|Abstract or Summary
- In radiation therapy it is very important to accurately measure the amount of
radiation delivered. The effectiveness of the treatment depends on delivering the dose with
an accuracy of 5% or better. The dosimetry in different clinics must also be consistent. For
these reasons national and international calibration protocols have been developed. In the
US, the American Association of Physicists in Medicine (AAPM) has published several
national dosimetry protocols for the calibration of high-energy photon and electron beams.
In this study the absorbed dose-to-water determined according to TG-21 and TG-51 protocols, developed by Task Group 21 and Task Group 51 of the Radiation Therapy of
AAPM, are compared. The older protocol, TG-21, is based on exposure calibrated
ionization chambers using a ⁶⁰Co beam. Many standards laboratories have started to
replace exposure standards with those involving absorbed dose-to-water. The new
protocol, TG-51, is based on absorbed dose-to-water calibrated ionization chambers using
a ⁶⁰Co beam. Also, there are some differences between the beam quality specifiers and data
proposed by the two protocols.
A comparison between TG-21 and TG-51 protocols was done by determining the
radiation dose rate at a designated distance for 6 and 18 MV photon beams, and 16 and 20
MeV electron beams, generated by Clinac a 2100 C linear accelerator. The cylindrical
ionization chambers used in this study were Capintec PR-06G and PR-05.
The results of the study show a discrepancy between the absorbed dose-to-water
determined according to TG-21 and TG-51 protocols of about 1.4% and 1.7% for 6 and 18
MV photon beams, respectively. Absorbed dose-to-water determined according to TG-21
and TG-51 protocols for 16 MeV energy electron beams agree within 1.8%, while the ones
of 20 MeV energy beams agree to within 2.4%. The change from exposure to absorbed
dose-to-water calibrated ionization chambers has the largest impact on the differences
between TG-21 and TG-51 absorbed dose-to-water, while the change in beam quality
specifier and stopping power ratios have only a very small effect on these differences.
The TG-51 protocol is very simple, minimizing the chance of mistakes, because it
starts with absorbed dose-to-water calibration, while the TG-21 is very complex, starting
with the calibration for exposure, which is different from the absorbed dose-to-water, the
clinical quantity of interest. The TG-51 protocol allows the determination of a more
accurate absorbed dose in a ⁶⁰Co beam than the TG-21 protocol since it uses an absorbed
dose-to-water calibration factor directly measured, while the exposure based dosimetry
system is dependent on external physical data which are not measured in clinics.