The dependence of computed tomography number to relative electron density conversion on phantom geometry and its impact on planned dose

Abstract

A computed tomography number to relative electron density (CT-RED) calibration is performed when commissioning a radiotherapy CT scanner by imaging a calibration phantom with inserts of specified RED and recording the CT number displayed. In this work, CT-RED calibrations were generated using several commercially available phantoms to observe the effect of phantom geometry on conversion to electron density and, ultimately, the dose calculation in a treatment planning system. Using an anthropomorphic phantom as a gold standard, the CT number of a material was found to depend strongly on the amount and type of scattering material surrounding the volume of interest, with the largest variation observed for the highest density material tested, cortical bone. Cortical bone gave a maximum CT number difference of 1,110 when a cylindrical insert of diameter 28 mm scanned free in air was compared to that in the form of a 30 × 30 cm(2) slab. The effect of using each CT-RED calibration on planned dose to a patient was quantified using a commercially available treatment planning system. When all calibrations were compared to the anthropomorphic calibration, the largest percentage dose difference was 4.2 % which occurred when the CT-RED calibration curve was acquired with heterogeneity inserts removed from the phantom and scanned free in air. The maximum dose difference observed between two dedicated CT-RED phantoms was ±2.1 %. A phantom that is to be used for CT-RED calibrations must have sufficient water equivalent scattering material surrounding the heterogeneous objects that are to be used for calibration.

Can CT scan protocols used for radiotherapy treatment planning be adjusted to optimize image quality and patient dose? A systematic review

“…This review has highlighted the need to use phantoms which approximately match the size and shape of patients when measuring HUs. 31,40 When reviewing the influence of scan protocol settings, published data is sparse. Considering the number of scanners and the variety of settings within CT protocols, the impact of scan parameters in radiotherapy CT is not well detailed in the literature.…”

“…Anthropomorphic phantoms which mimic patient size and shape are recommended. 31,40 Craig et al 41 used a new design of phantom to assess three radiotherapy TPSs and found an error in the CT calibration curve used in two of them. An incorrect assumption had been made when the calibration curve was initially defined in the TPS that Teflon could be used as a substitute for cortical bone.…”

Evaluation of CT calibration curves from stoichiometric and tissue substitute methods according to tissue characteristics

“…The averaged CT numbers were acquired for a manually defined region of interest (ROI) with an area of 3 cm 2 using E-film software (https://estore.merge.com/na/index.aspx), which helps users to display, analyze and process CT images. Each ROI was considered in the middle of each material away from the edges to avoid partial volume effects influencing the results (Inness et al, 2014). Given that two locations were assigned for each material, the average HUs of these two sites was considered as final HU of that material (Amini et al, 2018).…”

On the prediction of X-ray dose deviations and the influence of CT scan protocols

“…The DOSXYZnrc code was used and available photon energy spectra between 4 and 16 MeV included in the BEAMnrc Monte Carlo code were adopted as input spectra. Homogeneous phantoms were constructed consisting of water with densities of 1.0, 1.02, 1.05, 1.07, and 1.10 g/cm 3 …”

“…Reconstructed HUs also depends on the type of phantom and arrangements of inserts [3,4]. For soft tissue the tube potential between 100 and 140 kVp does not alter the relation between relative ED and HU significantly although differences will be observed for bone > 160 HU [1].…”