TITLE:
Measurement of Electron Return Effect and Skin Dose Reduction by a Bolus in an Anthropomorphic Physical Phantom under a Magnetic Resonance Guided Linear Accelerator (MR-LINAC) System
AUTHORS:
Eun Young Han, Zhifei Wen, Hannah J. Lee, Arnold dela Cruz Paulino, Choonsik Lee
KEYWORDS:
MR-Linac, Electron Return Effect, Skin Dose
JOURNAL NAME:
International Journal of Medical Physics, Clinical Engineering and Radiation Oncology,
Vol.7 No.3,
August
14,
2018
ABSTRACT: Background: Magnetic resonance image-guided radiation therapy (MR-IGRT) promises more precise and effective
radiation treatments compared to conventional IGRT by using real-time on-board
MR imaging. Under the influence of a magnetic field, however, secondary electrons
exiting a surface can be forced in a circular path and re-enter the medium,
resulting in dose increase at a beam-exit surface, called the electron return
effect (ERE). The purpose of the study is to compare the exit skin dose
computed by Monte Carlo dose calculation with measurements using an adult
anthropomorphic phantom and to measure the effect of skin dose reduction by
adding 1 cm-thick bolus. Method: The plan was compared with measurements
using an adult anthropomorphic phantom combined with radiochromic films and
thermoluminescent dosimeters. We also measured the skin dose reduction by
adding 1 cm-thick bolus on the frontal surface of the phantom. Results: We found that 1 cm-thick bolus reduced the skin dose by up to 20% both in
measurements and calculations. The plan was found to overestimate the measured
skin dose by about 10% and there was no significant difference in the bolus
effect between the breast skin and the skin (without breast attachment) doses. Conclusion: In conclusion, we confirmed the ERE effect on the anthropomorphic phantom under
the magnetic field and the exit
skin dose reduction by adding a bolus. Skin dose measurements using
anthropomorphic phantom may be helpful to evaluate more realistic skin dose and
the bolus effect in the magnetic field.