The retrospective cohort study aimed to describe the incidence of atrial fibrillation (AF) in patients with lung cancer and to identify predictive cardiac dosimetric parameters. The study included 239 patients with limited-stage small cell lung cancer (SCLC) and 321 patients with locally advanced non-small cell lung cancer (NSCLC) who received definitive chemoradiotherapy. The study found that incidental irradiation of the sinoatrial node (SAN) during chemoradiotherapy may be associated with the development of AF and increased mortality. The maximum dose delivered to the SAN (SAN Dmax) exhibited the highest predictive value for AF development in both cohorts. Increased SAN Dmax was significantly associated with poor overall survival in both cohorts. The coronary artery calcium score was automatically determined using a software (AVIEW CAC, Coreline Soft) and was expressed as the Agatston score. The results support minimizing radiation dose exposure to the SAN during radiotherapy planning and considering close follow-up for the early detection of AF in patients receiving thoracic irradiation.