This study is to keep the accuracy and stability of the output dose evaluations for linear accelerator photon beams by using the air ionization chambers (TM31010, 0.125 cc, PTW) through the Task Group 51 protocol. The absorbed dose to water calibration factor N_dw^Co-60 was delivered from the air kerma calibration factor N_k which was provided from manufacture through SSDL calibration for determination of output factor. The ionization chamber of TM31010 series was reviewed the calibration factor and other parameters for reduce the uncertainty within ±2% discrepancy and we found the supplied N_dw^Co-60 which was derived from N_k has shown a −2.8% uncertainty compare to that of PSDL. The authors provided the program to perform the output dosimetry with TG-51 protocol as it is composed same screen of TG-51 worksheets. The evaluated dose by determination of output factor delivered to postal TLD block for comparison the output dose to that of MDACC (RPC) in postal monitoring program. The results have shown the 1.001±0.013 for 6 MV and 0.997±0.012 discrepancy for 15 MV X rays for 5 years followed. This study shows the evaluated outputs for linear accelerate photon beams are very close to that of international output monitor with small discrepancy of ±1.3% with high reliability and showing the gradually stability after 2010.

KeywordsTG-51, Absorbed dose to water, N_dw^Co-60, Output factor, TLD monitor