A Monte Carlo analysis of the feasibility of a 3D structure build up with silicon photodiodes for in vivo dosimetry in radiotherapy

This study investigates the capabilities of a novel 3D cube device comprising six BPW34S photodiodes (Vishay Semiconductors), one on each side, recently proposed for in vivo dosimetry. The study has been conducted by using the Monte Carlo simulation codes FLUKA-CERN, TOPAS and PENH. In the simulations, the 3D dosimeter has been situated on top of a water phantom and has been irradiated with a 60 Co beam and two clinical photon beams of 6 MV and 15 MV. The dependence of the detector response with the angle of incidence of the beam has been investigated. Simulations without the phantom have been done to study the effect of the backscattered radiation in the detector response. The effect of an encapsulation of the detector on the angular dependence ofits response has been also analyzed. In addition, the changes that the presence of the detector produces on the absorbed dose in the phantom have been studied. Similar results have been obtained with the three Monte Carlo codes. The angular dependence of the 3D cube detector is reduced with respect to that found in case a single photodiode is used as dosimeter. The results of the simulations are in overall agreement with preliminary experimental data recently published. In the simulations carried out including the encapsulation, the angular dependence of the dosimeter response is significantly reduced. Finally, it has been found that the presence of the 3D detector produces a non-negligible increase of the absorbed dose at the phantom surface. The 3D cube detector constructed with six photodiodes, one in each of its faces, shows a significantly lower angular dependence in comparison to a single photodiode. The angular dependence can also be strongly reduced by embedding the dosimeter in an encapsulation with the appropriate thickness. The increasing of the absorbed dose at the surface must be considered because of the possible effects it could produce in the skin of patients in clinical applications. • Monte Carlo study of the angular dependence of a photodiode 3D structure. • The influence of the charge particle equilibrium is investigated. • Changes in the depth dose due to the presence of the device are studied.