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Output factor measurements for a kilovoltage X-ray therapy unit
B J Healy1, A Gibbs, R L Murry
1Southern Zone Radiation Oncology Service--Mater Centre, Princess Alexandra Hospital, Brisbane, Australia. brendan_j_healy@health.qld.gov.au
Australasian Physical & Engineering Sciences in Medicine
|August 3, 2005
Summary
This study evaluated measurement tools for superficial radiation therapy. Solid water phantoms and specific ionization chambers are suitable for output factor and depth dose measurements, though chamber energy dependence requires consideration.
Area of Science:
- Medical Physics
- Radiation Oncology
Background:
- Accurate dosimetry is crucial for superficial radiation therapy.
- Evaluating phantom materials and ionization chambers is essential for reliable output factor and percentage depth dose measurements.
Purpose of the Study:
- To assess the suitability of various phantom materials and ionization chambers for measuring output factors and percentage depth doses.
- To investigate discrepancies in measurements using different equipment for the Pantak Therapax SXT 150 unit.
Main Methods:
- Measurements of output factors and percentage depth doses were performed using a Pantak Therapax SXT 150 unit.
- Various phantom materials (Solid Water, Plastic Water, Perspex) and two ionization chambers (PTW Markus, NE 2532/3) were tested.
- X-ray beam qualities ranged from 1 to 13 mm Al HVL.
Main Results:
- Solid Water demonstrated good water equivalence, with discrepancies <2.3% for PDD and <0.6% for output factors.
- Larger discrepancies were observed with Plastic Water and Perspex.
- The PTW Markus chamber showed good agreement for PDD but up to 5% discrepancy for output factors with the NE 2532/3 chamber, especially for small fields.
- The Markus chamber exhibited an energy-dependent response in the kilovoltage range.
Conclusions:
- Solid Water is a suitable phantom material for superficial therapy dosimetry.
- The PTW Markus chamber's energy-dependent response can lead to significant output factor measurement discrepancies, particularly in small fields.
- Careful selection and calibration of measurement equipment are necessary for accurate superficial radiation therapy dosimetry.