Measurement of Dispersion of a Clinical Proton Therapy Beam

Abstract

Purpose: To measure dispersion of a clinical proton therapy beam. Methods and Materials: The proton center at our institution uses a Hitachi PROBEAT V synchrotron that delivers beams with energies ranging from 70 to 230 MeV to five treatment rooms, four of which have gantries and one fixed beam. For this study, the magnetic field strengths of all optics in the transport line were varied to measure the dispersion of 230 MeV beams in each of the four gantry rooms. Beam position was measured using the Spot Position Monitor (SPM), a multi-wire proportional counter (MWPC) in the nozzle approximately 0.5 meters upstream of the treatment isocenter. Measured beam positions as a function of momentum offset were fitted to a linear function to extract dispersion. In one gantry, dispersions for 70 and 140 MeV beams were also measured. Because spot position is continuously monitored during treatments and a position deviation will abort the beam delivery, the dispersion at the spot position monitor was used to set an upper limit of range uncertainty originating from momentum deviation. When dispersion was approximately zero, dose rate measurements were used to set an upper limit of momentum deviation that could reach the patient. Results: Dispersion was 0.45 m, 0.48 m and 0.30 m for 70 MeV, 140 MeV, and 230 MeV beams in a gantry, respectively, corresponding to 0.3 mm, 1.0 mm and 3.5 mm maximum range uncertainty during treatments. For 230 MeV beams, dispersion ranged from 0 to 0.73 m across the four gantries, corresponding to the upper limits of range uncertainty due to momentum deviation of 1.4 to 6.2 mm (0.4 to 1.9% in fractional range uncertainty). Conclusions: The measured dispersion was both energy- and gantry-dependent and ranged from 0 to 0.73 m.