The development and commissioning of a respiratory-gated treatment system for proton therapy

Abstract

Proton therapy offers excellent dose conformality and reduction in integral dose. The superior dose distribution is, however, much more sensitive to changes in radiological depths along the beam path than for photon fields. Respiratory motion can cause such changes for treatments sites like lung, liver, and mediastinum structures, and thus affect the proton dose distribution significantly. We have developed and commissioned a respiratory-gated beam control system for our range-modulated treatment beam. A Real-time Position Management (RPM, Varian Inc, Palo Alto, CA) system was used to monitor the patient’s breathing cycle and to provide the gating signal, which is combined with the range modulation signal to generate the required gating control of the cyclotron beam current. The gating circuit has been designed to ensure the full modulation cycle is completed so that for any beam-on segment, the delivered dose has the exact planned SOBP distribution. This inevitably introduces a delay of the beam on/off up to the length of the modulation cycle of 100 ms. Using a motion phantom and time resolved measurement, we have measured the response delay of the cyclotron beam control relative to the RPM gating signal, as well as the delay of the RPM system itself relative to the actual mechanical motion of the phantom. It was found that while the cyclotron beam current response delay is less than 5 ms (the lower limit of the our measurement capability), the RPM system poses a delay of 60–90ms with an average of 75 ms. The total gating delay of the beam is thus in the range of 65 to 195 ms. Film test using a motion phantom was conducted to verify the overall gating results.