Gafchromic EBT film dosimetry in proton beams.

The depth dose verification of active scanning proton beams is extremely time consuming with ion chamber measurements for beam data commissioning and patient specific measurements. With widespread use of Gafchromic EBT films, two-dimensional high-resolution dosimetry is explored in a uniform scanning proton beam. The EBT films were exposed parallel to the beam axis in a solid water phantom in order to obtain the depth-dose curve in a single measurement and compared with the gold standard measurement with a parallel plate ion chamber in water. Our results demonstrate that EBT films perform well in determining the proton beam range, with uncertainty of 0.5 mm. It is also found that EBT film response is a function of energy over the effective energy of 50-160 MeV proton beams with the variations less than 10%. However, an under-dosage of up to 20% was observed at the peak of the Bragg curve. An empirically derived correction factor is proposed to account for the EBT energy dependence. With corrections, EBT films can be a useful tool for the depth dose verification of active scanning proton beams, thus saving valuable proton beam time.