Air Kerma Calibration Factors and kch Values for PTW Soft X-ray, NACP and Roos Ionization Chambers at Very Low X-ray Energies

Several national and international protocols have been established for the dosimetry of x-ray beams used in radiotherapy. For the very low energy x-rays (0.035 mm1.0 mm Al HVL) only two codes are available: the UK IPEMB Code of Practice and the German standard, DIN 6809 Part 4. The measurement of very low energy x-ray beams is normally performed with parallel plate ionization chambers calibrated at a standards laboratory and characterized by an air kerma calibration factor Nk. According to the IPEMB Code of Practice the absorbed dose in the user’s beam should be determined by taking measurements with the parallel plate chamber positioned such that its entrance window is at the surface of a full-scatter water equivalent phantom. The absorbed dose to water can then be determined using an equation which includes a factor, kch, which accounts for the change in response of the ionization chamber between the calibration in air and measurement at the surface of the phantom. Nk and kch values for the PTW soft X-ray, NACP and Roos ionization chambers are reported. It was found that kch values varied from about 1.01 to 1.08 depending on the chamber, beam quality and phantom material. It is recommended that the IPEMB Code of Practice should be revised to incorporate these values.