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Further Understanding of DLP and DLI as Radiation Dosimetry of Cone-Beam CT

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Z Qi

Z Qi1*, J Zhang2, C Alsip3, L Lemen4, (1) University of Cincinnati, Cincinnati, OH, (2) University of Kentucky, Lexington, KY, (3) Cincinnati Childrens Hospital Medical Center, Cincinnati, OH, (4) University of Cincinnati, Cincinnati, OH

SU-E-I-6 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Traditional 100mm pencil-shaped ion-chamber cannot be directly applied to dose estimation of cone-beam CT. IEC recommended adjusting traditional CTDI₁₀₀ by nominal beam width and then calculating Dose-Length-Product (DLP) by multiplying scan length. An alternative approach is to measure dose in the central scan plane using a small volume ionization chamber. If the scan length is sufficiently long to achieve scattered equilibrium, the dose at the central location asymptotically approaches the equilibrium dose . Patient dose can then be estimated using Dose Line Integral (DLI= x scan length). This study aims to further understand these concepts, their limitations and the relationship.

Methods: A phantom study was performed on a Toshiba Aquilion One CT scanner (nominal beam width up to 160mm). Three CT head phantoms were joined to capture all possible scattered radiation. First a 100mm ion-chamber was used to measure CTDI with variable nominal beam width, from 16mm to 160mm. Adjusted CTDI was calculated according to IEC Standards (IEC 60601-2-44.ed3). Then an elongated 300mm ion-chamber was used for the same measurement. For DLI, a 0.6cc Farmer ion chamber was used to measure the equilibrium dose.

Results: With the increase of nominal beam width (from 16mm to 160mm), traditional CTDI₁₀₀ significantly underestimated CT radiation dose (12.6mGy vs. 7.5mGy). Adjusted CTDI₁₀₀ could account for this underestimation (12.6mGy vs. 11.9mGy) in some extent, though the estimation was still low compared to CTDI₃₀₀ measurements (approximately 30%). For measurements with Farmer chamber, the method using multi-scan with a narrow beam width to reach equilibrium scan length overestimated CT dose by approximately 40%. For the same scan length, DLI overestimated CT dose, compared to DLP calculated by CTDI₃₀₀ and CTDI₁₀₀.

Conclusion: Both DLP and DLI may be used for dose estimation of cone-beam CT but cannot be exchanged. A conversion factor is needed for cross comparison.

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