AAPM Task Group 21 protocol: dosimetric evaluation

The AAPM Task Group 21 protocol is the latest amongst the protocols introduced in recent years covering the dosimetry of high-energy x-ray and electron beams used in radiotherapy. We have found that the C lambda value for 15-MV x rays requires 1.6% upward adjustment, whereas the CE values for electrons in the range 7-18 MeV require downward adjustment of 1%-3% compared to our previous values. The adjustment values given in this paper pertain to the specific measuring systems and high-energy beams considered here and would not necessarily be applicable to other systems. The dose to medium, calculated for different modalities, was verified by thermoluminescent dosimetry and the results were found to be consistent. These values of C lambda and CE have also been compared with those derived using other protocols, particularly the Nordic Association of Clinical Physics 80 and 81, and the discrepancies discussed.