An experimental MOSFET approach to characterize¹⁹²Ir HDR source anisotropy

Abstract

The dose anisotropy around a (192)Ir HDR source in a water phantom has been measured using MOSFETs as relative dosimeters. In addition, modeling using the EGSnrc code has been performed to provide a complete dose distribution consistent with the MOSFET measurements. Doses around the Nucletron 'classic' (192)Ir HDR source were measured for a range of radial distances from 5 to 30 mm within a 40 x 30 x 30 cm(3) water phantom, using a TN-RD-50 MOSFET dosimetry system with an active area of 0.2 mm by 0.2 mm. For each successive measurement a linear stepper capable of movement in intervals of 0.0125 mm re-positioned the MOSFET at the required radial distance, while a rotational stepper enabled angular displacement of the source at intervals of 0.9 degrees . The source-dosimeter arrangement within the water phantom was modeled using the standardized cylindrical geometry of the DOSRZnrc user code. In general, the measured relative anisotropy at each radial distance from 5 mm to 30 mm is in good agreement with the EGSnrc simulations, benchmark Monte Carlo simulation and TLD measurements where they exist. The experimental approach employing a MOSFET detection system of small size, high spatial resolution and fast read out capability allowed a practical approach to the determination of dose anisotropy around a HDR source.

Quality assurance during interstitial brachytherapy: in vivo dosimetry using MOSFET dosimeters

“…This could have been caused by displacement of the source within the applicator tube up to a distance of 0.32 mm inaccuracy. Discrepancies in measurements close to the source could be attributed to a systematic error in the response of the MOSFET resulting from maximal dose gradient within very short distance [ 7 ]. Due to the inaccuracy of the measured value, it is not possible to issue a solid conclusion regarding the contribution of changes within the implanted tissues as edema and tissue inflammation.…”

“…The electrical voltage applied to the MOSFET detector is 7 volts. Therefore, it was suitable for measuring the accurate dose value at a distance from the 192 Ir source down to 5 mm [ 7 ]. The accuracy of measurements of radiation dose using the MOSFET detector is ± 4%.…”

Physics-aspects of dose accuracy in high dose rate (HDR) brachytherapy: source dosimetry, treatment planning, equipment performance and in vivo verification techniques

“…These include thermoluminescent dosimeters [17–20], ionisation chambers [21], semiconductor diodes [18], MOSFETS [22, 23], radiochromic film [24–28] and Polymer or Fricke gel dosimetry [29]. Theoretical Monte Carlo (MC) calculations have also been extensively utilised [17, 18, 30–35].…”

Comparative dosimetry of GammaMed Plus high-dose rate 192 Ir brachytherapy source

“…The literatures related to the Monte Carlo (MC) simulation dosimetry of GammaMed Plus and other high-dose rate brachytherapy sources have been cited. [4–11] Ballester et al . have used GEANT code for the complete dosimetry of the GammaMed Plus HDR source.…”

“…have made a comparative dosimetry of Nucletron ‘classic’ 192 Ir HDR source by MOSFET measurements in a water phantom and using the EGSnrc code. [11]…”