Asri, Nurul Hazwani Mohd
(2016)
Measurement of backscatter factors for static x-ray machine
using gafchromic film xr-qa2 and ionization chamber.
Technical Report.
Universiti Sains Malaysia.
(Submitted)
Abstract
A backscatter factor (BSF) was important parameters in the determination of dose for
kilovoltage X-ray beams. Various parameters that affected the contribution of BSF such as tube
voltage (kVp), phantom materials and field sizes were studied. The objectives were to study
dosimetry in diagnostic radiology range for Philips Optimus 80 machine based on TRS 457
protocol and to compare BSF value between ionization chamber and Gafchromic XR-QA2 film
by using PMMA and solid water phantom. The effective energy was measured to define the
monoenergetic beam of photons that has the same penetrating ability as the spectrum of photons.
For the film study, a piece of films with the size of 5x5 cm2 were exposed at the surface of
phantoms and in-air measurement for different field sizes. Net optical density (OD) and dose
(mGy) were determined in various field sizes for each phantom material. By using the formula,
the BSF value was obtained. The result from ionization chamber demonstrated that the bigger
field size will give higher BSF due to the contribution of scatter inside the area. As comparison
with TRS 457, in 10x10 cm2 field size, the percentage different (%) were within 18% and for
field size 20x20 cm2 was 9%. The material of phantom can affected the BSF value; PMMA
phantom (p = 1.190) gave different percentage 6% of BSF compared to solid water phantom (p =
1.00). The XR-QA2 film showed fluctuate result at the small field size because less effective
energy produced and the amount of photons scattered back to surface was lower. This
demonstrated that, XR-QA2 film was not suitable for small field size. This is because less
effective energy passes through the collimator and probability of photons to scatter back to the
surface of film was low compared to big field sizes.
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