Effects of Respiratory and Cardiac Motion on Dose to the Left Anterior Descending Artery in Patients Undergoing Radiation Therapy for Locally Advanced Lung Cancer

Abstract

Radiation dose to the left anterior descending artery (LAD) appears to be an independent predictor of major cardiac events and all-cause mortality following radiation therapy (RT) for lung cancer. In this study, we characterize the dosimetric effects of respiratory (RM) and cardiac (CM) motion on LAD. Ten patients with stage IIIA-IVA lung cancer received cine MRIs (25 phases) at expiration which were used to evaluate the CM, and 4D CT scans for the evaluation of RM. Following registration of the expiration images of 4D CTs with MRIs, RT dose was transferred from planning CT datasets to all phases of 4D CTs and cine MRIs. LAD was manually contoured on all scans. The contours from each 4D phase represent LADs over multiple cardiac cycles, thus they are referred to as cLAD from here on. Displacements and dose variations of cLAD between exhale/inhale on 4D CTs and of LAD between systolic and diastolic phases on cine MRIs were analyzed, and paired t-tests were performed to assess the significance of the differences observed. The following metrics are reported averaged over all patients. Clinical treatment planning CT metrics and doses: Mean dose, max dose, V5 Gy and V15 Gy were 15.0 (±11.3) Gy, 31.1 (±22.1) Gy, 53 (±33)%, 35 (±30)%. The cLAD metrics and dose variations due to respiratory motion (between exhale and inhale): RM amplitude was 0.3±0.2cm (RL 0.3 ± 0.1 cm, AP 0.2 ± 0.2 cm, SI 0.3 ± 0.2 cm). The variations in mean dose, max dose, V5 and V15 were 4.0 (±4.0) Gy, 1.6 (±1.5) Gy, 4.4 (±3.4)%, 4.0 (±3.4)%. Mean, max, V5 Gy and V15 Gy increased or remained unchanged with expiration in 8, 9, 5 and 5 patients, respectively. LAD metrics and dose variations due to cardiac motion (between systole and diastole): CM amplitude was 0.2 ± 0.1 cm (RL 0.2 ± 0.1 cm, AP 0.2 ± 0.1 cm, SI 0.3 ± 0.1 cm). The variations in mean dose, max dose, V5 and V15 were 1.7 (±1.4) Gy, 1.2 (±0.9) Gy, 5.2 (± 6.7)%, 4.6 (± 4.0)%. Mean, max, V5 Gy and V15 Gy increased or remained unchanged with diastole in 7, 7, 8 and 8 patients respectively. Statistical significance: The differences in displacement and dose between respiration and cardiac motion were not statistically significant. Respiratory and cardiac-induced physiological variations of the LAD position have dosimetric consequences comparable in magnitude. Expiration and diastole lead to increased LAD doses. This underscores the importance of considering both the respiratory and the cardiac motion when motion management techniques are considered. 4D CTs inherently include the cardiac motion. However, if breath-hold is used for respiratory motion mitigation, CM might not be captured appropriately due to short tube rotation times. In this case, understanding CM characteristics is important for more accurate assessment of LAD dose.