Kyle McMillan, Presenter: Institutional research agreement, Siemens AG Research support, Siemens AG

Maryam Bostani PhD, Abstract Co-Author: Research support, Siemens AG

Maria Zankl PhD, Abstract Co-Author: Nothing to Disclose

Christopher H. Cagnon PhD, Abstract Co-Author: Nothing to Disclose

John J. Demarco PhD, Abstract Co-Author: Nothing to Disclose

Michael F. McNitt-Gray PhD, Abstract Co-Author: Institutional research agreement, Siemens AG Research support, Siemens AG

To evaluate the relationship between partially irradiated organ dose and z-axis over-ranging and over-prescription.

Detailed Monte Carlo simulations of chest CT examinations were performed for one 64-slice multi-detector row CT scanner (Sensation 64, Siemens Healthcare) using a single voxelized patient model (GSF model “Irene”). Simulations were performed with a nominal collimation of 32 x 0.6 cm and a pitch of 1. As per the recommendations of the AAPM Routine Chest CT Protocol, an “ideal scan range” was set from the top through the bottom of the lungs. In order to model both z-axis over-ranging and scan over-prescription, scan start (beam on) and stop (beam off) locations were independently increased in 1 cm increments until full coverage of the thyroid and liver occurred. For each scan range, dose to the thyroid, liver and red bone marrow (RBM) was tallied, and relationships between organ dose and z-axis over-ranging and scan over-prescription were derived.

For all organs, dose as a function of over-ranging and over-prescription follows a cubic function. Correlation coefficients for these relationships were 0.99. The difference between dose from an “ideal scan range” and a scan range that included 2 cm of over-ranging only was 62.4%, 34.1% and 15.6% for the thyroid, liver and RBM, respectively. The difference between dose from an “ideal scan range” and a scan range that included over-ranging and maximum over-prescription (i.e. full coverage of the thyroid and liver) was 89.3%, 164.4% and 52.8% for the thyroid, liver and RBM, respectively.

Z-axis over-ranging and scan over-prescription can have a significant impact on dose to partially irradiated organs. Strong correlations between partially irradiated organ dose and over-ranging and over-prescription can be used to predict dose to organs outside the image range.

Results presented can be used to estimate the impact of over-ranging and over-prescription for dose to partially irradiated organs and help facilitate clinical decisions of scan range prescription.

McMillan, K, Bostani, M, Zankl, M, Cagnon, C, Demarco, J, McNitt-Gray, M, Dose to Partially Irradiated Organs as a Function of Z-Axis Over-Ranging and Scan Over-Prescription for Chest CT Examinations.  Radiological Society of North America 2014 Scientific Assembly and Annual Meeting, - ,Chicago IL. http://archive.rsna.org/2014/14010978.html