リニアックを用いた線量計の校正 Calibration of ionization chambers using a linear accelerator
Accuracy of cavity-gas calibration factors <I>N</I><SUB>gas</SUB> for ionization chambers using high-energy photon and electron beams from a linear accelerator was evaluated by comparing with in-air calibration using a <SUP>60</SUP>Co beam. For in calibration with a linear accelerator, fluctuations in beam energy and output, and changes in beam flatness and symmetry become important problems. To overcome these problems, we investigated a new method with a tip-to-tip calibration procedure in a phantom and with an exchange between a calibrated PTW-Farmer chamber and a calibration Fanner chamber. For the 8 PTW-chambers (with acrylic wall), <I>N</I><SUB>gas</SUB> ratios and the uncertainties (2 SD) of the improved in-phantom calibration for the in-air <SUP>60</SUP>Co calibration were 0.999± 0.005 for a <SUP>60</SUP>Co beam,1.000 ± 0.005 for 4 MV x-rays,0.999 ± 0.005 for 10 MV x-rays, and 0.998 ± 0.006 for 15 MeV electrons, respectively. The uncertainties (2 SD) in <I>N</I><SUB>gas</SUB> ratios among beam energies were within ±0.3%. On the other hand, for 2 NE-chambers (with graphite wall), the deviations among the beam energies were observed because of errors of the physical data for the wall correction factors <I>P</I><SUB>wall</SUB>. Therefore, it was concluded that when the discrepancies in <I>P</I><SUB>wall</SUB> values do not occur, from the results for the PTW -chambers, the accuracy of our calibra tion method for ionization chambers with a linear accelerator is equivalent to the in-air <SUP>60</SUP>Co calibration.
放射線医学物理 18(4), 356-362, 1998-12-01
Japan Society of Medical Physics