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SU‐E‐T‐532: Measurements of the Doses Delivered during Radiotherapy CT Exams Using AAPM Task Group Report N°111
Author(s) -
Descamps C,
Gonzalez M,
Garrigo E,
Germanier A,
Venencia C
Publication year - 2011
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3612494
Subject(s) - ionization chamber , imaging phantom , nuclear medicine , dosimetry , scanner , electrometer , dose profile , thermoluminescent dosimeter , pencil (optics) , medicine , physics , optics , ionization , dosimeter , ion , quantum mechanics
Purpose: The current paradigm used to evaluate the doses delivered during CT procedure is the computed tomography dose index (CTDI). It is measured with a 100mm‐long pencil ionization chamber placed in a cylindrical PMMA phantom (14cm‐long and 16 or 32cm‐diameter) but this method excludes contribution of radiation scattered beyond the 100mm‐ range of integration along z. The purpose of this work was to measured CT radiation dose following the new method described in the AAPM TG111 report using small volume ionization chamber positioned in a phantom. Methods: A Siemens, SOMATOM Spirit Power 2‐slice CT scanner was used. A PTW Farmer‐type chamber (0.6cm3) connected to a PTW UnidosE electrometer was calibrated by a Secondary Standard Dosimetry Laboratory for beam quality ranges associated with those of CT scanner spectra. A 30cm‐diameter, 50cm‐long water phantom was designed to allow the chamber position at the center or at peripheral axis. Measurements were realized for each clinically CT protocols following the AAPM recommendations. The minimum scanning length needed to obtain cumulative dose equilibrium was evaluated. The equilibrium dose was determined and compared to CTDI values informed by the CT scanner. In order to validate the measurement set, the dose values were confirmed with TLD measurements. Results: The measurements showed that the scanning lengths needed to reach the cumulated dose equilibrium were 450mm and 380mm for central and peripheral axis respectively that justify the phantom length. The difference between doses measured with TLD and ionization chamber was 2%. For each clinical protocol, the doses measured were about 30% higher than those informed by the CT scanner. Conclusions: For new generations of CT systems with wider longitudinal detector size or cone‐ beam technology, the CTDI informed by the CT scanner tends to undervalue the dose delivered. It is therefore important to evaluate CT radiation dose with new methodology.