Ionization chamber dosimetry based on Co absorbed dose to water calibration for diagnostic kilovoltage x-ray beams.

The purpose of this study was to develop a new ionization chamber dosimetry formalism for diagnostic kilovoltage x-ray beams based on a Co absorbed dose-to-water calibration coefficient [Formula: see text]. To validate the new chamber dosimetry, 39 fluence spectra of kilovoltage x-rays were calculated, using a SpekCalc program, for Al half-value-layer (Al-HVL) values of 1.4-8.5 mm and tube voltages of 50-137.6 kVp. We used these spectra to calculate the beam quality conversion factor [Formula: see text] for kilovoltage x-ray beams Q to a Co beam for a PTW 30013 Farmer ionization chamber in a reference geometrical setup (depth  =  1 cm in water, field size  =  20  ×  20 cm). The absorbed dose to water for kilovoltage x-rays was obtained using [Formula: see text] and [Formula: see text]. Meanwhile, the water surface dose based on the air kerma calibration coefficient N was obtained using a Monte Carlo-calculated correction factor CF (corresponding to [Formula: see text] in AAPM TG-61) for the 39 x-ray fluence spectra. The absorbed dose based on [Formula: see text] was validated by comparing the water surface dose for the 39 x-ray fluence spectra with the water surface dose based on N . The water surface dose based on [Formula: see text] was obtained using Monte Carlo-calculated percentage depth doses. For Al-HVL ranging from 1.4 to 8.5 mm, [Formula: see text] ranged from 0.884 to 0.956 and CF ranged from 1.264 to 1.663. The water surface dose based on [Formula: see text] for the x-ray fluence spectra was 1.0% higher on average than that based on N , except for Al-HVLs of 1.4 and 1.5 mm and 1% was within their uncertainty ranges. Therefore, it is possible to use the absorbed dose-to-water value based on [Formula: see text] instead of on N . With the new chamber dosimetry, the chamber reading can be easily converted to the absorbed dose using [Formula: see text] and the calculated [Formula: see text].