Reference dosimetry in clinical high-energy electron beams: Comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols

Citation
Ms. Huq et al., Reference dosimetry in clinical high-energy electron beams: Comparison of the AAPM TG-51 and AAPM TG-21 dosimetry protocols, MED PHYS, 28(10), 2001, pp. 2077-2087
Citations number
25
Categorie Soggetti
Radiology ,Nuclear Medicine & Imaging","Medical Research Diagnosis & Treatment
Journal title
MEDICAL PHYSICS
ISSN journal
00942405 → ACNP
Volume
28
Issue
10
Year of publication
2001
Pages
2077 - 2087
Database
ISI
SICI code
0094-2405(200110)28:10<2077:RDICHE>2.0.ZU;2-F
Abstract
A comparison of the determination of absorbed dose to water in reference co nditions with high-energy electron beams ( E-nominal of 6, 8, 10, 12, 15, a nd 18 MeV) following the recommendations L given in the AAPM TG-51 and in t he original TG-21 dosimetry protocols has been made. Six different ionizati on chamber types have been used, two Farmer-type cylindrical (PTW 30001, PM MA wall; NE 2571, graphite wall) and four plane parallel (PTW Markus, and S canditronix-Wellhofer NACP, PPC-05 and Roos PPC-40). Depending upon the cyl indrical chamber type used and the beam energy, the doses at d(max) determi ned with TG-51 were higher than with TG-21 by about 1%-3%. Approximately 1% of this difference is due to the differences in the data given in the two protocols another 1.1%-1.2% difference is due to the change of standards, f rom air-kerma to absorbed dose to water. For plane-parallel chambers, absor bed doses were determined by using two chamber calibration methods: (i) dir ect use of the ADCL calibration factors N-D,w(60Co) and N-X for each chambe r type in the appropriate equations for dose determination recommended by e ach protocol, and (fi) cross-calibration techniques in a high-energy electr on beam, as recommended by TG-21, TG-39, and TG-51. Depending upon the plan e-parallel chamber type used and the beam energy, the doses at dm,, determi ned with TG-51 were higher than with TG-21 by about 0.7%-2.9% for the direc t calibration procedures and by 0.8%-3.2% for the c ro ss-calibration techn iques. Measured values of photon-electron conversion k(ecal), for the NACP and Markus chambers were found to be 0.3% hi-her and 1.7% lower than the co rresponding values given in TG-51. For the PPC-05 and PPC-40 (Roos) chamber types, the values of k(ecal) were measured to be 0.889 and 0.893, respecti vely. The uncertainty for the entire calibration chain, starting from the c alibration of the ionization chamber in the standards laboratory to the det ermination of absorbed dose to water in the user beam, has been analyzed fo r the two formalisms. For cylindrical chambers, the observed differences be tween the two protocols are within the estimated combined uncertainty of th e ratios of absorbed doses for 6 and 8 MeV; however, at higher energies (10 less than or equal toE less than or equal to 18 MeV), the differences are larger than the estimated combined uncertainties by about 1%. For plane-par allel chambers, the observed differences are within the estimated combined uncertainties for the direct calibration technique; for the cross-calibrati on technique the differences are within the uncertainty estimates at low en ergies whereas they are comparable to the uncertainty estimates at higher e nergies. A detailed analysis of the reasons for the discrepancies is made w hich includes comparing the formalisms, correction factors, and quantities in the two protocols, as well as the influence of the implementation of the different standards for chamber calibration. Q 2001 American Association o f Physicists in Medicine.