CTDI versus New AAPM Metrics to assess Doses in CT: a case study
DOI:
https://doi.org/10.15392/bjrs.v4i2.194Keywords:
Computed tomography, AAPM TG 111, CTDIAbstract
In modern CT, CTDI100 measurements would underestimate accumulated dose at the gantry center. AAPM TG 111 report proposed improved metrics for CT dosimetry, mainly for helical and wide beam width scanning. In this study, a methodology to assess CT dose, inspired on TG 111, was applied. Dosimeters were firstly calibrated in lab in beams like those utilized clinically. Using a reference 0.6cc Farmer chamber, two CT “pencil” chambers were calibrated in PKL by substitution method. Results showed differences ≤ 2% in the calibration coefficients, for three collimation apertures. A small 0.6cc chamber was calibrated in air kerma with this setup, without any collimator. After this, in a private Brazilian hospital, the small chamber was applied in dosimetry tests of a CT scanner, according to TG 111, determining Dose profiles and Equilibrium dose free-in-air (Deq,air) for some protocols and pitch values. Results showed that Deq,air increased when reducing pitch and Equilibrium dose-pitch product free-in-air (p.Deq,air) remain constant. In measurements with a 450mm CT phantom, differences between Planar Average Equilibrium Dose (Deq,p) and CTDIvol ranged between 30-37%. This occurs because CTDIvol cannot include dose profile "tail" contribution, caused by scattering in phantom, especially for wide beam widths.
Downloads
References
IAEA - International Atomic Energy Agency. Dosimetry in Radiology: An International Code of Practice. Technical Reports Series n° 457. Vienna: IAEA, 2007.
SMITH-BINDMAN R. et al.. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch. Intern. Med., v.169(22), p. 2078-2086, 2009.
UNSCEAR - United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and Effects of Ionizing Radiation. UNSCEAR 2008 Report, vol. I, Annex A: Med-ical Radiation Exposures, NY: UNSCEAR, 2010.
MERIMAA HK, TAPIOVAARA M, KOSUNEN A, TOROI P. Calibration and features of air kerma-length product meters. Rad. Prot. Dos., v. 152(4), p. 384-392, 2012.
AAPM - American Association of Physics in Medicine. Comprehensive methodology for the evaluation of radiation dose in X-ray computed tomography. AAPM report no. 111, Task Group 111. College Park, MD: AAPM, 2010.
BOONE J M. The trouble with CTDI100. Med. Phys., v. 34, p. 1364–1371, 2007.
DIXON R L. and BALLARD, A. C.. Experimental validation of a versatile system of CT do-simetry using a conventional ion chamber: Beyond CTDI100. Med. Phys., v. 34, p. 3399–3413, 2007.
KRAMER HM, SELBACH HJ, ILES WJ. The Practical Peak Voltage of Diagnostic X-Ray Generators, Br. J. Radiol., v. 71, p. 200-209, 1998.
TERINI RA, POTIENS, MPA, HERDADE, SB, PEREIRA, MAG, PIRES, JSJ, VIDEIRA, HS. Measurement of the quantity Practical Peak Voltage in the radiology practice. Radiologia Brasileira, v. 42, p. 389-394, 2009.
DESCAMPS C, GONZALEZ M. Measurements of the dose delivered during CT exams using AAPM Task Group Report No. 111, Journal of Applied Clinical Medical Physics, vol. 13, N. 6, 2012.
HOURDAKIS CJ, BOZIARI A, KOUMBOULI E, Calibration of pencil type ionization chambers at various irradiation lengths and beam qualities. Proceedings IAEA IDOS 2010, vol.2, 21-33, IAEA STI/PUB/1514, 2010.
BOONE JM, Dose spread functions in computed tomography: A Monte Carlo study. Med. Phys., v. 36, p. 4547–4554, 2009.
LI X, ZHANG D, LIU B, Calculations of two new dose metrics proposed by AAPM Task Group 111 using the measurements with standard CT dosimetry phantoms. Med. Phys., v. 40, 081914, 2013. Available at http://dx.doi.org/10.1118/1.4813899.
PLATTEN DJ et al. Radiation dosimetry for wide-beam CT scanners: recommendations of a working party of the Institute of Physics and Engineering in Medicine, Br. J. Radiol., 86:20130089, 2013.
CARVALHO ACP. Medical knowledge of ionizing radiations, Radiol Bras. v. Mar/Abr. 47(2):X. 2014. Available at http://dx.doi.org/10.1590/S0100-39842014000200004 .
Published
Issue
Section
License
Licensing: The BJRS articles are licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/