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

Purpose: To determine the surface dose of a water phantom using a semiconductor detector for diagnostic kilovoltage x-ray beams. Methods: 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 2.2-9.7 mm 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 x 10 and 20 x 20 cm(2) 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. Results: 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. Conclusion: 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.