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Electron disequilibrium in high-energy x-ray beams
D Hannallah1, T C Zhu, B E Bjärngard
1Department of Radiation Oncology, Roger Williams Medical Center, Brown University, Providence, Rhode Island 02908-4735, USA.
Medical Physics
|November 1, 1996
Summary
Monte Carlo calculations show scatter dose to kerma ratios (beta'S) differ from primary dose to kerma ratios (beta'p) for 15-MV x-ray beams. The approximation beta'S = 1 is clinically acceptable for dose calculations.
Area of Science:
- Medical Physics
- Radiation Oncology
- Computational Physics
Background:
- Accurate dose calculation in radiation therapy is crucial.
- Understanding the scatter dose to kerma ratio (beta'S) is important for 15-MV x-ray beams.
- Previous studies have focused on primary dose to kerma ratios (beta'p).
Purpose of the Study:
- To investigate the scatter dose to kerma ratio (beta'S) for 15-MV x-ray beams.
- To compare the behavior of beta'S with beta'p.
- To evaluate the clinical applicability of approximations for beta'S.
Main Methods:
- Utilized Monte Carlo simulations to calculate dose and kerma.
- Examined points on the central axis of 15-MV x-ray beams.
- Analyzed the spatial variation of scatter kerma.
Main Results:
- Beta'S exhibits different behavior compared to beta'p.
- Beta'S shows a higher surface value and requires greater depth for longitudinal electron equilibrium.
- The approximation beta'S = 1 results in <0.5% error in total dose for clinical calculations.
Conclusions:
- The approximation beta'S = 1 is sufficient for clinical dose calculations at 15 MV.
- Assuming beta'S = beta'p can lead to significant errors (up to 2.5%) near the surface.
- Spatial variations in scatter kerma influence the behavior of beta'S.