Introduction <p>Loss to follow-up after radiotherapy presents a critical challenge in cancer care, undermining treatment effectiveness and efficient use of healthcare resources. Understanding predictors of follow-up non-adherence in the Thai context is essential to improving patient outcomes.</p> Research objective <p>This study aimed to determine the rate, underlying causes, and predictive factors associated with loss to follow-up appointments among cancer patients after completing radiotherapy.</p> Methods <p>A predictive correlational design was employed with 294 cancer patients who had completed radiotherapy and were scheduled for follow-up appointments. Participants were selected using systematic random sampling. Data were collected through questionnaires, medical record reviews, and telephone interviews for patients who missed appointments. Research instruments included a personal and clinical data form, a radiotherapy service quality assessment based on the SERVQUAL model, and a researcher-developed questionnaire assessing knowledge of follow-up care. Data analysis involved descriptive statistics, chi-square tests, Spearman’s correlation, and multiple logistic regression.</p> Results <p>The loss to follow-up rate was 20%. The most common reasons were forgetting appointments (38%), feeling unwell or bedridden (21%), and hospitalization (13%). Multivariate analysis identified two significant predictors: distance from residence to hospital (OR = 1.011, 95% CI 1.003–1.018, <i>p</i> = 0.007) and Eastern Cooperative Oncology Group (ECOG) performance status (OR = 1.973, 95% CI 1.355–2.871, <i>p</i> &lt; 0.001).</p> Conclusion <p>Distance to hospital and poorer physical performance status are key predictors of loss to follow-up. Interventions such as telemedicine, multi-channel reminder systems, and case management for high-risk patients are recommended to strengthen continuity of care and reduce missed appointments.</p>