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Transmission measurements in air using the ESTRO mini-phantom
S A Johnsson1, C P Ceberg, T Knöös
1Radiation Physics, Lund University Hospital, Sweden. stefan.johnsson@skane.se
Physics in Medicine and Biology
|October 26, 1999
Summary
Using the ESTRO mini-phantom for in-air primary kerma measurements is inaccurate. Relative measurements deviate significantly from free-in-air values due to beam hardening effects.
Area of Science:
- Medical Physics
- Radiation Dosimetry
Background:
- Accurate measurement of primary kerma in water is crucial for radiotherapy.
- Transmission measurements using mini-phantoms are explored as a method for in-air dosimetry.
Purpose of the Study:
- To evaluate the feasibility of using the ESTRO mini-phantom for transmission measurements of primary kerma in water, free in air.
- To assess the accuracy of in-air equivalent measurements using a water-equivalent mini-phantom.
Main Methods:
- Monte Carlo simulations using the EGS4 code were performed.
- Simulations covered four photon energies (4, 6, 10, 18 MV) in narrow beam geometry.
- Dose and primary kerma were scored within the mini-phantom.
Main Results:
- Relative measurements at 10 cm depth in the mini-phantom showed significant deviations (up to 7% at 4 MV) from free-in-air primary kerma.
- Deviations were minimized at depths exceeding the depth of dose maximum.
- Beam hardening in the water absorber caused changes in attenuation and scatter, leading to observed discrepancies.
Conclusions:
- The ESTRO mini-phantom is not suitable for accurate transmission measurements of primary kerma in water free in air.
- Beam hardening effects significantly impact the accuracy of these measurements.
- Careful consideration of phantom depth and beam characteristics is necessary for reliable dosimetry.