Energy dependence corrections to MOSFET dosimetric sensitivity

Abstract

Metal Oxide Semiconductor Field Effect Transistors (MOSFET's) are dosimeters which are now frequently utilized in radiotherapy treatment applications. An improved MOSFET, clinical semiconductor dosimetry system (CSDS) which utilizes improved packaging for the MOSFET device has been studied for energy dependence of sensitivity to x-ray radiation measurement. Energy dependence from 50 kVp to 10 MV x-rays has been studied and found to vary by up to a factor of 3.2 with 75 kVp producing the highest sensitivity response. The detectors average life span in high sensitivity mode is energy related and ranges from approximately 100 Gy for 75 kVp x-rays to approximately 300 Gy at 6 MV x-ray energy. The MOSFET detector has also been studied for sensitivity variations with integrated dose history. It was found to become less sensitive to radiation with age and the magnitude of this effect is dependant on radiation energy with lower energies producing a larger sensitivity reduction with integrated dose. The reduction in sensitivity is however approximated reproducibly by a slightly non linear, second order polynomial function allowing corrections to be made to readings to account for this effect to provide more accurate dose assessments both in phantom and in-vivo.

Small field electron beam dosimetry using MOSFET detector

“…MOSFET dosimeters have been widely studied for photon beam dosimetry ( 8 – 14 ) and have been used extensively for in vivo clinical dose measurements, ( 11 , 12 , 15 – 19 ) as well as for small field photon dosimetry. ( 20 ) MOSFET systems have recently been investigated for use in electron beam dosimetry, ( 21 – 23 ) as well.…”

Development of an applicator for eye lens dosimetry during radiotherapy

“…[18][19][20][21] Since previous studies have demonstrated that the energy dependency of MOSFET dosemeters is negligible in the energy range of this study, it was not evaluated. [23][24][25][26] Full paper: Applicator development of eye lens dosemeter BJR As shown in the results, the measured dose on the surface of the eyelid does not accurately represent the delivered dose to the lens. The difference between the measured dose at the surface and the calculated lens dose could be up to 35 cGy for a single fraction delivery.…”

Lack of backscatter factor measurements in HDR applications with MOSkins

“…3). MOSFETs are well known to have an energy response [4,[7][8][9]. The spectrum from an Ir-192 source changes according to its depth in water and the presence of backscatter material.…”

“…Long term stability is an important consideration for MOSFETs as they are known to display a decreased response with total dose accumulated by the detector [7,9,10]. Long term stability was measured by comparing measurements from three MOSkins, from the same batch, at the start and end of a sequence of measurements using the Ir-192 source.…”

“…The dose delivered, both in the presence and absence of backscatter material, was measured using MOSkins MOSFETs are well known to vary in response due to a range of physical properties such as accumulated dose and angular and energy response [4][5][6][7][8]. To estimate the uncertainty and/or determine appropriate correction factors to be applied in the backscatter measurements, we first determined the change in response of the MOSkins under a variety of conditions.…”