SU‐E‐T‐334: Clinical Implementation of Gating and Dose Verification with Scanned Ion Beams at HIT

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Purpose: Medical physics aspects of gated irradiation to moving tumors using scanned ion beams. Methods: At our institute, more than 700 patients have been treated with proton and carbon ion beams since its start. The first patient has now been treated with the application of gating. For this purpose, the Anzai respiratory motion detection and gating system was interfaced with the treatment control system such as to control spill pausing. The patient presented with a liver tumor moving with peak‐to‐peak amplitude of ∼1cm due to breathing. Treatment planning was performed using the Siemens Syngo RT‐Planning system on a 3DCT dataset. The gating window was determined from 4DCT. Plan verification was performed in a motion phantom, consisting of a PMMA block equipped with a 3D set of 24 pinpoint chambers mounted on the Quasar moving platform. The breathing motion perpendicular to the beam was simulated based on the trajectory obtained from the Anzai system during 4DCT acquisition. Dose was measured at three conditions: static phantom, moving phantom with continuous beam delivery, and moving phantom with gated irradiation. The statically delivered dose distribution was used as a reference. Further, a 4D dose calculation was performed on the 4DCT dataset using a research treatment planning system. Results: The relative rms deviations in the homogenous dose region were ∼18% without gating, and ∼6% with gating. The 4D dose calculation showed a decrease in target coverage by ∼20% and ∼6%, respectively, for the 95% isodose line. Conclusions: Gating of a scanned carbon ion beam was applied for treatment of a moving liver tumor and a workflow was developed to verify the dose to be actually delivered. With respect to continuous dose delivery, the target dose improvement achieved due to gating is considered to be significant for this patient.