A Study Of Experimental Measurement Of Dosimetric Parameters In HDR Ir-192 Source

The role of intracavitary brachytherapy (ICBT) with or without external beam radiation in the management of cervical caner is well established. High-dose-rate brachytherapy is the ICBT technique recommended by the American Brachytherapy Society. The application of Indium-192 in HDR afterloader equipment is a common practice in clinical brachytherapy. According to the updadate protocol of the TG43 one (TG-43U1), accurate dose distribution data of the brachytherapy sources in use should be obtained experimentally or/and by Monte Carlo (MC), to be used as input in the HDR treatment planning system. The purpose of this study is to obtain critical dosimetric parameters with Thermoluminescent (TLD). An improved phantom was used to perform a more efficient and accurate measurementA 30×30×0.6 cm3 polystyrene plate was prepared for the development of phantom. Small cylindrical holes to accommodate the TLD rods were drilled vertically through the plate along polar anglesθ=0～11π/12 at radial distances r=1.0,3.0,5.0 and 7.0 cm from the origin of the coordinate system. Holes with different radial distances lay in different sides of z-axis, which made it possible to obtain dose distribution of two radiuses in one radiation. Thus we would have better measurement efficiency and increase distances between adjacent holes. A hole for plastic catheter (2 mm in outer diameter) was drilled horizontally at mid-plane along the negative z-axis with its tip extending just beyond the coordinate origin. To provide full backscatter conditions, this slab was sandwiched between other polystyrene plates to build a 30×30×30 cm phantom.To avoid interTLD attenuation effects, polar TLD measurements were performed in one exposure only for two radial distances whose holes were located on two sides of the z-axis. TLD rods were placed in the holes along the polar angles at the radial distances of just measurement. First of all, D (1 cm,π/2), the dose at reference point, was measured. Secondly D (r,π/2), by which gL(r) was calculated, was measured at r=1～14 cm. Thirdly D (r,θ),by which F (r,θ) was calculated,was measured alongθ=0～11π/12 at r=1,3,5 cm. All the exposed TLD rods were numbered and were read on a FJ-427A1 TLD system.All data obtained by TLD were listed and compared with that by MC. Paired t test between Monte carlo calculations and TLD measurement is t=1.847 (p=088>0.05). the measured data agree well with Monte Carlo calculations, and no difference is seen between them.these dosimetric parameters can be used as input data or to verify the calculations of TPS used in HDR brachytherapy.