Detemination of Distal Dose Edge with Array-type Prompt Gamma Detection System for 39 MeV Proton Beam

Abstract

To verify experimentally the distal dose edge or proton beam range, an array-type prompt gamma measurement system was constructed and applied to a 39 MeV proton beam. The proton beam was delivered to a water phantom and then the prompt gammas, generated in the passage of the proton beam were measured with an array-type prompt gamma detection system` The measurement system Is composed of a collimator with multiple collimation slits, an CsI(Tl) scintillator and a photodiode, and lead shielding` The distal dose edge was also measured with a plane-parallel ionization chamber and, then, the result was compared with the distribution of the prompt gammas. The prompt gamma energy spectra around the distal dose edge was measured and compared for the validity of the energy window of 4-10 MeV. The distribution of the prompt gammas, measured by the array-type prompt gamma detection system, sharply decreased at the distal dose edge, within the range of a few millimeters. The effect of the background gammas was not very significant at least [01` the 39 MeV proton beam case. The results were very encouraging, and demonstrated that the distal dose of the proton beam in the patient can be determined by directly measuring the distribution of the prompt gammas with an array-type prompt gamma detection system.