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
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.