WE‐E‐BRB‐03: Precision of Respiratory‐Gated Delivery of Synchrotron‐Based Proton Therapy

Purpose: To investigate the precision of synchrotron based passively scattered respiratory gated delivery of proton irradiation. A simulation study was undertaken for the analysis of the residual target motion uncertainty during synchrotron based respiratory gated proton treatment. Method and Materials: In‐house software was developed to simulate a synchrotron based respiratory gated proton treatment. The interactions of respiratory motion traces (70 traces, 22 patients), respiratory gate threshold levels (10%, 20% and 30% duty cycle around peak exhalation) and synchrotron T_cyc patterns (fixed T_cyc =2.7, 3.0 ∼ 6.0 second, average patient breathing‐cycle and variable T_cyc) were plotted along the same time scale, similar to an oscilloscope display. Proton beam delivery within a gate threshold only occurred during a portion of each T_cyc pattern. A specific pattern of T_cyc acts as a “gate‐within‐a‐gate”, which produces a smaller effective gating window. Precision of respiratory gated proton delivery was analyzed by examining distribution of distance from the respiratory gate threshold where 95% of gated beam delivery (DGT₉₅) occurred. Results: With shorter fixed T_cyc (< 4 sec), average DGT₉₅ values were 0.30 cm, 0.23 cm and 0.17 cm respectively for 30%, 20% and 10% respiratory gate duty cycles. With longer fixed T_cycs (> 4 sec) average DGT₉₅ values were 0.25 cm, 0.19 cm and 0.14 cm respectively for 30%, 20% and 10% respiratory gate duty cycles. With T_cyc ≈ average patient breathing cycle, average DGT₉₅ values were 0.27 cm, 0.20 cm and 0.15 cm for 30%, 20% and 10% respiratory gate duty cycles. With variable T_cyc, DGT₉₅ values were 0.21 cm, 0.17 cm and 0.14 cm for 30%, 20% and 10% respiratory gate duty cycles. Conclusion: Variable T_cyc mode offered the greatest precision of respiratory gated delivery for passively scattered synchrotron proton irradiation.