalized lifestyle interventions and systematic eval- uation of treatment outcomes, particularly in terms of Health-Related Quality Of Life (HRQoL).4,5 HRQoL is defined as the value assigned to the duration of life, modified by impairments, functional states, percep- tions, and social opportunities influenced by disease, injury, treatment, or policy.6 HRQoL has emerged as a critical out- come measure in oncology research and practice, prompt- ing the development of numerous cross-culturally validated instruments. Among the most widely used cancer-specific tools are the European Organisation for Research and Treat- ment of Cancer Quality of Life Questionnaire-Core 30 (EORTC QLQ-C30) 7 and the Functional Assessment of Cancer Therapy—General (FACT-G).8 To our knowledge, no published studies have detailed the cross-cultural adaptation and psychometric validation of the EORTC QLQ-C30 and FACT-G questionnaires for use among Bulgarian CS.9,10 This gap is particularly relevant in light of the growing commitment among Bulgarian oncol- ogists, Surgical Oncologists (SO), Physical and Rehabili- tation Medicine (PRM) specialists, and other Healthcare Professionals (HCPs) to systematically evaluate treatment outcomes, rehabilitation strategies, and Patient-Reported Outcomes (PRO) such as HRQoL. The cross-cultural adap- tation and validation of the Bulgarian versions of EORTC QLQ-C30 and FACT-G are essential to ensure accurate, re- liable, and culturally appropriate tools for evaluating PRO in clinical practice and research. Aim of the study The aim of this study was to translate, culturally adapt, and psychometrically validate the Bulgarian versions of the EORTC QLQ-C30 and FACT-G questionnaires, focusing on Internal Consistency (IC), Test–Retest Reliability (TRR), and Construct Validity (CV) in CS diagnosed with breast, lung, or colorectal cancer. Materials and Methods Data collection and study participants This cross-sectional validation study was conducted be- tween November 2023 and May 2025 among adult CS re- ceiving follow-up care at tertiary care hospitals in Pleven, Haskovo, Burgas, Plovdiv, and Ruse, Bulgaria. A total of 235 participants were recruited, with a mean age of 62.3 ± 11.2 years; the sample included 118 males (50.2%) and 117 females (49.8%). The cohort comprised 93 lung CS (39.5%), 77 colorectal CS (32.7%), and 65 breast CS (27.8%). Eligibility criteria included: i) age ≥18 years; ii) confirmed diagnosis of breast, lung, or colorectal cancer, regardless of disease stage or time since diagnosis; iii) re- ceipt of any form of cancer treatment (e.g., surgery, chemo- therapy, radiotherapy, or immunotherapy); iv) life expectancy greater than six months; and v) ability to read and understand Bulgarian. Exclusion criteria were: i) dia- gnosis of a severe psychiatric disorder (e.g., schizophrenia or active psychosis) or cognitive impairment that could in- terfere with questionnaire completion; ii) receipt of end-of- life or hospice care; and 3) insufficient proficiency in Bulgarian to independently complete study instruments. Breast, lung, and colorectal CS were selected due to their high prevalence and substantial contribution to the national cancer burden in Bulgaria,2 thereby ensuring that the study population was representative and appropriate for the cross- cultural validation of Quality Of Life (QoL) measures.12A subsample of 78 study participants (33.2%) of the total 235 CS completed the EORTC QLQ-C30 and FACT-G ques- tionnaires twice, with a 14-day interval to assess TRR. This time frame was chosen to minimize recall bias while ensur- ing that participants’ Health Status (HS) was unlikely to have changed significantly. Both test and retest administra- tions were conducted using the same mode (paper-based, in-person), and consistent settings were used, including Sur- gery Departments, PRM Departments, and the Medical Diagnostic Consultative Center. CS of our study were in- structed to complete the questionnaires independently with- out consulting other individuals, and were asked to reflect on their experiences over the previous seven days. The pro- cess was supervised by trained study administrators (H.B., J.P., D.S., Y.K.). All study participants signed written in- formed consent prior to enrollment, and all procedures were conducted in accordance with the Declaration of Helsinki (2013) and was approved by the Ethics Committee at the University Hospital «Sveta Marina», Pleven, Bulgaria, (Registry No. #223/13.10.2023). - 65 - Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 Questionnaires The EORTC QLQ-C30 is a widely used questionnaire for assessing HRQoL in cancer clinical trials.9 It consists of 30 items, which are scored and linearly transformed to yield values ranging from 0 to 100. These scores encom- pass five multi-item Functional Scales (FS), three multi- item symptom scales, six single-item symptom measures, and one global HS/QoL scale.7,12 Higher scores on the FS of the EORTC QLQ-C30 indicate better HRQoL, while higher scores on the symptom scales and individual items reflect a greater symptom burden. The EORTC QLQ-C30 has been psychometrically validated based on classical test theory, demonstrating satisfactory reliability and va- lidity.13 The FACT-G is a validated cancer-specific instrument de- signed to measure general cancer HRQoL across four core domains, Social/Family Well-Being (SWB), Physical Well- Being (PWB), Functional Well-Being (FWB) and Emo- tional Well-Being (EWB). SWB refers to the entire family dynamics and functioning.8 The FACT-G questionnaire comprises 27 items, each scored on a 5-point Likert scale reflecting experiences over the past 7 days, and demon- strates robust psychometric properties.8 Higher scores on the FACT-G indicate better QoL, while lower scores reflect greater impairment.14 Translation and cross-cultural adaption of the EORTC QLQ-C30 and FACT-G The translation and cross-cultural adaptation of the EORTC QLQ-C30 and FACT-G into Bulgarian were conducted in accordance with the standardized procedures outlined by the European Organisation for Research and Treatment of Cancer (EORTC)¹5 and the Functional Assessment of Chronic Illness Therapy (FACIT) system, respectively.16 Two native Bulgarian speakers (H.B., Y.K.) with advanced academic and clinical backgrounds independently produced forward translations of the original English-language in- struments. For items presenting linguistic or conceptual challenges, multiple translation options (typically two or three alternatives) were proposed. The translation team con- ducted a consensus review to resolve discrepancies, ensur- ing semantic and cultural equivalence, as well as clinical appropriateness. Final item wording for the first Bulgarian version was determined by the translation coordinator, based on group consensus and alignment with the concep- tual intent of the original items. Backward translation and development of the second in- termediate versions The first intermediate Bulgarian versions of the EORTC QLQ-C30 and FACT-G were independently back-translated into English by two bilingual, bicultural native English speakers fluent in Bulgarian (J.P., D.S.). Neither had prior exposure to the original English versions of the instruments. Both translators were informed that the questionnaires were intended to assess HRQoL among Bulgarian CSs diagnosed with lung, colorectal, or breast cancer. Following the back- translation, the translation coordinator conducted structured review sessions with all translators to reconcile discrep- ancies and ensure semantic, conceptual, and cultural equiv- alence. As part of the cultural adaptation process, minor modifications were implemented to enhance item clarity, cultural appropriateness, and respondent comprehension, while preserving the original conceptual meaning of each item. Statistical analysis The statistical analysis of the psychometric properties of the EORTC QLQ-C30 was conducted among Bulgarian CSs who had received various forms of cancer treatment (e.g., surgery, chemotherapy, or radiotherapy). Data were analysed by language version, Bulgarian and English. Anal- ysis of data quality included distribution of scale scores (item means and standard deviations, floor and Ceiling Ef- fects (CE), and missing item responses). To evaluate the presence of floor and CE, we calculated the proportion of respondents scoring the lowest (floor) and highest (ceiling) possible values on each EORTC QLQ-C30 and FACT-G domain. Both, floor and CE were considered significant if more than 15% of respondents scored at the minimum or maximum scale range. IC reliability was assessed using Cronbach’s α, with a value of ≥ 0.70 considered acceptable, and ≥ 0.90 considered ideal for clinical decision-making.17 TRR was assessed using the intraclass Correlation Coefficient (ICC), specifically the ICC (3,1) form based on a two-way mixed- effects model with absolute agreement. A threshold of ≥ 0.75 was was interpreted as indicative of good temporal sta- bility. CV was evaluated through both convergent and Dis- criminant Validity (DV), using Spearman’s rank-order and Pearson correlation coefficients, depending on the level of measurement and distributional assumptions. Convergent Validity (CVg) was considered satisfactory when an item demonstrated a correlation of ≥ 0.40 with its hypothesized scale. DV was confirmed when each item correlated more strongly with its own scale than with other, unrelated scales.12 At the domain level, construct alignment between the EORTC QLQ-C30 and the FACT-G was assessed by com- puting Pearson correlation coefficients between concep- tually related subscales. Correlations of r ≥ 0.60 were considered evidence of moderate to strong agreement be- tween instruments, supporting CVg.18 Bland–Altman anal- ysis was applied to assess agreement between the EORTC QLQ-C30 and FACT-G instruments, with a mean differ- ence (bias) and 95% limits of agreement calculated. All sta- tistical analyses were performed using IBM SPSS Statistics for Windows, Version 24.0 (IBM Corp., Armonk, NY, USA). A p-value < 0.05 was considered statistically signif- icant for all tests. Results Descriptive statistics Of the 290 initially enrolled CS, 235 completed the full study (81.1% retention), with 55 (18.9%) lost to follow- up. Finally, the total sample included 235 participants (mean age 62.3 ± 11.2 years), comprising 118 males - 66 - Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 (50.2%) and 117 females (49.8%). The most common cancer types were lung (n = 93; 39.6%), followed by col- orectal (n = 77; 32.8%) and breast cancer (n = 65; 27.7%). A total of 195 participants (83.0%) completed the survey approximately one month after surgery and before the in- itiation of chemotherapy. Additionally, 162 participants (68.9%) completed the assessment following chemother- apy, and 113 (48.1%) after radiation therapy. None of the participants had evidence of metastatic disease or comor- bid conditions likely to significantly impair HRQoL. Pa- tient sociodemographic and clinical characteristics are presented in Table 1. Construct validity CV of the instrument was supported through evidence of both convergent and DV, as assessed through item-scale and domain-level correlations. Following the EORTC QLQ-C30 guidelines,7,15 CVg is established when an item correlates moderately to strongly (r ≥ 0.40) with its hy- pothesized scale. DV is confirmed when the correlation between an item and its intended scale is significantly greater than its correlation with other unrelated scales.17 In our study, Spearman’s rank-order correlations were em- ployed to examine both convergence and discrimination at the item level. At the domain level, Pearson correlations between the EORTC QLQ-C30 and a comparator instru- ment ranged from 0.62 to 0.81, indicating strong associa- tions and consistent with findings from prior cross-cultural validation studies (Figure 1).19 The strongest correlations were observed for Physical Functioning (PF) (r = 0.81) and Role Functioning (RF) (r = 0.79). Moderate-to-strong correlations were also evident in Emotional Functioning (EF) (r = 0.75) and Cognitive Functioning (CF) (r = 0.70) (Table 2).20 Table 3 summarizes the results of the Spear- man correlation analysis between individual items and their respective scales on the EORTC QLQ-C30. All items demonstrated correlation coefficients equal to or greater than the minimum acceptable threshold (ρ ≥ 0.68), with the majority surpassing the CVg criterion of ρ ≥ 0.70 sup- - 67 - Table 1. Patient sociodemographic and clinical characteristics. Sample Size (n) n=235 Age (years, mean±SD) 62.3±11.2 Gender (%) Female (%) n=117 (49.8) Male (%) n=118 (50.2) Cancer type (%) Lung cancer (%) n=93 (39.6) Colorectal cancer (%) n=77 (32.8) Breast cancer (%) n=65 (27.7) Treatment received (%) Surgery n=195 (83.0) Chemotherapy n=162 (68.9) Radiation therapy n=113 (48.1) Figure 1. Pearson correlation coefficients between EORTC QLQ-C30 domains and the comparator instrument among Bulgarian cancer survivors. Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 porting the CV of the instrument (Figure 2). Notably, the Global HS items (GH1 and GH2) showed particularly strong item-scale associations (ρ = 0.79–0.81), reflecting the stability and robustness of this domain across cultural contexts. Items within the Fatigue, Pain, and Nausea/Vom- iting scales also exhibited strong internal alignment (ρ ≥ 0.70), indicating sound scale-specific coherence even for symptom-based domains (Figure 3). Floor and ceiling effects For the EORTC QLQ-C30, CE were observed in the SF domain, with 17.4% of participants reporting the maxi- mum score. Similarly, CE we measured in the GH with 16.2% of CS of our study achieving the highest score, re- flecting perceived stability or improvement in overall well-being post-treatment. No significant Floor Effects (FE) were observed across any EORTC domains, with all values falling below the 10% threshold. Regarding the FACT-G questionnaire, CE were observed in the SWB (18.3%) and EWB (15.7%) domains, suggest- ing a potential clustering of positive responses in these do- mains. No domains showed evidence of notable FE. - 68 - Table 2. Pearson correlations between EORTC QLQ-C30 and FACT-G domains. Domain Pearson correlation (r) Interpretation PF 0.81 Strong correlation RF 0.79 Strong correlation EF 0.75 Moderate-to-strong correlation CF 0.70 Moderate correlation Correlations reflect the degree of association between conceptually aligned domains across the EORTC QLQ-C30 and FACT-G. All correlations were statistically significant at p <0.001. CF, cognitive functioning; EF, emotional functioning; PF, physical functioning; RF, role functioning. Table 3. Spearman correlation of EORTC QLQ-C30 items with their own scales (Functional, Symptom, and Global QoL). Item(s) Corresponding scale Spearman correlation (ρ) PF1–PF5 Physical functioning 0.77–0.82 RF1–RF2 Role functioning 0.76–0.78 EF1–EF4 Emotional functioning 0.68–0.74 CF1–CF2 Cognitive functioning 0.69–0.71 FA1–FA3 Fatigue 0.73–0.75 PA1–PA2 Pain 0.76–0.78 NV1–NV2 Nausea/vomiting 0.70–0.72 GH1–GH2 Global health status/QoL 0.79–0.81 All item-scale correlations exceeded the recommended threshold for convergent validity (ρ ≥0.40), with most items demonstrating moderate to strong correlations (ρ ≥0.70), indicating high internal alignment and scale coherence. Item-to-own-scale correlations were higher than correlations with non-hypothesized scales, supporting DV. CF, cognitive functioning; EF, emotional functioning; GH1–GH2, global health status/QoL; NV1–NV2, nausea/vomiting; PA1–PA2, pain; PF, physical functioning; RF, role functioning. Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 Psychometric properties Reliability To evaluate the IC of the Bulgarian version of the EORTC QLQ-C30, we calculated Cronbach’s α coefficients across its five core Functional Domains (FD).17 IC was assessed separately for both questionnaires, with thresholds of ≥0.70 for acceptability and ≥0.90 for clinical applications. As presented in Table 4, Cronbach’s α values ranged from 0.92 to 0.95, demonstrating excellent IC across all sub- scales. Specifically, Cronbach’s α values were 0.94 for PF, 0.92 for RF, 0.93 for EF, 0.95 for CF, and 0.92 for Social Functioning (SF). These values exceed the commonly ac- cepted threshold of 0.70 and meet the stricter benchmark of 0.90 recommended for instruments used in individual- level clinical decision-making.18-20 Similarly, the IC of the Bulgarian version of the α FACT-G, were evaluated across its four domains. Estimates were high, ranging from 0.88 to 0.91, indicating good to IC across all subscales (Table 5). Specifically, Cronbach’s α was 0.91 for PWB, 0.88 for SWB, 0.89 for EWB, and 0.90 for FWB. These findings exceed the widely accepted cutoff of 0.70, and meet or ex- - 69 - Figure 2. Pearson correlations between EORTC QLQ-C30 and FACT Domains. Figure 3. Spearman correlations of items with their own scale (EORTC QLQ-C30 extended). Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 ceed the 0.90 benchmark for clinical use of QoL instru- ments.10 TRR for both instruments was assessed by using the ICC based on a two-way mixed-effects model with absolute agreement. For the EORTC QLQ-C30, ICC values ranged from 0.75 to 0.89, indicating good to excellent temporal sta- bility (Figure 4).21Αs seen in Table 4 the ICC was 0.88 for PF, 0.85 for RF, 0.86 for EF, 0.89 for CF, and 0.75 for SF. For the FACT-G, ICCs ranged from 0.82 to 0.87, also sup- porting good to excellent TRT. Specifically, the ICC was 0.86 for PWB, 0.82 for SWB, 0.84 for EWB, and 0.87 for FWB (Table 5). The Bland–Altman analysis revealed a mean difference (bias) of 23.20 (95% CI: 21.87 to 24.53), indicating that, on average, the EORTC QLQ-C30 yielded higher HRQoL scores compared to the FACT-G across cor- responding FDs (Figure 5). 22 Discussion To our knowledge, this is the first study to cross-culturally adapt and psychometrically validate both the EORTC QLQ- C30 and FACT-G for use in Bulgaria, including CSs from five tertiary hospitals. The findings provide robust evidence for the validity and reliability of the Bulgarian versions of both instruments in assessing HRQoL among Bulgarian CSs. While both questionnaires are widely used for eval- uating HRQoL in cancer research, they are grounded in dis- tinct conceptual frameworks. The EORTC QLQ-C30 was developed primarily for use in international clinical trials and emphasizes functional status and symptom burden, in- cluding physical, role, emotional, cognitive, and SF, along with key symptom domains such as fatigue, pain, and nau- sea.12,15 Its structure is aligned with the clinical trajectory of cancer treatment and recovery, making it highly sensitive to disease- and treatment-related impairments. In contrast, the FACT-G adopts a broader well-being-centered ap- proach, focusing on patients’ subjective perceptions of their physical, social/family, emotional, and FWB.8,16 This frame- work reflects a PRO concept that prioritizes emotional ad- justment and QoL beyond purely clinical outcomes, which may be especially relevant in survivorship care. As such, FACT-G may be more suitable for assessing long-term - 70 - Table 4. Reliability of the EORTC QLQ-C30 domains in Bulgarian cancer survivors. Domain Cronbach’s α IC ICC TRR PF 0.94 Excellent 0.88 Excellent RF 0.92 Excellent 0.85 Excellent EF 0.93 Excellent 0.86 Excellent CF 0.95 Excellent 0.89 Excellent SF 0.92 Excellent 0.75 Good Cronbach’s α values ≥0.90 indicate excellent internal consistency. ICC values ≥0.75 reflect good to excellent TRR. CF, cognitive functioning; EF, emotional functioning; IC, internal consistency; ICC, intraclass correlation coefficient; PF, physical functioning; RF, role functioning, SF, social functioning; TRR, test–retest reliability. Table 5. Reliability of FACT-G domains in Bulgarian cancer survivors. Domain Cronbach’s α IC ICC TRR PWB 0.91 Excellent 0.86 Excellent SWB 0.88 Good 0.82 Good EWB 0.89 Good 0.84 Good FWB 0.90 Excellent 0.87 Excellent Cronbach’s α values ≥0.90 indicate excellent IC; values between 0.80–0.89 are considered good. ICC values ≥0.75 reflect good to excellent TRR. EWB, emotional well-being; FWB, functional well-being; IC, internal consistency; ICC, intraclass correlation coefficient; PWB, physical well-being; SWB, social/family well-being; TRR, test–retest reliability. Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 adaptation and well-being, whereas the EORTC QLQ-C30 may be more responsive to short-term treatment effects.14,20 These differences underline the complementary nature of both HRQoL instruments, and their combined use may pro- vide a more comprehensive understanding of PRO across the cancer care continuum.23 Psychometric evaluation revealed excellent IC across all domains, with Cronbach’s α coefficients exceeding the recommended threshold of 0.70 and meeting or surpass- ing the 0.90 benchmark for clinical decision-making (Table 4; Table 5).24 These results are consistent with prior validation studies conducted in diverse inter- - 71 - Figure 4. Internal consistency (IC) and test–retest reliability (TRR) for each of the five functional domains in the Bul- garian version of the EORTC QLQ-C30. Figure 5. Bland–Altman analysis comparing the EORTC QLQ-C30 and FACT-G scores. Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 national settings, reinforcing the cross-cultural robust- ness of both instruments.13,14,18-20 CV was supported through strong convergent and DV. High domain-level associations between the EORTC QLQ-C30 and FACT-G were particularly evident in the domains of PF, RF, and EF, indicating strong conceptual alignment across instruments (Table 2).24 While these do- mains are inherently more subjective and potentially in- fluenced by cultural attitudes toward psychological well-being and memory or concentration, the observed values suggest that both instruments assess these con- structs with substantial agreement. Furthermore, the strong positive correlation (r ≈ 0.99, p < 0.001) observed between the Bulgarian versions of the EORTC QLQ-C30 and the FACT-G reinforces their substantial CVg in eval- uating HRQoL among CS. This finding aligns with pre- vious cross-sectional validation studies that reported similarly high correlations between the instruments, sup- porting their complementary utility in both research and clinical settings.25 While FE and CE are not direct indicators of responsive- ness, substantial clustering at score extremes (typically >15%) suggests potential limitations in detecting further deterioration or improvement.26 Moreover, the presence of mild CE in selected domains may attenuate respon- siveness well-functioning Cancer Populations (CP). TRR further confirmed the temporal stability of both in- struments, with ICC values ranging from 0.75 to 0.89. Specifically, the EORTC QLQ-C30 demonstrated excel- lent temporal stability across most FD, with ICCs of 0.88 for PF, 0.85 for RF, 0.86 for EF, 0.89 for CF, and 0.75 for SF (Table 4). The FACT-G showed similar results, with ICCs ranging from 0.82 to 0.87 across its domains, also supporting its temporal reliability (Table 5). The rel- atively lower ICC observed for the SF domain in our study (ICC = 0.75) is consistent with findings from El Alami et al.27, who reported an ICC of 0.64 for SF in the Moroccan version of the EORTC QLQ-C30, signifi- cantly lower than other FD. This trend underscores the transient and context-dependent nature of interpersonal and social experiences among CS.20 The observed variability in SF scores of our study is thought to reflect the domain’s heightened sensitivity to short-term emotional states, interpersonal dynamics, and contextual factors such as cultural and environmental in- fluences.5 These findings are further supported by cross- cultural validation studies in diverse CP. Hoopman et al.28and Tomaszewski et al.,29 reported similar patterns of lower TRR for SF domains, underscoring their suscep- tibility to transient psychosocial and cultural fluctuations. The measured 95% limits of agreement between the two HRQoL instruments, as revealed through Bland–Altman analysis, were within clinically acceptable bounds, indi- cating that the EORTC QLQ-C30 consistently yielded slightly higher HRQoL scores compared to the FACT-G (Figure 5). This discrepancy likely reflects inherent dif- ferences in scale structure, item phrasing, and response range, but it does not compromise the validity or clinical utility of either instrument.22 This study has important implications for clinical prac- tice in Bulgaria. First, the findings of this study offer Bulgarian oncologists, Surgical Oncologists SO, Physi- cal and Rehabilitation Medicine PRM physicians, and other Healthcare Professionals HCPs involved in cancer care and survivorship a set of standardized, psychomet- rically robust, and culturally validated instruments for the assessment of PRO.11,23 Second, the study supports alignment with international recommendations, such as those from the European Society for Medical Oncology (ESMO), which advocate for the routine incorporation of HRQoL assessments into comprehensive cancer care and clinical decision-making. Third, it enhances the comparability of Bulgarian oncology data in multi- national trials and collaborative studies. Moreover, this validation effort supports the ongoing de- velopment of survivorship care in Bulgaria, which is in- creasingly prioritized due to improving survival rates. Reliable and culturally adapted tools for assessing HRQoL are essential for guiding individualized care, re- habilitation outcomes, and public health strategies. From a research standpoint, the availability of validated instru- ments lays the groundwork for future studies on symp- tom burden, late effects, and the long-term efficacy of behavioral and rehabilitation interventions. The Bulgarian versions of the EORTC QLQ-C30 and FACT-G demonstrate strong psychometric properties, supporting their use in both clinical practice and research for CS in Bulgaria. Their combined application can pro- vide a multidimensional perspective on PRO across dif- ferent phases of the cancer care continuum. Limitations However, our study faced several limitations that should be acknowledged. First, the cross-sectional design pre- cludes assessment of responsiveness to clinical changes over time.26 Second, the sample included only CSs with lung, colorectal, and breast cancer, limiting the general- izability of findings to other cancer populations. Third, while FACT-G was used as a comparator instru- ment, inclusion of additional validated HRQoL tools could have provided a more comprehensive concurrent validity assessment.14 Finally, another important limita- tion was that both the EORTC QLQ-C30 and FACT-G were simultaneously translated, adapted, and validated in this study. As neither tool had been previously val- idated in the Bulgarian CP, the CVg results must be in- terpreted with caution. Future research should aim to confirm these findings using external instruments that have already undergone psychometric evaluation in this context. As neither instrument had been previously validated in this context, our assessment of CVg is constrained by the absence of an established reference standard. This par- allel validation approach may limit the strength of con- clusions regarding the observed cross-instrument correlations. Future studies should aim to validate these HRQoL instruments against other internationally recog- nized and previously validated tools, such as the SF-36 or EQ-5D, to further support their psychometric robust- ness and CVg. - 72 - Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 Conclusions The Bulgarian versions of the EORTC QLQ-C30 and FACT-G are valid, reliable, and culturally appropriate in- struments for assessing HRQoL in Bulgarian CS. Both tools demonstrated excellent IC, strong CV, and good-to-excel- lent TRR. These findings support their use in clinical prac- tice, survivorship programs, and research settings across Bulgaria. Future research should explore the responsiveness of both instruments over time and their applicability in broader CP. List of abbreviations BC, breast cancer CCA, cross-cultural adaptation CC, colorectal cancer CCDs, cancer related deaths CCV, cross-culturally validated CF, cognitive functioning CS, cancer survivors CP, cancer populations CV, construct validity CVg, convergent validity DV, discriminant validity EF, emotional functioning EORTC QLQ-C30, European Organisation for Research and Treatment of Cancer Quality of Life Questionnaire- Core 30 ESMO, European Society for Medical Oncology EWB, emotional well-being FACT-G, Functional Assessment of Cancer Therapy– General FD, functional domains FS-functional scales FWB, functional well-being HCPs, healthcare professionals HRQoL, health-related quality of life HS, health status ICC, intraclass correlation coefficient IC, internal consistency LC, lung cancer PRM, physical and rehabilitation medicine PRO, patient-reported outcomes PWB, physical well-being QoL, quality of life RF, role functioning SF, social functioning SO, surgical oncologists SWB, social or family well-being TRR, test–retest reliability Acknowledgments The authors sincerely thank all the participants and the dedicated staff of University Hospital «Sveta Marina», Pleven; the Surgical Department of the Multiprofile Hospital for Active Treatment, Haskovo; the Complex Oncology Center of Burgas; and University Hospital “Medica Rousse”, Bulgaria, for their invaluable contrib- utions to this study. We also extend our special gratitude to Academician Prof. Grigor Gorchev, MD, PhD, DSc, and Prof. Dr. Georgi Paskalev, MD, PhD, for their insightful guidance and valuable advice on the study design. Funding The authors received no specific funding for this work. Conflict of interest The authors declare no financial, personal, or other conflicts of interest. Ethics approval and consent to particpate All study participants signed written informed consent prior to enrollment, and all procedures were conducted in accor- dance with the Declaration of Helsinki (2013) and was ap- proved by the Ethics Committee at the University Hospital «Sveta Marina», Pleven, Bulgaria, (Registry No. #223/13.10.2023). Contributions JP, GP, and HB contributed to the design, conception, data acquisition, analysis, and interpretation, as well as the writing and critical revision of the manuscript; JP, HB, and AU contributed to the writing and critical revision of the manuscript; GP, AU, PU contributed to the design and has been involved in the critical revision of the manuscript; JP, YK, GG and KP contributed to data analysis and in- terpretation and has been involved in the critical revision of the manuscript; MB, YK, GG, DS contributed to the analysis and interpretation as well as critical revision of the manuscript; ND, HB, DS, and YK contributed to the analysis and interpretation as well as critical revision of the manuscript. All authors read and approved the final manuscript. Corresponding author Jannis Papathanasiou, Head of the Department of Physical and Rehabilitation Medicine, Faculty of Medicine, Medical University of Plovdiv, Bulgaria. Tel: +359889 101178. ORCID ID: 0000-0003-2557-5508 E-mail: giannipap@yahoo.co.uk Co-authors Gospodin Peykov ORCID ID: 0009-0008-5992-8125 E-mail: dr_peykov@mail.bg Hristo Bozov ORCID ID: 0000-0001-7897-2051 E-mail: hr_bozov@yahoo.com - 73 - mailto:dr_peykov@mail.bg Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 Angel Uchikov ORCID ID: 0000-0002-1596-3956 E-mail: angel.uchikov@mu-plovdiv.bg Petar Uchikov ORCID ID: 0000-0003-2064-173X E-mail: petar.uchikov@mu-plovdiv.bg Georgi Gelov ORCID ID: 0009-0009-3362-5852 E-mail: ggelov@mail.bg Desislava Stambolova ORCID ID: 0009-0002-0040-0883 E-mail: stambolovadesislava@gmail.com Yanna Kashilska ORCID ID: 0000-0002-8859-7934 E-mail: iana_kashilska@abv.bg Maria Bojhkova ORCID ID: 0000-0002-9115-3437 E-mail: mariya.bozhkova@mu-plovdiv.bg Nigyar Dzhafer ORCID ID: 0000-0003-3196-8359 E-mail: nigdzhafer@gmail.com Kiril Panayotov ORCID ID: 0000-0002-2737-0023 E-mail: zkm4@abv.bg References 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. 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. Ac- cessed May 27, 2025. 4. Miller KD, Nogueira L, Devasia T, et al. Cancer treat- ment and survivorship statistics, 2022. CA Cancer J Clin 2022;72:409-36. 5. Olson JL, Conroy DE, Mama SK, Schmitz KH. Life- style behaviors and health-related quality of life in cancer survivors: a latent class analysis. Health Educ Behav 2024;51:341-51. 6. Vetter TR. Health-related quality of life in pain medi- cine: 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. 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/mea- sures/fact-g. Accessed May 27, 2025. 9. Aaronson NK, Ahmedzai S, Bergman B, et al. The Eu- ropean 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. 10. Cella DF, Tulsky DS, Gray G, et al. The Functional As- sessment of Cancer Therapy scale: development and validation of the general measure J Clin Oncol 1993; 11:570-9. 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. 12. Giesinger JM, Loth FLC, Aaronson NK, et al. EORTC Quality of Life Group. Thresholds for clinical impor- tance were established to improve interpretation of the EORTC QLQ-C30 in clinical practice and research. J Clin Epidemiol 2020:118:1-8. 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. 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. 15. Koller M, Aaronson NK, Blazeby J, et al; EORTC Quality of Life Group. Translation procedures for standardised quality of life questionnaires: The Eu- ropean Organisation for Research and Treatment of Cancer (EORTC) approach. Eur J Cancer 2007;43: 1810-20. 16. Webster K, Cella D, Yost K. The Functional Assessment of Chronic Illness Therapy (FACIT) Measurement Sys- tem: properties, applications, and interpretation. Health Qual Life Outcomes 2003:1:79. 17. Tavakol M, Dennick R. Making sense of Cronbach’s alpha. Int J Med Educ 2011;27:2:53-5. 18. Luckett T, King MT, Butow PN, et al. Choosing be- tween the EORTC QLQ-C30 and FACT-G for measur- ing health-related quality of life in cancer clinical research: issues, evidence and recommendations. Ann Oncol 2011;22:2179-90. 19. Holzner B, Bode RK, Hahn EA, et al. Equating EORTC QLQ-C30 and FACT-G scores and its use in oncologi- cal research. Eur J Cancer 2006;42:3169-77. 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 Inter- national Classification of Functioning, Disability and Health. Psychooncology 2023 32:1372-84. 21. Koo TK, Li MY. A Guideline of Selecting and Report- ing Intraclass Correlation Coefficients for Reliability Research. J Chiropr Med 2016;15:155-63. 22. Euser AM, Dekker FW, le Cessie S. A practical ap- proach to Bland-Altman plots and variation coefficients for log transformed variables. J Clin Epidemiol 2008; 61:978-82. 23. Schurr T, Loth F, Lidington E, et al; European Organi- - 74 - https://ec.europa.eu/eurostat https://www.eortc Validation of the Bulgarian versions of the EORTC QLQ-C30 and FACT-G Eur J Transl Myol 35 (4) 14121, 2025 doi: 10.4081/ejtm.2025.14121 sation for Research, Treatment of Cancer Quality of Life Group (EORTC QLG). Patient-reported outcome meas- ures 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. 24. Oerlemans S, Lodder P, van der Baan F, Giesinger JM, Ezendam NPM. Development and validation of cross- walks between the EORTC QLQ-C30 physical, role, social and emotional functioning, fatigue and global health status/quality of life scales and their correspon- ding PROMIS scales. J Clin Epidemiol 2025:184: 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. 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. 27. El Alami Y, Essangri H, Majbar MA, et al. Psychomet- ric 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. 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. 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. 30. ESMO. European Society for Medical Oncology. Avail- able from: https://www.esmo.org/. Accessed May 27, 2025. Disclaimer All claims expressed in this article are solely those of the authors and do not necessarily represent those of their af- filiated 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. Submitted: 6 July 2025. Accepted: 25 July 2025. Early access: 3 September 2025. - 75 - Online supplementary material: EORTC QLQ-C30 – Bulgarian version. FACT-G – Bulgarian version.