An optimized calibration method for surface measurements with MOSFETs in shaped-beam radiosurgery

Abstract

Nowadays MOSFET dosimeters are widely used for dose verification in radiotherapy procedures. Although their sensitive area satisfies size requirements for small field dosimetry, their use in radiosurgery has rarely been reported. The aim of this study is to propose and optimize a calibration method to perform surface measurements in 6 MV shaped-beam radiosurgery for field sizes down to 18 × 18 mm(2). The effect of different parameters such as recovery time between 2 readings, batch uniformity and build-up cap attenuation was studied. Batch uniformity was found to be within 2% and isocenter dose attenuation due to the build-up cap over the MOSFET was near 2% irrespective of field size. Two sets of sensitivity coefficients (SC) were determined for TN-502RD MOSFET dosimeters using experimental and calculated calibration; the latter being developed using an inverse square law model. Validation measurements were performed on a realistic head phantom in irregular fields. MOSFET dose values obtained by applying either measured or calculated SC were compared. For calibration, optimal results were obtained for an inter-measurement time lapse of 5 min. We also found that fitting the SC values with the inverse square law reduced the number of measurements required for calibration. The study demonstrated that combining inverse square law and Sterling-Worthley formula resulted in an underestimation of up to 4% of the dose measured by MOSFETs for complex beam geometries. With the inverse square law, it is possible to reduce the number of measurements required for calibration for multiple field-SSD combinations. Our results suggested that MOSFETs are suitable sensors for dosimetry when used at the surface in shaped-beam radiosurgery down to 18 × 18 mm(2).

In-vivo dosimetry for conformal arc therapy using several MOSFET in stereotactic radiosurgery computed by an inverse model

“…At 18×18 mm², the attenuation is maximal using Constraint 2, where all the detectors are positioned at the beam entry. The attenuation thus becomes comparable with that attained with fixed beams [25]. For comparison, the attenuation of OSLs equipped with electronic water-equivalent caps was estimated at 4.6 %, and for the diodes the attenuation varies between 5.4 % and 12 % [31].…”

“…The constraints were investigated from 1 to 4 for 30×30 mm² field size, and from 4 to 1 for the 18×18 mm² field size. MOSFETs (calibrated by the optimised calibration method previously described) were readout 30 s after each irradiation in order to obtain a stable readout without fading effect [25]. For all constraint geometries the time between two consecutive measurements was 3.5 minutes and never more than 5 minutes.…”

“…The detectors were calibrated [25] at the surface of phantom XWU-IMRT (ρ = 1.18 g.cm -3 ; 20×20×20 cm…”

Surface Radiation Dose Measurement with Mosfet for Head and Neck Cancer using Phantom

“…Electronhole pairs are formed by incident radiation within the depletion layer [10]. Electron mobility in SiO2 at room temperature is around four orders larger than holes, move rapidly out of the gate electrode while holes move stochastically to the Si/SiO 2 interface where they are stuck in long-term locations, creating a negative voltage shift (VTH) that can persist for years [11]. The difference in the voltage change can be measured before and after exposure and is proportional to the dose.…”