Implementation of TG-51: Practical considerations

The AAPM Task Group 51 (TG-51) recently published its protocol for clinical reference dosimetry of megavoltage radiation therapy. Explicit application of TG-51 to a high energy accelerator with electron capability requires the clinical physicist to have a 1mm thick sheet of lead and a parallel plate chamber. Many clinical physicists have neither, therefore we have studied the impact of alternative measurement techniques on output calibration to solve this problem. Depth dose measurements with a lead sheet in the beam are required to determine the beam quality (k_Q) by obtaining the %dd(10)_x (depth dose with electron contamination removed) for energies ≥ 10 MV. TG-51 also states that a parallel plate chamber is recommended for electron calibration and is required for reference dosimetry for electron energies ≤ 6 MeV. To determine if these requirements are necessary, the Radiological Physics Center (RPC) made measurements on Varian, GE, and Siemens units for 10 and 18 MV photons and 5 - 20 MeV electrons. The comparison of lead versus no lead revealed that the k_Q value, thus the calibration of the beam, will vary by no more than 0.2%. The comparison between a parallel plate and a cylindrical Farmer chamber showed no measurable difference in the calibration across the range of electron energies. Omission of the lead to determine k_Q and use of a cylindrical chamber for low electron energies have an insignificant effect on the calibration.