1116 - Assessing Dose Uncertainties in Cardiac SBRT due to Cardiorespiratory Motion: Photon vs. Proton Therapy Comparison

G. Li¹, W. Wei², Q. Xiao³, J. Li², X. Zhang², and S. Bai⁴; ¹Radiation Physics Center, State Key Laboratory of Biotherapy and Cancer Center, West China Hospital, Sichuan University, Chengdu, China, ²West china hospital, Sichuan university, chengdu, China, ³Radiotherapy Physics and Technology Center, Cancer Center, West China Hospital, Sichuan University, Chengdu, Sichuan, China, ⁴Division of Radiation Physics, Cancer Center, West China Hospital, Sichuan University, Chengdu, Sichuan, China

Purpose/Objective(s): To evaluate the influence of cardiorespiratory motion on the quality of Cardiac Stereotactic Body Radiation Therapy (CSBRT) plans in photon and proton therapies. This study aimed to quantify the dose uncertainties induced by cardiorespiratory motion and to assess the efficacy of the motion-encompassing method. Materials/

Methods: This retrospective study analyzed 12 patients with refractory arrhythmia who underwent CSBRT. A 25-Gy prescription dose was delivered to 95% of the Internal target volume(ITV) in a single fraction while ignoring setup errors to render cardiorespiratory motion effects fully visible. Patient-specific cardiac and respiratory motion characteristics were assessed through four-dimensional cardiac computed tomography (4DcCT) and 4DCT, respectively. The study utilized 4D dose reconstruction techniques with in-house Python scripts to evaluate the dose uncertainties caused by cardiorespiratory motion. All plans were normalized to cover 95% of the ITV. For the 4D dose, V₂₅ above 95% and D₉₅ exceeding 25 Gy were considered to meet the prescription.

Results: The study found considerable variability in cardiorespiratory motion characteristics, with respiratory motion generally showing greater displacement than cardiac motion. Specifically, cardiac pulsation led to maximum displacement (DMX) values between 0.2 and 0.8 cm, whereas respiratory motion resulted in DMX values ranging from 0.3 to 2.0 cm. This difference highlights the varying impacts of heartbeats and breathing patterns on the movement of the target and surrounding cardiac structures during treatment. In photon therapy, the average V₂₅ and D₉₅ values were 94.3% and 24.94 Gy in 4DcCT (P>0.05), and 93.5% and 24.66 Gy in 4DCT, respectively (P>0.05). Cardiac pulsation caused a slight decrease in V₂₅ by an average of 1% and up to 4%. Conversely, respiratory motion had a more significant effect on V₂₅, reducing it by an average of 2% and up to 11%.6 patients in the 4DCT and 8 in the 4DcCT failed to meet the prescribed requirements. The variations in homogeneity index (HI) and gradient index (GI) due to cardiorespiratory motions did not exhibit a significant difference (P>0.05), with respiratory motion causing more notable deviations. For proton therapy, the average V₂₅ and D₉₅ values were 94.4% and 24.99 Gy in 4DcCT (P>0.05) and 89.2% and 24.69 Gy in 4DCT (P <0.05), respectively. 11 patients in the 4DCT and 6 in the 4DcCT failed to meet the prescribed requirements. The dose uncertainties were significant for HI and GI in both 4DCT and 4DcCT (P <0.05).

Conclusion: Cardiorespiratory motion marginally affects photon therapy plans but has a significant impact on proton therapy. While motion-encompassing techniques can somewhat mitigate dose uncertainties in photon therapy, they are less effective in proton therapy, where significant dose deviations remain in some patients. This underscores the critical importance of 4D dose evaluations in ensuring the precision and safety of CSBRT, especially for proton therapy.