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Comparative dosimetry in narrow high-energy photon beams
M Westermark1, J Arndt, B Nilsson
1Department of Medical Radiation Physics, Karolinska Institutet and Stockholm University, Sweden.
Physics in Medicine and Biology
|March 24, 2000
Summary
This study compared various dosimeters for narrow photon beams, finding significant differences in output factors and beam profiles. Detector characteristics influence accuracy, especially in small fields, impacting radiation therapy precision.
Area of Science:
- Medical Physics
- Radiation Oncology
- Dosimetry
Background:
- Accurate dosimetry is crucial for effective radiation therapy.
- Narrow photon beams present unique challenges for detector response.
Purpose of the Study:
- To compare the performance of different dosimeters in narrow photon beams (≥ 4 mm) at 6 and 18 MV.
- To evaluate dosimeter accuracy for dose distribution and output factor measurements.
Main Methods:
- Comparison of natural diamond, liquid ionization chamber, plastic scintillator, and two silicon diodes.
- Measurements included depth-dose curves, lateral beam profiles, and output factors.
- Dose rate and angular dependence were also assessed.
Main Results:
- Depth-dose distributions agreed well for diamond, scintillator, and single diode; double diode and liquid chamber showed minor deviations.
- Penumbra width varied, with finite-sized detectors broadening it and small, dense diodes potentially underestimating it.
- Output factors showed significant differences, with diodes overestimating and diamond detector showing field-size dependent deviations.
Conclusions:
- Detector characteristics (size, material, construction) significantly impact measurements in narrow photon beams.
- Silicon diodes may overestimate output factors due to low-energy photons and electron disequilibrium.
- Corrections for detector volume and Monte Carlo simulations are essential for accurate beam profile assessment.