Consideration of Surface Dose and Depth of Maximum Dose Using Various Detectors for High Energy X-rays

Lee, Yong Ha; Park, Kyung Ran; Lee, Jong Young; Lee, Ik Jae; Vahc, Young Woo; Lee, Kang Kyoo

Abstract

PURPOSE: It is difficult to exactly determine the surface dose and the dose distribution in buildup region of high energy X-rays by using the conventional ion chamber. The aim of this study is to evaluate the accuracy of widely used dosimetry systems to measure the surface dose and the depth of maximum dose (dmax).
MATERIALS AND METHODS
We measured the percent depth dose (PDD) from the surface to the dmax in either a water phantom or in a solid water phantom using TLD-100 chips, thimble type ion chamber, diode detector, diamond detector and Markus parallel plate ion chamber for 6 MV and 15 MV X-rays, 10 10 cm2, at SSD=100 cm. We analysed the surface dose and the dmax. In order to verify the accuracy of the TLD data, we executed the Monte Carlo simulation for 6 MV X-ray beams.
RESULTS
The surface doses in 6 MV and 15 MV X-rays were 29.31% and 23.36% for Markus parallel plate ion chamber, 37.17% and 24.01% for TLD, 34.87% and 24.06% for diamond detector, 38.13% and 27.8% for diode detector, and 47.92% and 36.01% for thimble type ion chamber, respectively. In Monte Carlo simulation for 6 MV X-rays, the surface dose was 36.22%, which is similar to the 37.17% of the TLD measurement data. The dmax in 6 MV and 15 MV X-rays was 14~16 mm and 27~29 mm, respectively. There was no significant difference in the dmax among the detectors.
CONCLUSION
There was a remarkable difference in the surface dose among the detectors. The Markus parallel plate chamber showed the most accurate result. The surface dose of the thimble ion chamber was 10% higher than that of other detectors. We suggest that the correction should be made when the surface dose of the thimble ion chamber is used for the treatment planning for the superficial tumors. All the detectors used in our study showed no difference in the dmax.