Optimizing Cardiac Dose Sparing in Lung Cancer SBRT Through 4D-CT-Based Respiratory Phase Selection

Cardiac toxicity remains a significant concern in stereotactic body radiotherapy (SBRT) for lung cancer, particularly when tumors are adjacent to the heart. This study aimed to assess the dosimetric impact of different respiratory phases on heart dose using four-dimensional computed tomography (4D-CT), and to determine the optimal phase for gated SBRT planning. We retrospectively analyzed lung cancer patients treated with SBRT who had 10-phase 4D-CT scans. Cardiac dose metrics, including mean heart dose (Dmean), V30 (volume receiving ≥30 Gy), and V25 (volume receiving ≥25 Gy), were evaluated across all respiratory phases. The heart was delineated on each phase, and the internal target volume (ITV) and displacement were also recorded. Tumor location was considered to assess its influence on heart exposure. Statistical comparisons were performed using paired t-tests. All cardiac dose metrics showed a decreasing trend from inspiration (0-20%) to expiration (80- 90%). The lowest Dmean (19.2 Gy), V30 (38.1%), and V25 (25.6%) were observed during the end-expiratory phase (90%). This phase also demonstrated minimal heart displacement (2.1 mm) and the smallest ITV (60 cm³), suggesting enhanced stability. Tumors located in the lower lobes or central regions exhibited greater sensitivity to respiratory phase, especially during inspiration. Expiratory gating significantly reduced heart dose in these cases. End-expiration appears to be the most favorable respiratory phase for cardiac sparing in SBRT for lung cancer. The integration of 4D-CT-based respiratory gating into clinical workflows enables individualized treatment planning, reduced target margins, and improved delivery safety. Tumor location should also be considered when selecting the optimal respiratory phase to optimize dosimetric outcomes. Although the present analysis focused primarily on cardiac dosimetry, this emphasis is clinically justified, as the heart is a critical and dose-limiting organ at risk with a well-established impact on long-term survival in lung cancer patients. Optimizing cardiac dose through respiratory phase selection therefore represents a clinically meaningful step toward safer SBRT delivery. Nevertheless, comprehensive evaluation of additional organs at risk is required before these findings can be generalized to broader clinical practice.