Room: AAPM ePoster Library
Purpose: The AAPM has published an addendum in the AAPM Task Group 51 report, which showed a new correction factor to account for the contributions from leakage current. This study aimed to determine the influence of leakage current from chamber–cable–electrometer systems (dosimeters) on the determination of the absorbed dose to water for high-energy photon and electron beams. Thus, this work assessed the leakage currents from dosimeters.
Methods: Sixteen dosimeters were used. The ionization chambers of the dosimeters were (the number of detectors of each type is given in parentheses): PTW30013 (9); IBA PPC40 (5); and IBA NACP-02 (2). The electrometers of the dosimeters were (the number of electrometers investigated is given in parentheses): TOYO MEDIC RAMTEC Duo (2); TOYO MEDIC RAMTEC Smart (6); and TOYO MEDIC RAMTEC 1000plus (1). We determined the leakage currents according to the addendum to TG-51; thus, they were measured with all the equipment used for the absorbed dose measurement in place and the accelerator and dosimeters on, but there was no beam. The measurement times for the leakage currents correspond to the irradiation times during the calibration of the clinical treatment machine output.
Results: The leakage currents for the dosimeters containing cylindrical chambers were 12 fA or less, except for the PTW 30013-RAMTEC 1000plus system, where they were as high as 32 fA. By contrast, those for dosimeters that contained plane-parallel chambers were 13 fA or less.
Conclusion: The leakage currents investigated contributed much less than 0.1% to the charge readings in determining the absorbed dose for external photon and electron beams. These findings suggest that the leakage effect can be considered negligible in the clinical measurement of the dose.?