earticle

영어

When the PDD (percentage depth dose) in the megavoltage beams is measured in the water phantom, the polarity and ionrecombination effects of ionization chambers with depth in water are not usually taken into consideration. We try toinvestigate if those variations with depth should be taken into consideration or could be ignored for the thimble type semi-flex ionization chamber (PTW 31010TM, SN 1551). According to the recommendation of IAEA TRS-398, the 4 representativedepths of ds, dmax, d90and d50were used for the electron beams. For the photon beams, the 4 depths were arbitrarily chosenfor the photon beams, which were ds, dmax, d10and d20. For the high energy photon beam both polarity and ion recombinationfactors of the chamber with depth in water gives the good agreements within the maximum ±0.2%, while the Cpols withdepth came within the maximum ± 0.4% and the CIRs within the maximum ±0.6% in every electron beam used. This studyshows that PDI (percentage depth ionization) could be a good approximation to PDD for the chamber used.