Energy and integrated dose dependence of MOSFET dosimeter sensitivity for irradiation energies between and

Abstract

Since metal-oxide-semiconductor field effect transistors (MOSFETs) medical applications in radiotherapy and radiology are gaining popularity, evaluating them under radiation of different energies is of major interest. This study aims at a characterization of MOSFET sensitivity with regard to total integrated dose. Sensitivity is expressed by the water calibration factor (CFw) and allows the user to associate the voltage difference reading displayed by the device to a dose value in water at the MOSFET location. The CFw of seven p-type dual-bias MOSFETs were measured for several accumulated doses. The radiation sources used were a 60Co unit ((E)gamma: 1.25 MeV), an 192Ir high dose rate unit ((E)gamma: 380 keV), and an orthovoltage unit providing two x-ray energy spectra for tube voltages of 30 kV((E)gamma:14.8 KeV) and 150 kV((E)gamma:70.1 keV). The CFw value diminishes with increasing threshold voltage, especially for low-energy radiation. It was stable for 60Co irradiations, while it decreased 6%, 5%, and 15% for beam energies of 192Ir, 150 kV, and 30 kV, respectively. The decrease rate is higher for the first half of the device lifetime. This behavior is explained by an alteration of the effective electric field applied to the MOSFET during irradiation, caused by the accumulation of holes at the Si-SiO2 interface. It is strongly dependent on the nature of the radiation, and particularly affects low x-ray energies. A frequent calibration of the device for this radiation type is essential in order to achieve adequate measurement accuracy, especially in low-energy applications, such as superficial therapy, brachytherapy, and diagnostic and interventional radiology.

Energy dependence corrections to MOSFET dosimetric sensitivity

“…One aspect of MOSFET dosimetry is an inherent decrease in sensitivity with applied dose that is seen for any given dosimeter. This is known to be caused by alterations in the effective electric field applied to the MOSFET during irradiations which causes an accumulation of holes at the Si-SiO2 interface 28 . Depending on the performance characteristics of the MOSFET device, eg, dual versus singular bias voltage) this effect can be larger or smaller for an individual type of device 29 .…”

“…Another aspect of MOSFET dosimetry is the sensitivity dependence to integrated dose history of a MOSFET device. It is known in general that the sensitivity of the device can change with dose history [28][29] . We also plan to investigate this effect using the newly designed CSDS MOSFET devices at superficial, orthovoltage and megavoltage x-ray energies for comparison.…”

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.…”

In Vivo Dosimetry Using a Linear Mosfet-Array Dosimeter to Determine the Urethra Dose In 125I Permanent Prostate Implants

“…The reproducibility of the MOS-FET detector depends on the energy of the radiation and the integrated dose measured by the MOSFET detector. Lavallee et al (10) reported uncertainties of about 6% (1 SD) using an orthovoltage beam with a mean energy of 30 keV and dose values of about 55 cGy. During the clinical measurements, the uncertainty may be even larger because of the lower signal and positional inaccuracies of the detector.…”

“…The calibration coefficient, which converts the radiation-induced voltage shift of the detector into dose, decreases with increasing energy in the keV range (10). Therefore, the energies used for the clinical measurements should be used to assess the calibration coefficient in this low energy range.…”