Dosimetric Characteristics of Standard and Micro MOSFET Dosimeters as In-vivo Dosimeter for Clinical Electron Beam

The metal oxide semiconductor field effect transistor (MOSFET) dosimeters have recently become commercially available. The purpose of this study was to investigate the fundamental dosimetric characteristics of MOSFET dosimeters for clinical electron beams and to compare standard MOSFET and micro MOSFET dosimeters. In this study, five identical standard MOSFET (TN-502-RD) and micro MOSFET (TN-502-RDM) dosimeters were used for measurements. All measurements, with the exception of angular dependence, were performed in a slab-shaped PMMA phantom. For determining the angular dependence of MOSFET dosimeters, a cylindrically shaped PMMA phantom was used. Both MOSFET dosimeters showed excellent linearity against doses measured in the dose range of 50-600 cGy for a electron beam of 9 MeV. The reproducibility of all MOSFETs, excepted one standard MOSFET, was less than ±2 %. The dose-rate dependence of the two types MOSFET was within ±3 %. However, for the angular dependence, standard and micro MOSFETs show remarkable differences relative to gantry angles. This study shows the dosimetric characteristic of standard and micro MOSFET dosimeters for clinical electron beams. Both MOSFET dosimeters are suitable for dosimetry of electron beams in the energy range of 6 −20 MeV. However, the dose verification of radiation therapy using multidirectional electron beam treatments allows for better use of micro MOSFETs which have a reduced directional dependence compared to standard MOSFETs.