Study of dosimetry consistency for kilovoltage x-ray beams

In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phantom measurement has been studied. The procedures for the measurement of the central-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 uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane-parallel ionization chamber at different depths in a water phantom irradiated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. The depth-dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to 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 range if the depth-dose curve can be measured accurately. To this end, measured depth ionization curves require depth-dependent correction factors.