TH‐SAMS‐332‐04: Dose Descriptors and Dose Assessment for Cone Beam CT Scanners

Abstract

Computed tomography (CT) dosimetry should be adapted to the fast developments in CT in order to keep pace with new CT technology. Recently, a cone beam CT scanner that challenges the existing Computed Tomography Dose Index (CTDI) dosimetry paradigm was introduced. The appropriateness of existing CT dose metrics for a cone beam CT scanner will be assessed and to new approaches for CT dosimetry will be suggested.Dose measurements with a small Farmer type ionization chamber, and 100mm and 300mm long pencil ionization chambers can be performed free‐in‐ air to characterize the cone beam. According to the most common dose metric in CT, measurements can also performed in 150mm and 350mm long CT head and CT body dose phantoms with 100mm and 300mm long ionization chambers. Effects that cannot be measured with ionization chambers can be explored with Carlo calculations of the dose distribution in 150mm, 350mm and 700mm long CT head and CT body phantoms. To overcome inconsistencies in the definition of CTDI, doses are also expressed as CT_Dose100 (CT_Dose100=160/100 × CTDI100). CTDI300 is chosen as the standard of reference against which results were assessed.Measurements free‐in‐air reveal good‐to‐excellent correspondence between CTDI300air and CT_Dose100air; CTDI100air substantially underestimates CTDI300air. Measurements in phantoms and Monte Carlo calculations reveal good correspondence between CTDI300w, CT_Dose100w and CTDI600w; CTDI100w substantially underestimates CTDI300w. Measurements at different positions within CT dose phantoms with an ionization chamber that is smaller than the cone beam are fundamentally different compared to readings of pencil chambers that are longer than the cone beam.CT_Dose100air and CT_Dose100w are pragmatic metrics for characterizing the dose of the cone beam CT scanner; these quantities can be measured with the widely available 100mm pencil ionization chambers and 150mm long CT dose phantoms. CTDI300air and CTDI300w in a 350mm long CT dose phantom can serve as appropriate standards of reference for characterizing the dose of the cone beam CT scanner. Simple geometrical considerations, supported by Monte Carlo calculations, explain the fundamental differences of different CT dose metrics when measurements are performed at different position within CT dose phantoms.