A novel technique for measuring the low-dose envelope of pencil-beam scanning spot profiles.

To investigate the profile measurement capabilities of an IBA-Dosimetry scintillation detector and to assess its feasibility for determining the low-intensity tails of pencil-beam scanning spots, the responses of the scintillation detector and Gafchromic EBT2 film to a 115 MeV proton spot were measured in-air at the isocenter. Pairs of irradiations were made: one lower-level irradiation insufficient to cause saturation, and one higher-level irradiation which deliberately saturated the central region of the spot, but provided magnification of the tails. By employing the pair/magnification technique, agreement between the film and scintillation detector measurements of the spot profile can be extended from 4% of the central spot dose down to 0.