Ionizing
Radiation Standards
Institute
for National Measurement Standards
National Research Council
of Canada
The protocol was published in Medical Physics, Volume 26 (1999) 1847 -- 1870.
These are the slides used for the above refresher course.
The slides from a talk entitled ``Why to Use TG-51 instead of TG-21'', which was presented at the Chicago 2000 World Congress on Medical Physics, are also available
and 50 MV and electron beams with nominal energies between 4 and 50 MeV.
The protocol was written by Task Group 51 (TG-51) of the Radiation Therapy
Committee of the American Association of Physicists in Medicine (AAPM)
and has been formally approved by the AAPM for clinical use. The protocol
uses ion chambers with absorbed-dose-to-water calibration factors, ND,w60Co,
which are traceable to national primary standards, and the equation: DQw=
M
kQND,w60Co
where Q is the beam quality of the clinical beam, DQw
is the absorbed dose to water at the point of measurement of the ion chamber
placed under reference conditions, M is the fully corrected ion chamber
reading and kQ is the quality conversion factor which
converts the calibration factor for a
beam to that for the beam quality Q. Values of kQ are
presented as a function of Q for many ion chambers. The value of
M is given by 
where 
is the raw, uncorrected ion chamber reading and 
corrects for ion recombination, PTP for temperature and
pressure variations, 
for inaccuracy of the electrometer if calibrated separately and 
for chamber polarity effects. Beam quality, Q, is specified: (i) for photon
beams, by 
,
which is the photon component of the percentage depth dose at 10 cm depth
for a field size of 10
10 
on the surface of a phantom at an SSD of 100 cm ; and, (ii) for electron
beams, by 
,
the depth at which the absorbed-dose falls to 50% of the maximum dose in
a beam with field size 
1010
on the surface of the phantom (2020
for 
cm) at an SSD of 100 cm.
is determined directly from the measured value of 
,
the depth at which the ionization falls to 50% of its maximum value. All
clinical reference dosimetry is performed in a water phantom. The reference
depth for calibration purposes is 10 cm for photon beams and 0.6 
cm for electron beams. For photon beams clinical reference dosimetry is
performed in either an SSD or SAD setup with a 1010
field size defined on the phantom surface for an SSD setup or at the depth
of the detector for an SAD setup. For electron beams clinical reference
dosimetry is performed with a field size of
1010
(
2020
for 
cm) at an SSD between 90 and 110 cm. This protocol represents a major simplification
compared to the AAPM's TG-21 protocol in the sense that large tables of
stopping-power ratios and mass-energy absorption coefficients are not needed
and the user does not need to calculate any theoretical dosimetry factors.
The protocol will be published in Medical Physics, most likely in the September 1999 issue. The protocol provides little background information but is, by design, more of a cookbook. For a general introduction and details about how the calculations have been done, please see the following two papers and references therein:
D.W.O. Rogers:
Fundamentals of Dosimetry Based on Absorbed-Dose Standards pp
319--356 in
Teletherapy Physics, Present and Future (1996 AAPM Summer
School proceedings).
and
D. W. O. Rogers: A new approach to electron beam reference dosimetry, Medical Physics, 25 (1998) 310-320.
If you would like a blown-up hard copy of the figures, they are available
at
http://www.irs.inms.nrc.ca/inms/irs/tg51_figures/tg51_figures.html
People are welcome to make use of the slides for their own presentations on the condition that the NRC-CNRC logo not be covered over or removed. For individual slides, one can download the individual slide (using the right mouse button) and save it and then import into your own powerpoint presentation. If you want the whole set, it is available here but be warned, it is 848896 bytes long!.
For access to all of the IRS web site please go to our index.
Table of ContentsThe AAPM TG-51 ProtocolProblems with current practice |
Author: Rogers
Email: dave@irs.phy.nrc.ca Home Page: http://www.irs.inms.nrc.ca/inms/irs/irs.html Other information:
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