Feasibility of a Pair-Production Positron Annihilation Coincidence Counting during Photon Radiotherapy
Abstract
Purpose
PET imaging of positrons emitted during megavoltage radiotherapy has not been implemented due to concern of high-count rates. This computational study explores the feasibility of coincidence-counting experiments using two scintillators inside the linac vault by quantifying the rate of photons entering one scintillator.
Methods
The simulation was performed in GATE using a continuous point source emitting a 6.5 MV linac photon spectrum. Two field sizes (FS) were modeled, 10cmx10cm and 1cmx1cm, incident on a 30cmx30cmx30cm water cube at 90cm SSD. A 2.5cm diameter cylindrical LaBr3 scintillator was positioned 2.5 m from the water cube at a 90-degree scattering angle from the incident beam. Linac vault concrete shielding with inner dimensions of 5.5mx5.7mx2.8m and 3 feet thickness was included. Absorbed dose in the water with 2mmx2mmx2mm voxels and energy spectrum of photons entering the detector crystal were scored.
Results
For 2x109 primaries, a 10 cm central dose of 0.0107 cGy and 0.78 cGy was scored for 10cmx10cm and 1cmx1cm FS. A TMR(10cm, 10cmx10cm) of 0.775 and a 1cmx1cm OF of 0.756 helped estimate 1.45x1011 primaries/MU and 1.5x109 primaries/MU. A linac macropulse frequency of 60Hz for 100 MU/min and micropulse frequency of 3 GHz yields 7.2x105 micropulses/second was assumed. A rate of 5x10-5 and 3.22x10-5 photons entering the crystal/primary were scored, yielding, on average, 1 photon entering the detector every 1 ps and 3.01 ns during a macropulse for 10cmx10cm and 1cmx1cm FS.
Conclusion
Under these and assuming a typical scintillator decay time of approximately 20 ns, the simulated count rates exceeded conventional scintillator limits. In contrast, ultrafast scintillators, such as BaF2, with decay times below 1 ns, would make this experiment possible for a 1cmx1cm FS. Additional measures, such as shielding or collimating the scintillator and a smaller phantom, would reduce the expected count rates and improve experimental viability.