A portable secondary dose monitoring system using scintillating fibers for proton therapy of prostate cancer: A Geant4 Monte Carlo simulation study

Abstract

Purpose: The main purpose of this study was to monitor the secondary dose distribution originating from a water phantom during proton therapy of prostate cancer using scintillating fibers. Methods: The Geant4 Monte Carlo toolkit version 9.6.p02 was used to simulate a proton therapy of prostate cancer. Two cases were studied. In the first case, 8 × 8 = 64 equally spaced fibers inside three 4 × 4 × 2.54 cm 3 Delrin® blocks were used to monitor the emission of secondary particles in the transverse (left and right) and distal regions relative to the beam direction. In the second case, a scintillating block with a thickness of 2.54 cm and equal vertical and longitudinal dimensions as the water phantom was used. Geometrical cuts were implemented to extract the energy deposited in each fiber and inside the scintillating block. Results: The transverse dose distributions from the detected secondary particles in both cases are symmetric and agree to within <3.6%. The energy deposited gradually increases as one moves from the peripheral row of fibers towards the center of the block (aligned with the center of the prostate) by a factor of approximately 5. The energy deposited was also observed to decrease as one goes from the frontal to distal region of the block. The ratio of the energy deposited in the prostate to the energy deposited in the middle two rows of fibers showed a linear relationship with a slope of (-3.55±2.26) × 10 -5 MeV per treatment Gy delivered. The distal detectors recorded a negligible amount of energy deposited due to higher attenuation of the secondary particles by the water in that direction. Conclusion: With a good calibration and with the ability to define a good correlation between the radiation flux recorded by the external fibers and the dose delivered to the prostate, such fibers can be used for real time dose verification to the target. The system was also observed to respond to the series of Bragg Peaks used to generate the Spread Out Bragg Peak inside the water phantom. Such Bragg Peaks were detected by the fibers. The energy deposited inside the lateral blocks were also observed to decrease as one goes away from the beam nozzle due to increased attenuation.