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Off-axis chamber response in the depth of photon dose maximum
Oliver S Dohm1, Matthias Fippel, Gunter Christ
1Sektion für Biomedizinische Physik, Universitätsklinik für Radioonkologie Tübingen, Hoppe-Seyler Str 3, 72076 Tübingen, Germany. ordohm@med.uni-tuebingen.de
Physics in Medicine and Biology
|March 31, 2005
Summary
Ionization chambers may underestimate radiation dose by up to 2.5% at shallow depths and off-axis positions. This underresponse is due to variations in the wall correction factor, impacting accurate dose measurements.
Area of Science:
- Medical Physics
- Radiation Dosimetry
Background:
- Ionization chambers are standard devices for measuring radiation dose.
- Accurate dose determination is critical in radiation therapy and diagnostic imaging.
Purpose of the Study:
- To investigate the deviation between dose to water and ionization chamber measurements.
- To quantify underresponse at specific depths and off-axis positions.
Main Methods:
- Performed measurements and Monte Carlo simulations.
- Evaluated deviations at depths of 1.5 cm and 10 cm.
- Assessed measurements at an off-axis position of 15 cm.
Main Results:
- Ionization chambers showed an underresponse of up to 2.5% compared to the dose to water.
- This deviation was observed at shallow depths and far off-axis positions.
- The primary cause identified was the variation in the wall correction factor.
Conclusions:
- The wall correction factor is sensitive to changes in radiation spectra with depth and position.
- Variations in the wall-originated dose to the air volume affect the correction factor.
- Accurate dose measurements require consideration of these positional and spectral effects.