Development of Phantom and Comparison Analysis for Performance Characteristics of MOSFET Dosimeter

Chung, Jin Beom; Lee, Jeong Woo; Kim, Yon Lae; Lee, Doo Hyun; Choi, Kyoung Sik; Kim, Jae Sung; Kim, In Ah; Hong, Semie; Suh, Tae Suk

Abstract

This study is to develope a phantom for MOSFET (Metal Oxide Semiconductors Field Effect Transistors) dosimetry and compare the dosimetric properties of standard MOSFET and microMOSFET with the phantom. In this study, the developed phantom have two shape: one is the shape of semi-sphere with 10 cm diameters and the other one is the flat slab of 30 cm x 30 cm with 1 cm thickness. The slab phantom was used for calibration and characterization measurements of reproducibility, linearity and dose rate dependency. The semi-sphere phantom was used for angular and directional dependence on the types of MOSFETs. The measurements were conducted under 10 x 10 cm2 fields at 100 cm SSD with 6 MV photon of Clinac (21EX, Varian, USA). For calibration and reproducibility, five standard MOSFETs and microMOSFETs were repeatedly irradiated by 200 cGy five times. The average calibration factor was a range of 1.09+/-0.01~1.12+/-0.02 mV/cGy for standard MOSFETs and 2.81+/-0.03~2.85+/-0.04 mV/cGy for microMOSFETs. The response of reproducibility in the two types of MOSFETs was found to be maximum 2% variation. Dose linearity was evaluated in the range of 5 to 600 cGy and showed good linear response with R2 value of 0.997 and 0.999. The dose rate dependence of standard MOSFET and microMOSFET was within 1% for 200 cGy from 100 to 600 MU/min. For linearity, reproducibility and calibration factor, two types of MOSFETs showed similar results. On the other hand, the standard MOSFET and microMOSFET were found to be remarkable difference in angular and directional dependence. The measured angular dependence of standard MOSFET and microMOSFET was also found to be the variation of 13%, 10% and standard deviation of +/-4.4%, +/-2.1%. The directional dependence was found to be the variation of 5%, 2% and standard deviation of +/-2.1%, +/-1.5%. Therefore, dose verification of radiation therapy used multidirectional X-ray beam treatments allows for better the use of microMOSFET which has a reduced angular and directional dependence than that of standard MOSFET.