Keywords:
Radiation physics, Radioprotection / Radiation dose, CT, CT-Quantitative, Cone beam CT, Physics, Radiation safety, Education, Quality assurance, Education and training
Authors:
A. Albngali, A. Shearer, W. van der Putten, B. Tuohy, N. Colgan; Galway/IE
DOI:
10.1594/ecr2018/C-1445
Conclusion
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A study conducted by the Radiological Protection Institute of Ireland (now EPA) found that artificial sources such as medical ionizing radiation was by far the largest contribution to the population dose,
particularly diagnostic X rays (above 90%); moreover,
one of the largest contributors is CT,
which contributes up to 55% of the collective dose(5).
Currently,
assessments of radiation dose in CT are based on the metric CTDI,
introduced more than 30 years ago.
CTDI underestimates the dose because the 100-mm-long pencil ionization chamber dose not measure the tails of the scattered dose distribution.
Our find is a good agreement with the published data from “Dixon” method (3) .
The current work demonstrated the limitation of the CTDI,
especially CTDI 100.
A 100-mm pencil chamber is too short and it underestimates the limiting equilibrium dose D(eq),
as well as the accumulated dose,
for any scan length above 100 mm.
Furthermore,
the equilibrium dose actually delivered to the three protocols during CT exam as measured by the farmer chamber was underestimated with the pencil chamber by 25% - 35% (table 2).
The current methodology based on measurement of the single axial scan slice profile using a 100-mm long ion chamber is no longer adequate.
Alternately,
the “Dixon” methodology uses an ion chamber shorter than 1 cm,
and a phantom long enough to establish dose equilibrium in the center.
This method is more flexible and not significantly more time consuming than the current method that uses a long chamber.
The TLD measurements independently validated the farmer chamber measurement and Dixon methodology.