Theme: Medical Physics 
InPhantom and WellType Ionization Chamber Calibration Techniques Comparison for Ir192 HDR BrachyTherapy  


The Welltype ionization chamber (WIC) is the standard method to measure the Air Kerma strength K_{a,100}for the Ir192 source, the aim of this study is to validate the K_{a,100} using PTW Al T9193 phantom.
For phantom measurement, Farmer chamber PTW 300121, with volume of 0.6 cc and electrometer PTW UNIDOS E were used. The chamber has a calibration factor in terms of N_{D,w}at a reference beam quality Co60.
The phantom is 20cm 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 HDR1000 plus with its electrometer used in this work.
the following formula was applied to calculate the K_{a,100}
Ka,100 = M_{PMMA }x N_{D,w }x K_{Q }x (1g_{w})^{1} x t_{a,w }x K_{wp x }K'_{zp }x K_{v }x K_{r }
Where MPMMA is the collected charge per time interval (nC/min), K_{Q} correction factor for beam quality, (1g_{w})^{1} correction factor for energy loss due to bremsstrahlung, t_{a,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, K_{v} correction factor for volume effect of the ionization chamber. K_{r} 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 K_{a,100} was measured for five Ir192 sources.
The ratio of K_{A,100,PMMA}/K_{A,100,HDR1000} 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 inphantom calibration method to the WIC. The K_{a,100} are estimated from readings of the ionization chamber calibrated in terms of N_{D, W}. The mean deviations were 0.63% ± 0.1. However, a high degree of agreement was demonstrated over the five sources.