The South African national calibration facility is currently not equipped for mammography,
dose-measuring equipment. A therapy X-ray machine was used as a calibration unit at the
national secondary standards dosimetry laboratory (SSDL) for medium and low energy X-ray,
therapy calibrations. It is not necessarily intuitive that the latter calibrations are applicable to
diagnostic X-ray beams generated by high frequency generators. The response of measuring
equipment calibrated in a therapy X-ray beam, compared to its response in a diagnostic or
clinical mammography unit, is unknown.
The aim of the research was to investigate whether there was a measurable difference between
the X-ray beam qualities available for low energy diagnostic radiology and radiation therapy,
i.e. up to 100 kV. The beam qualities studied included both mammography and conventional
diagnostic radiography, i.e. nominally 20 kV to 100 kV. The diagnostic and therapy X-ray
tubes under investigation had different target-filter combinations, inherent filtration and
theoretically, different X-ray spectra.
Practically, spectrometry of X-ray beams is not possible because of the sophistication of the
instrumentation, comprehensive analyses being very time consuming and not practically
applicable to the clinical environment (Kharatti and Zarrad, 2003). Furthermore, not all
SSDL’s or Hospitals have access to spectral analysers. Clinical beam quality is instead
specified in terms of both the tube peak voltage and the half-value layer (HVL), the thickness
of material that will reduce the maximum output of the X-ray beam to 50%.
The goal was to compare measured HVLl’s to the ones recommended by the International
Electro technical Commission (IEC-61267, 2005) for available mammography beam qualities.
The method was validated using attenuation curves. The attenuation curves were then used to
derive the suitability of the X-ray spectra for calibration of mammography ionisation chambers
(Waggener and Blough, 1999). One of the low energy therapy units was found to be suitable
for introducing a regional calibration service for mammography.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/5767 |
| Date | 20 October 2008 |
| Creators | Steenkamp, Maria |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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