4D patient dose reconstruction using online measured EPID cine images for lung SBRT treatment validation

Purpose: This study aims to develop an EPID-guided 4D patient dose reconstruction framework and to investigate its feasibility for lung SBRT treatment validation. Methods: Both the beam apertures and tumor movements were detected based on the continuously acquired EPID images during the treatment. Instead of directly using the transit photon fluence measured by the EPID, this method reconstructed the entrance fluence with the measured beam apertures and the delivered MUs. The entrance fluence distributions were sorted into their corresponding phases based on the detected tumor motion pattern and then accumulated for each phase. Together with the in-room 4DCT taken before every treatment to consider the interfractional-motion, the entrance fluence was then used for the patient dose calculation. Deformable registration was performed to sum up the phase doses for final treatment assessment. The feasibility of using the transit EPID images for entrance fluence reconstruction was evaluated against EPID in-air measurements. The accuracy of 3D- and 4D-dose reconstruction was validated by experiments with a motor-driven cylindrical diode array for six clinical-SBRT plans. Results: The average difference between the measured and reconstructed fluence maps was within 0.16. The reconstructed 3D-dose showed a less than 1.4 difference for the CAX-dose and at least a 98.3 gamma-passing-rate (22 mm) for the peripheral dose. Distorted dose distributions were observed in the measurement with the moving phantom. The comparison between the measured and the reconstructed 4D-dose without considering temporal information failed the gamma-evaluation for most cases. In contrast, when temporal information was considered, the dose distortion phenomena were successfully represented in the reconstructed dose (97.6-99.7 gamma-passing rate). Conclusions: The proposed method considered uncertainties of the beam delivery system, the interfractional- and intrafractional-motion, and the interplay effect. The experimental validation demonstrates that this method is practical and accurate for online or offline SBRT patient dose verification.