Calibration of photon and electron beams
JOURNAL OF APPLIED CLINICAL MEDICAL PHYSICS, VOLUME 1, NUMBER 3, SUMMER 2000
Comparison between TG-51 and TG-21: Calibration of photon and electron beams in water using cylindrical chambers
S. H. Cho,a) J. R. Lowenstein,b) P. A. Balter,c) N. H. Wells,d) and W. F.
Hansone)
Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center,
1515 Holcombe Boulevard, Box 547, Houston, Texas 77030
͑Received 31 March 2000; accepted for publication 8 June 2000͒
A new calibration protocol, developed by the AAPM Task Group 51 ͑TG-51͒ to replace the TG-21 protocol, is based on an absorbed-dose to water standard and calibration factor (N D,w ), while the TG-21 protocol is based on an exposure ͑or air-kerma͒ standard and calibration factor (N x ). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG-51 may be somewhat different from those based on TG-21. The
Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG-51 report, covering the majority of current manufacturers. Measured ratios between absorbed-dose and air-kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG-21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed-dose to water determined by TG-51 and that determined by TG-21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG-51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV
Comparison between TG-51 and TG-21: Calibration of photon and electron beams in water using cylindrical chambers
S. H. Cho,a) J. R. Lowenstein,b) P. A. Balter,c) N. H. Wells,d) and W. F.
Hansone)
Department of Radiation Physics, The University of Texas M.D. Anderson Cancer Center,
1515 Holcombe Boulevard, Box 547, Houston, Texas 77030
͑Received 31 March 2000; accepted for publication 8 June 2000͒
A new calibration protocol, developed by the AAPM Task Group 51 ͑TG-51͒ to replace the TG-21 protocol, is based on an absorbed-dose to water standard and calibration factor (N D,w ), while the TG-21 protocol is based on an exposure ͑or air-kerma͒ standard and calibration factor (N x ). Because of differences between these standards and the two protocols, the results of clinical reference dosimetry based on TG-51 may be somewhat different from those based on TG-21. The
Radiological Physics Center has conducted a systematic comparison between the two protocols, in which photon and electron beam outputs following both protocols were compared under identical conditions. Cylindrical chambers used in this study were selected from the list given in the TG-51 report, covering the majority of current manufacturers. Measured ratios between absorbed-dose and air-kerma calibration factors, derived from the standards traceable to the NIST, were compared with calculated values using the TG-21 protocol. The comparison suggests that there is roughly a 1% discrepancy between measured and calculated ratios. This discrepancy may provide a reasonable measure of possible changes between the absorbed-dose to water determined by TG-51 and that determined by TG-21 for photon beam calibrations. The typical change in a 6 MV photon beam calibration following the implementation of the TG-51 protocol was about 1%, regardless of the chamber used, and the change was somewhat smaller for an 18 MV