Characteristics of mobile MOSFET dosimetry system for megavoltage photon beams

Abstract

The characteristics of a mobile metal oxide semiconductor field effect transistor (mobile MOSFET) detector for standard bias were investigated for megavoltage photon beams. This study was performed with a brass alloy build-up cap for three energies namely Co-60, 6 and 15 MV photon beams. The MOSFETs were calibrated and the performance characteristics were analyzed with respect to dose rate dependence, energy dependence, field size dependence, linearity, build-up factor, and angular dependence for all the three energies. A linear dose-response curve was noted for Co-60, 6 MV, and 15 MV photons. The calibration factors were found to be 1.03, 1, and 0.79 cGy/mV for Co-60, 6 MV, and 15 MV photon energies, respectively. The calibration graph has been obtained to the dose up to 600 cGy, and the dose-response curve was found to be linear. The MOSFETs were found to be energy independent both for measurements performed at depth as well as on the surface with build-up. However, field size dependence was also analyzed for variable field sizes and found to be field size independent. Angular dependence was analyzed by keeping the MOSFET dosimeter in parallel and perpendicular orientation to the angle of incidence of the radiation with and without build-up on the surface of the phantom. The maximum variation for the three energies was found to be within ± 2% for the gantry angles 90° and 270°, the deviations without the build-up for the same gantry angles were found to be 6%, 25%, and 60%, respectively. The MOSFET response was found to be independent of dose rate for all three energies. The dosimetric characteristics of the MOSFET detector make it a suitable in vivo dosimeter for megavoltage photon beams.

Advances in Radiotherapy Dosimetry Techniques and Pre-Treatment Verification

“…The "threshold voltage" required for these devices is very low, and there is no risk of electrical shock [69]. The readings obtained from MOSFET indicate consistent results, displaying high dose linearity and insignificant fading [70,71]. Zilio et al [72] conducted a comparison between the dosimetry result of MOSFET and Monte Carlo calculations and confirmed the accuracy of the MOSFET dosimeter as an absolute dosimeter for brachytherapy.…”

“…The thickness of the silicon dioxide insulator can be varied between 0.1 to 1 μm, which affects the MOSFET's sensitivity [67]. Using a MOSFET dosimeter has several benefits, including its ability to accurately measure radiation doses in areas with steep dose gradients and cases of electronic disequilibrium due to its small detector size, lightweight design that poses no harm to patients, low power requirements, user-friendly operation, and real-time online readout [68][69][70][71]. The threshold voltage of its gate varies in proportion to the amount of absorption of radiation.…”

Different dosimeters/detectors used in small-field dosimetry: Pros and cons

“…The major disadvantage of MOSFETs is their angular dependency that can lead to uncertainties in dosimetry. [ 64 65 ] A solution for this problem is obtaining quantitative correction factors in the commissioning stage. MOSFET represents a temperature dependency and need to the correction factor if applied at a temperature different from the temperature that are calibrated.…”

Análise de parâmetros π-híbridos de um MOSFET com encapsulamento TO-220 sob feixes de raios X para diagnóstico médico

“…Além disso, desde a década de 1980 o MOSFET é utilizado como detector de radiação ionizante, como dosímetro em radioterapia. O método para medir a dose em radioterapia é baseado na correlação entre a variação da tensão de limiar do MOSFET com a dose acumulada de radiação recebida pelo dispositivo [3][4][5][6][7][8]. A tensão de limiar é um parâmetro elétrico característico do MOSFET.…”