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Radiation Protection Dosimetry 2005 114(1-3):317-320; doi:10.1093/rpd/nch522
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© The Author 2005. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oupjournals.org

Estimation of the peak entrance surface air kerma for patients undergoing computed tomography-guided procedures

P. Avilés Lucas1,2,3, D. R. Dance1,*, I. A. Castellano1 and E. Vañó2,3

1 Physics Department, The Royal Marsden NHS Trust, Fulham Road. London SW3 6JJ, UK
2 Radiology Department, Complutense University Madrid, Madrid 28040, Spain
3 Medical Physics Service, San Carlos University Hospital, Madrid 28040, Spain

* Corresponding author: david.dance{at}rmh.nhs.uk

The purpose of this work was to develop a method for estimating the patient peak entrance surface air kerma from measurements using a pencil ionisation chamber on dosimetry phantoms exposed in a computed tomography (CT) scanner. The method described is especially relevant for CT fluoroscopy and CT perfusion procedures where the peak entrance surface air kerma is the risk-related quantity of primary concern. Pencil ionisation chamber measurements include scattered radiation, which is outside the primary radiation field, and that must be subtracted in order to derive the peak entrance surface air kerma. A Monte Carlo computer model has therefore been used to calculate correction factors, which may be applied to measurements of the CT dose index obtained using a pencil ionisation chamber in order to estimate the peak entrance surface air kerma. The calculations were made for beam widths of 5, 7, 10 and 20 mm, for seven positions of the phantom, and for the geometry of a GE HiSpeed CT/i scanner. The program was validated by comparing measurements and calculations of CTDI for various vertical positions of the phantom and by directly estimating the peak ESAK using the program. Both validations showed agreement within statistical uncertainties (standard deviation of 2.3% or less). For the GE machine, the correction factors vary by ~10% with slice width for a fixed phantom position, being largest for the 20 mm beam width, and at that beam width range from 0.87 when the phantom surface is at the isocentre to 1.23 when it is displaced vertically by 24 cm.


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