Optical range determination of clinical proton beams in water. A comparison with standard measurement methods.

As recently discovered, water emits a weak luminescence when it is irradiated with protons even with energies below the Cerenkov light threshold. In this work it was investigated if this phenomenon could be exploited for range measurements in proton therapy. A measurement setup based on a scientific CMOS camera that can be operated under normal room light was built and tested in a proof-of-principle experiment at the West German Proton Therapy Center, Essen. The luminescence depth profiles were analyzed to obtain the range information and the method was compared with ionization chamber based depth dose measurements. The noise caused by scattered radiation hitting the camera chip could be removed with a simple threshold-based median filter. The influence of Cerenkov radiation produced by delta electrons was analyzed by FLUKA simulations and it was shown that it does not affect the range measurements. It could be shown that the luminescence method is as fast as the multi-layer ionization chamber measurement (a few seconds) but with a higher depth resolution that is comparable with the Bragg peak chamber method. The proton ranges determined with the luminescence method agree with the reference methods better than 0.2% over the whole energy range 100-226MeV. The sensitivity of the method regarding detectable range shifts was tested. It was shown, that energy shifts of 0.5MeV (at 151MeV), leading to a range shift of ∼0.9mm, were clearly detectable.