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Determination of the Phantom Surface Dose Using a Diode Detector for Diagnostic X-Ray Beams

R Tanabe*,

Presentations

PO-GePV-M-226 (Sunday, 7/25/2021)   [Eastern Time (GMT-4)]

Purpose: To determine the surface dose for diagnostic kilovoltage x-ray beams from the air kerma and a half-value layer (HVL, mm Al) measured using a diode detector.

Methods: A kV/dose multi-sensor (AGMS-DM+ detector, Radcal Co. USA) was used as a diode detector, and calibrated in terms of the air kerma measured with a PTW 30013 Farmer-type ionization chamber. The air kerma was measured for 20 x-ray beams with tube voltages of 50-140 kVp and HVL of 2.2-9.7 mm Al for a quality index (QI) of 0.4, 0.5 and 0.6, which is specified by the ratio of the effective energy to maximum energy corresponding to the tube voltage. The air kerma was converted into the phantom surface dose using the backscatter factor and the ratio of mass-energy absorption coefficient for water to air. Finally, the air kerma measured by the AGMS-DM+ detector was expressed as a ratio of the surface dose for 10 × 10 and 20 × 20 cm² 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 for the AGMS-DM+ detector ranged from 0.986 to 1.016 (0.9% in k = 1), and was 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 30013 chamber and the AGMS-DM+ detector was less than 1.8% for the values calculated using the Monte Carlo method, showed a good correlation with HVL for the given QI values.

Conclusion: It is possible to determine the surface dose from the air kerma and HVL measured by a diode detector for the given QI values for diagnostic kilovoltage x-ray beams.

Funding Support, Disclosures, and Conflict of Interest: The diode detector, AGMS-DM+ detector used in this study were provided by Toyo Medic Co. in Japan.

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