Optimization of a novel Cerenkov detector for radiotherapy applications using GEANT4 and FLUKA

GEANT 4 and FLUKA are two well-established Montecarlo (MC) simulation codes used in high energy and medical physics fields. The main goals of this work are to optimize the design of a novel Cerenkov detector for radiotherapy applications and to compare the output of the two MC codes. The detector is based on a lead glass instead of a conventional scintillator, the Cerenkov photons generated by the secondary electrons with an energy greater than Cerenkov threshold inside the lead glass are collected by an inclined paraboloidal mirror and then focused on an off axis CMOS detector. The dimensions of the proposed Cerenkov detector are quite compact and, thus, we recon that it can be placed below the LINAC multi-leaf collimator. This will allow the online verification of IMRT treatments. A first preliminary step involved the optimization of the geometry of the paraboloidal mirror with respect to the position of the CMOS detector and lead glass. In order to do that a parallel beam of optical photons coming from the lead glass were simulated and their focusing on the CMOS was optimized. The spectrum of the photons delivered by a clinical LINAC (Varian Clinac 2100) was included in both GEANT4 and FLUKA. A radiation beam containing 10
⁴
photons was simulated and the resulting beam shape derived from the two MC codes was compared. For GEANT4 and FLUKA the modelling of the electromagnetic interactions was made respectively using the EM Standard class and EMF command, to request a detailed electrons, positrons and photons transport. The same electromagnetic processes and the same energy cuts was considered in the simulation setup. Both MC allows the simulation of the transport and boundary effects for optical photons generated by Cerenkov photons in the lead glass. The paraboloidal mirror was simulated as a spike reflector with a reflectivity fixed to 95% and the CMOS active surface was set as dielectric-metal interface with an efficiency of 0.85.