Study of dosimetry consistency for kilovoltage x-ray beams

In this paper, the consistency of kilovoltage (tube potentials between 40 a nd 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phanto m measurement has been studied. The procedures for the measurement of the c entral-axis depth-dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The unc ertainties on the measured dose distributions were analyzed with the emphas is on the surface dose measurement. The Monte Carlo method was used to calc ulate the perturbation correction factors for a photon diode and a NACP pla ne-parallel ionization chamber at different depths in a water phantom irrad iated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. T he depth-dose curves measured with these two detectors, after correcting fo r the perturbation effect (up to 15% corrections), agreed with each other t o within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100-300 kV tube potential obtained using the "backscatter" method and those using the "in-phantom" measurement have shown that the ''in-air'' method can be equally applied to this energy ran ge if the depth-dose curve can be measured accurately. To this end, measure d depth ionization curves require depth-dependent correction factors. (C) 1 998 American Association of Physicists in Medicine. [S0094-2405(98)02712-6] .