SU-F-T-162: Comparison of Beam Data for Two Varian ProBeam Pencil Beam Scanning Proton Systems.

PURPOSE: To compare the beam data for proton spot scanning beams for the first two Varian ProBeam sites in the US at the Maryland Proton Treatment Center (MPTC) and Scripps Proton Therapy Center (Scripps). To investigate how well beams can be matched between rooms.

METHOD: Beam data were acquired with independent dosimetry systems and compared for two sites. Integrated depth dose curves (IDDs) were acquired with large volume PTW Bragg peak chambers in a 3D water tank for pencil beams at both sites. Spot profiles were acquired at different distances from the isocenter at a gantry angle of 0 degrees as well as function of gantry angle.

RESULTS: IDDs for the two Varian ProBeam proton sites are similar, except in the plateau region were the Scripps data is on average 4.5% higher relative to each other, but only 1.5% relative to the maximum at the Bragg Peak. This increase in the plateau region decrease as energy increase and for energies larger than 180 MeV there is no marked difference. Range in water coincide for all energies within 0.5mm. The sigma of the spot profiles in air are within 10% of each other at isocenter. This difference increase as the detector distance from the isocenter is increased.

CONCLUSION: Beam data of the Varian ProBeam proton spot scanning beams for two sites are very well matched. Differences will not be clinically relevant.