The Comparison the Output dose of a Linear Accelerator Photon Beams by Using the Ionization Chamber IC-10 Series Through TG-21 Protocol to Postal Monitoring Output of MDACC for One and Half Decade Tae Jin Choi, Ph.D., Jin Hee Kim, M.D., Ok Bae Kim, M.D. Department of Medical Engineering and Physics 1, Radiation Oncology, Keimyung University School of Medicine, Daegu, Korea Abstract
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20 ADCL Exposure calibration factor Nx INSTITUTE Cavity-gas calibration factor Ngas Phantom, calibration depth Dose to water Dmed Fig. 1. Flow chart shows the determination of water absorbed dose from exposure calibration factor which can be obtained from an Accredited Dosimetry Calibration Laboratory (ADCL) at time of the 60 Co exposure calibration. gas ( ) wall gas ( ) wall air wall Dgas = Jgas (W/e) Ngas = Dgas A ion / M Ngas = Nx Ngas = Nx Nx = X/M ( ) bldcap gas
21 ( ) bldcap air ) ND = D water /M Ngas = N D D med/dgas = ( ) med gas =
22 Fig. 2. Schematic diagram of IC-10 Ionization Chamber. Table 1. The composition of % weight for chamber wall material C-552 of IC-10 series % Element H C O F Si density atomic No.% 26.860 45.810 0.310 26.810 0.150 % mass 2.473 50.161 0.453 46.529 0.384 1.76 g/cc Table 2. The parameters of ionization chamber IC-10 and IC-15 are provided from the manufacture and NBS for the exposure calibration factor Nx of which Co-60 gamma ray Chamber type inner radius (cm) chamber wall material Wall thickness (g/cm 2 ) Buildup material/thick (g/cm 2 ) SN active volume length Nx R/nc To ( o C) Po (mmhg) IC-10 0.3 C-552 0.07 Acrylic 11.70 351 0.63 21.746 22 760 IC-15 0.3 C-552 0.07 PMMA 5.97 3352 0.58 26.860 22 760
23 A wall = 1 - t /100
24 ( ) med gas D med = M Ngas ( ) med gas P ion P repl P wall P wall =
25 Table 3. The parameter of ionization chamber IC-10 and IC-15 for getting the Ngas factor Nx from exposure calibration Chamber type SN Nx R/nc Aion Awall uen/ L/ L/ 1 uen/ Ngas IC-10 351 21.746 0.995 0.988 0.57 1.000 1.000 0.43 1.103 0.925 1.852x10 8 IC-15 3352 26.860 0.995 0.988 0.57 1.000 1.000 0.43 1.103 0.925 2.288x10 8 Table 4. The parameters of ionization chambers IC-10 and IC-15 in pulsed radiation beam for water absorbed dose (Gy) Chamber type SN Ngas (Gy/C) Xray Energy (MV) IC-10 351 1.852x10 8 6 1.128 1.01 1.00 0.995 23 1.095 1.01 1.00 0.995 IC-15 3352 2.288x10 8 6 1.128 1.01 1.00 0.995 Pion Prepl Pwall 23 1.095 1.01 1.00 0.995 Monitoring the output dose by TG-21 using the IC 10 and IC 15 ionization chambers. Fig. 3. The dose evaluations of output dose of linear accelerator (Mevatron, Siemens) verified constancy for 15 years long term from 1989 to 2004. The constancy have shown the 1.000 0.012 for 6 MV and 1.001 0.013 for 23 MV X rays.
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27 Table 5. Determination of Pwall of ionization chambers IC-10 and IC-15 for 6 MV X and 23 MV X rays, respectivelly Chamber E (MV) 1 Pwall IC-10 IC-15 6 0.25 1.128 1.111 1.000 1.111 0.75 0.993 0.995 23 0.15 1.095 1.094 0.992 1.103 0.85 0.963 0.995 6 0.25 1.128 1.111 1.000 1.111 0.75 0.993 0.995 23 0.15 1.095 1.094 0.992 1.103 0.85 0.963 0.995
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