Radiation shielding assessment of high-energy proton imaging at a proton therapy facility

Proton imaging makes use of high-energy, low-intensity proton beams that fully traverse the patient and has been suggested to reduce range uncertainty in proton therapy. Upright patient positioning with proton imaging is being considered for a fixed beam room of a new proton therapy facility currently under construction. Considering that the yield and energy spectrum of secondary radiation from high-energy proton beams is proton beam energy dependent, an assessment of radiation shielding at the energies required for proton imaging should be performed prior to use. Furthermore, NCRP 144 recommends that pion production be considered for proton energies greater than 300 MeV, which are not typically utilized for proton therapy but may be required for proton imaging.

The purpose of this work was to determine whether proton treatment and imaging with an upright patient positioning system on a fixed beamline were acceptable from a radiation shielding perspective. This is the first report on radiation shielding assessment of proton imaging applications and includes consideration of pion production at the proton beam energy of 330 MeV.

The Geant4 Monte Carlo toolkit was used for the radiation shielding assessment. The calculations consisted of the generation of secondary particle phase-space files by simulating the passage of high-energy proton beams in two target materials, and subsequent simulation of the secondary particles in the proton therapy facility geometry. Particle fluence was converted to operational and protection radiation safety quantities with a custom python script for assessment of instantaneous and annual doses, respectively.

The total yields of pions from a 330-MeV proton beam were many orders of magnitude less than that of neutrons and photons. Three-dimensional maps of ambient dose rate for a 330-MeV proton beam showed doses arising from secondary neutrons and photons far exceed those arising from pion production. Incorporating representative annual workloads into the calculation demonstrated that proton imaging doses outside the shielded area were negligible compared to those arising from proton therapy.

Pion production has a negligible impact on the radiation shielding of proton imaging at 330 MeV relative to neutron and photon production. Radiation shielding designed for proton therapy is adequate for high-energy proton imaging applications.