Theme: Medical Physics |
In-Phantom and Well-Type Ionization Chamber Calibration Techniques Comparison for Ir-192 HDR BrachyTherapy | ||||||
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The Well-type ionization chamber (WIC) is the standard method to measure the Air- Kerma strength Ka,100for the Ir-192 source, the aim of this study is to validate the Ka,100 using PTW Al T9193 phantom.
For phantom measurement, Farmer chamber PTW 30012-1, with volume of 0.6 cc and electrometer PTW UNIDOS E were used. The chamber has a calibration factor in terms of ND,wat a reference beam quality Co-60.
The phantom is 20-cm diameter with 4 holes at the edges for detector adaptors and one hole at the center for afterloading applicator. For the WIC measurements, Standard Imaging HDR-1000 plus with its electrometer used in this work.
the following formula was applied to calculate the Ka,100
Ka,100 = MPMMA x ND,w x KQ x (1-gw)-1 x ta,w x Kwp x K'zp x Kv x Kr
Where MPMMA is the collected charge per time interval (nC/min), KQ correction factor for beam quality, (1-gw)-1 correction factor for energy loss due to bremsstrahlung, ta,w is the mass energy absorption coefficients of air to water, Kwp correction factor for perturbation of radiation field between water and PMMA, K'zp correction factor for the influence of the PTW phantom, Kv correction factor for volume effect of the ionization chamber. Kr correction factor to account for the different distances between measurement (8cm) and reference point (100cm). All factors were calculated using Monte Carlo simulation EGSnrc code (Ubrich F. 2013). The Ka,100 was measured for five Ir-192 sources.
The ratio of KA,100,PMMA/KA,100,HDR-1000 is approximately constant from one source to another with a mean value equal to 0.997±0.005 (-0.28%).
The results underline the equivalence of in-phantom calibration method to the WIC. The Ka,100 are estimated from readings of the ionization chamber calibrated in terms of ND, W. The mean deviations were 0.63% ± 0.1. However, a high degree of agreement was demonstrated over the five sources.