Preliminary study for determination of distal dose edge by measuring 90-deg prompt gammas with an array-type prompt gamma detection system

A scanning-type prompt gamma measurement system, called prompt gamma scanner (PGS), was constructed and used to determine the relationship between the proton dose distribution and the longitudinal profile of the prompt gammas generated by the nuclear interaction from the proton beam passage in a medium. However, the PGS system entails insuperable difficulties when used in clinical proton therapy owing to its scanning process. In order to measure the prompt gamma distribution without the scanning process, it was proposed to develop an array-type prompt gamma measurement system that can measure the prompt gammas with a linear array of radiation detectors through multiple collimation slits. Prior to constructing a full-scale measurement system with many detectors and multiple data acquisition channels, a simplified prototype measurement system, using only one detector moving from one measurement location to the next, was constructed in the present study and applied to a 39-MeV proton beam. The results are very encouraging, as the prototype measurement system predicted the distal dose edge very accurately within a few millimeters of error despite the fact that the level of background gammas increased as a result of reduced collimator shielding.