H. Qi1, Q. Chen1, F. Yu1, D. Han1, M. Kang1, J. Zhou2, S. Lazarev3, N. Y. Lee4, M. K. Garg5, C. Hajj4, I. J. Choi1, S. Hasan1, A. M. Chhabra1, C. B. Simone II1, and H. Lin1; 1New York Proton Center, New York, NY, 2Department of Radiation Oncology, Winship Cancer Institute of Emory University, Atlanta, GA, 3Department of Radiation Oncology, Icahn School of Medicine at Mount Sinai, New York, NY, 4Memorial Sloan Kettering Cancer Center, New York, NY, 5Montefiore Einstein Comprehensive Cancer Center, Bronx, NY
Purpose/Objective(s): A spirometer-based respiratory management system has demonstrated high reproducibility for internal organs in proton therapy. This study explores free-breathing respiratory-gated delivery (FBRGT) experimentally. The effect of gating-window widths (GW) and breathing speed on dose distribution are assessed for single spot and IMPT delivery. Materials/
Methods: The cranial-caudal movement of a dynamic lung phantom (3-cm diameter target) was synchronized with a QA syringe to generate the airflow. 4DCT images were acquired with 5 or 2 cm peak-to-peak amplitudes for single-spot or IMPT deliveries. Beam directions were perpendicular to the motion, and the gating window was selected towards the end of the exhalation phase. To study the interplay effect of different GW, a single spot (8 Gy center dose) was repeatedly delivered on seven GW: from 0% (static) to 50% (gated with 10% step), and 100% (non-gated). For IMPT delivery, motion analysis was applied to determine the optimal gating window of 50% (< 1cm motion) for respiratory periods of 3 and 5 seconds and irregular breathing (over-exhale) scenarios. An averaged CT was reconstructed within the optimal GW. ITV was generated and overridden with an averaged HU of the target. The four-field IMPT plan (30 Gy in 5 fractions) was robustly optimized (SFO) on the phase-averaged CT with perturbations of 3 mm setup and 3.5% range uncertainties. Films were sandwiched inside the target and moved with the phantom during delivery under different scenarios. Results: Profiles for the single-spot film measurements showed that with an increase in GW width (e.g., 40% GW), the center of the dose shifted more towards inhalation (1 cm), the dose spread-out increased (55%), and the peak dose dropped (10%). The dose profiles for IMPT deliveries showed that the target was well covered for the static and 12 breaths/min scenarios but not for faster and irregular breathing conditions. The profile widths at 90%/50% of prescription dose level were 39.7/53.8, 38.9/54.5, 36.6/55.2, 36.2/54.9 and 30.5/57.9 mm for static, 12 breaths/min, 20 breaths/min, irregular breathing and non-gated scenarios, respectively. Due to the communication delay between the gating and delivery system, the dose shifted towards inhalation for faster breathing and irregular breathing scenarios, resulting in dose variation around ±10% at the plateau. The total beam-on times for four-field IMPT deliveries were 79 seconds for static delivery and 129/119 seconds for gated delivery of 20/12 breaths per minute. Conclusion: FBRGT is successfully demonstrated experimentally. Gated delivery achieves target coverage for free breathing treatment with the cost of a longer delivery time. A series of clinical validations are warranted in the future.