Determination of an ionization chamber response using quality index for kilovoltage x-ray beams

Abstract

This study aims to investigate the energy response of a Farmer-type ionization chamber for kilovoltage x-ray beams. The chamber response in free air and at depths of 1 to 15 cm in a water phantom was calculated using the Monte Carlo method. The response in the phantom was evaluated using a beam quality conversion factor, kQx, for x-ray beams Qx to a 60Co beam. The response was expressed as a function of the effective energy or a half-value layer (HVL, mm Al) for x-ray beams with given quality index (QI) values of 0.4, 0.5, and 0.6 and for monoenergetic photon beams. QI is defined as the ratio of the effective energy to the tube voltage. The response for 25 fluence spectra of x-ray beams was calculated for the beam qualities with HVL of 1.39-10.4 mm Al and tube voltages of 50-137.6 kVp, as classified by QI. The chamber response calculated in free air for the given QI values agreed with 1% of that provided by secondary standard laboratories. The kQx values depended on depth at the lower energy levels. The depth dependence was less than 1% for an HVL (mm Al) of more than 6 mm. The dose to water obtained from kQx and a 60Co absorbed dose-to-water calibration coefficient ND,wC60o was within 1.5% at depths of 1 to 15 cm with that determined by the AAPM TG-61 for HVL of 2.9 to 10.8 mm Al. This method can reduce the uncertainty of doses in the phantom without any correction using PQ,cham and [(μ‾en/ρ)airw]w, and can directly determine the absorbed dose to water from the ionization chamber reading for medium-energy x-ray beams, including kilovoltage x-ray CT and image-guided cone beam CT in radiotherapy.