Determination of the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams.

Abstract

To determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams. An AGMS-DM+semiconductor detector was calibrated in terms of air kerma measured with an ionization chamber. Air kerma was measured for 20 x-ray beams with tube voltages of 50-140 kVp and a half-value layer (HVL) of 2.2-9.7mm Al for given quality index (QI) values of 0.4, 0.5, and 0.6, and converted to the surface dose. Finally, the air kerma and HVL measured by the AGMS-DM+detector were expressed as a ratio of the surface dose for 10×10 and 20×20cm2 fields. The ratio of both was represented as a function of HVL for the given QI values and verified by comparing it with that calculated using the Monte Carlo method. The air kerma calibration factor, CF, for the AGMS-DM+detector ranged from 0.986 to 1.016 (0.9% in k=1). The CF values were almost independent of the x-ray fluence spectra for the given QI values. The ratio of the surface dose to the air kerma determined by the PTW 30,013 chamber and the AGMS-DM+detector was less than 1.8% for the values calculated using the Monte Carlo method, and showed a good correlation with the HVL for the given QI values. It is possible to determine the surface dose of a water phantom from the air kerma and HVL measured by a semiconductor detector for given QI values.