Cone-beam CT (CBCT) images are routinely acquired to verify patient position in radiotherapy (RT), but are
typically not calibrated in Hounsfield Units (HU) and feature non-uniformity due to X-ray scatter and detector
persistence effects. This prevents direct use of CBCT for re-calculation of RT delivered dose. We previously
developed a prior-image based correction method to restore HU values and improve uniformity of CBCT images.
Here we validate the accuracy with which corrected CBCT can be used for dosimetric assessment of RT delivery,
using CBCT images and RT plans for 45 patients including pelvis, lung and head sites. Dose distributions were
calculated based on each patient's original RT plan and using CBCT image values for tissue heterogeneity
correction. Clinically relevant dose metrics were calculated (e.g. median and minimum target dose, maximum
organ at risk dose). Accuracy of CBCT based dose metrics was determined using an "override ratio" method
where the ratio of the dose metric to that calculated on a bulk-density assigned version of the image is assumed
to be constant for each patient, allowing comparison to “gold standard” CT. For pelvis and head images the
proportion of dose errors >2% was reduced from 40% to 1.3% after applying shading correction. For lung
images the proportion of dose errors >3% was reduced from 66% to 2.2%. Application of shading correction to
CBCT images greatly improves their utility for dosimetric assessment of RT delivery, allowing high confidence
that CBCT dose calculations are accurate within 2-3%.
T. E. Marchant, K. D. Joshi, and C. J. Moore, "Shading correction for cone-beam CT in radiotherapy: validation of dose calculation accuracy using clinical images," Proc. SPIE 10132, Medical Imaging 2017: Physics of Medical Imaging, 101320J (Presented at SPIE Medical Imaging: February 14, 2017; Published: 9 March 2017); https://doi.org/10.1117/12.2254059.
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