DOS 514 - Week 5 Discussion
Initial Post: TG-51, TG-21, and TRS 398
The American Association of Physics in Medicine (AAPM) and the international Atomic Energy Agency (IAEA) are two organizations that publish guidelines for the calibration of linear accelerators and other devices that deliver photon or electron beams. The AAPM has formed task groups to formalize these guidelines on several occasions, with Task Group 21 (TG-21) and Task Group 51 (TG-51) publishing recommendations in 1983 and 1999 respectively.1
The purpose of a standard for beam calibration is to ensure that delivery of radiation is predictable and accurately measurable, so that the correct dose can be safely delivered. Before TG-21 and TG-51, several first-generation protocols existed, including ICRU 14 (1969), SCRAD (1971), and ICRU 21 (1972).2 These first-generation protocols did not take into account the type of chamber used or the quality of the beam, so they could have errors of up to 5% in their calculated dose. TG-21 is considered to be a second-generation protocol, and TG-51 is a third-generation protocol.
TG-51 offers the advantage of simplicity when compared to TG-21.1 TG-21 requires the use of chamber-dependent correction factors that are themselves dependent on beam quality, resulting in the need for lookup tables and complex calculations to take all of these factors into account. The complexity of the math was a potential source of errors that TG-51's simplification seeks to address. Instead of converting charge to exposure, and then converting exposure to dose, taking into account yet more factors such as the type of phantom material, TG-51 requires the use of an actual water phantom with real water (not a stand-in material) so that charge can be converted directly to dose with a single conversion factor. TG-51 also offers worksheets for electron and photon beams that can simply be filled out in a stepwise fashion, making the process of calibration easier to follow.
SCCA Proton Therapy can not use TG-51 because we are a proton-only facility and TG-51 does not cover the calibration of proton beams. An international calibration protocol, TRS 398, was published by the IAEA in 2005. TRS 398 covers electron and photon beams in the same energy ranges as TG-51, but it also adds support for calibration of kilovoltage x-ray beams, proton beams, and other heavy ion beams.3 Since there is no AAPM task group report that covers proton beams, and since TRS 398 is an international standard used in many member nations, we are comfortable using it as our standard for proton beam calibration.
- Almond PR, Biggs PJ, Coursey BM, Hanson WF, Huq MS, Nath R, Rogers DWO. AAPM’s TG-51 protocol for clinical reference dosimetry of high-energy photon and electron beams. Med Phys. 1999;26(9):1847-1870.
- Dosimetry protocols [PowerPoint]. https://uqu.edu.sa/files2/tiny_mce/plugins/filemanager/files/4092434/DOSIMETRY%20PROTOCOLS_1.ppt. Umm al-Qura University. Modified December 22, 2009. Accessed October 1, 2014.
- Implementation of the international code of practice on dosimetry in radiotherapy (TRS 398): Review of testing results. Vienna, Austria: Internation Atomic Energy Agency; 2005. IAEA-TECDOC-1455