Study design and participants
This was a longitudinal study to investigate the QOL trajectory patterns of breast cancer patients for 12 months from the end of primary treatment. In total, 140 breast cancer patients at a University hospital, who underwent a mastectomy and had completed adjuvant therapy such as chemotherapy or radiotherapy not more than a month previously, were selected through random sampling. The inclusion criteria were as follows: adults (1) aged 19 to 64 years, (2) with stage 1, 2, or 3 breast cancer who had completed chemotherapy or radiotherapy following surgery, (3) had no recurrence or metastasis, and (4) could communicate in Korean and fill out questionnaires. Patients receiving hormone therapy or targeted therapy to prevent recurrence were included in the study. The exclusion criteria were patients (1) diagnosed with a psychiatric disorder, such as adjustment disorder, obsessive–compulsive disorder, or anxiety disorder, or taking related drugs, and (2) those with health problems that might have caused cognitive impairment, such as stroke or dementia. Using the patient list registered at the hospital, from April to August 2018, breast cancer patients who met the selection criteria and gave their written consent to participate in the study were selected. Data were collected at four time points: immediately after the end of primary treatment (T1) and at 3 months (T2), 6 months (T3), and 12 months (T4) after the end of primary treatment.
On calculating with a significance level of 0.05, an odds ratio of 2.0 , and a power of 0.80 using G power analysis , the number of participants required was 113; therefore, 140 patients were recruited, considering a dropout rate of approximately 20%. Sixteen patients were lost to follow-up; 124 patients were included in the final analysis.
Quality of life
Quality of life was measured using 27 items of the Korean version of the Functional Assessment of Cancer Therapy Scale-G (FACT-G) developed by the Functional Assessment of Chronic Illness Therapy Measurement System (FACIT). This scale assesses physical (7 items), social/family (7 items), emotional (6 items), and functional well-being (7 items) over the previous week; responses for each item range from 0 points for "strongly disagree" to 4 points for "strongly agree," and the higher the score, the higher the QOL. The validity and reliability of the Korean version of the FACT-G  had a Cronbach's ɑ of 0.78 − 0.90, while Cronbach's ɑ in this study was 0.85 − 0.91.
Symptom experience was measured using the Memorial Symptom Assessment Scale-Short Form (MSAS-SF), in which patients rate symptom distress associated with 28 physical and four psychological symptoms . Physical symptoms are included with two written description items and rated on a Likert scale (not at all: 0.8; a little bit: 1.6; somewhat: 2.4; quite a bit: 3.2; and very much: 4.0). Psychological symptoms are a total of four items rated as 0 points for "rarely" and 4 points for "almost constantly." The scores are compared using the mean of each item, and the higher the score, the more severe the symptom distress. At the time of development, this scale had Cronbach's α of 0.80 , while in this study, Cronbach's α was 0.80.
In this study, self-efficacy was measured using the Self-Efficacy Scale for Self-Management of Breast Cancer (SESSM-B), developed by Lee et al. . This tool consists of 13 items, including coping with psycho-informational demand (3 items), maintenance of a healthy lifestyle (3 items), management of side effects (3 items), therapeutic compliance (2 items), and sexual life (2 items). The items were rated on a Likert scale ranging from 1 (strongly disagree) to 5 (strongly agree). The higher the sum of the scores, the higher the self-efficacy for self-management. The scale's reliability at the time of development was Cronbach's α = 0.78 , and the reliability in this study was Cronbach's α = 0.80.
Social support was assessed using the Korean version of the Interpersonal Support Evaluation List-12 (ISEL-12)  developed by Cohen . This tool comprises 12 questions, with four response options: "not true at all," “not true,” “true,” and “very true” is to be selected. The ISEL-12 yields a total score that describes overall perceived social support and three subscales representing perceived availability of appraisal (advice or guidance), belonging (empathy, acceptance, concern), and tangible (help or assistance, such as material or financial aid) social support . The higher the score, the higher the level of social support. The reliability of the Korean version of the ISEL-12 at the time of development was Cronbach's α = 0.87 , and the reliability in this study was Cronbach's α = 0.87.
Descriptive statistics were used to summarize all participants' sociodemographic and disease-related characteristics and major study variables. Normality assumptions of the dependent variable was checked using the Kolmogorov–Smirnov test. Group-based trajectory modeling was used to identify different patterns of the overall QOL trajectory over time. The SAS 9.4 software (SAS Institute, Inc., Cary, NC) was used to estimate the model and calculate model performance indexes for alternative models based on the Bayesian information criterion (BIC) . Low BIC values are interpreted as a good model fit to the data when an additional latent class is included . Additionally, a model with more than 10% of the sample was selected for the minimum number of participants in a trajectory group. A value of 0.7 or higher was selected as the probability of a participant belonging to a specific trajectory group .
The Chi-square test was conducted to examine differences in participants’ sociodemographic and disease-related characteristics according to trajectory membership. Analysis of variance was conducted for differences in symptom experience, social support, and self-efficacy, and post hoc analysis was performed using Scheffé’s multiple comparison analysis. A multinomial logistic regression (MLR) was performed to identify the factors influencing the changes in patients’ QOL by introducing the variables that showed statistically significant univariate analysis variables, including perceived economic burden, symptom experience, self-efficacy, and social support. The MLRs results were reported as odds ratios (ORs) and 95% confidence intervals (95% CIs).
This study was conducted with the approval of the Institutional Review Board of Ajou University Hospital (AJIRB-SBR-SUR-18–122). The study was conducted according to the principles expressed in the Declaration of Helsinki. Prior to the data collection, a research assistant explained the purpose and content of the study, study ethics, and data collection methods when the patients visited the outpatient clinic. The survey was conducted only with patients who understood the study process and provided written consent to participate.