Cone-beam computed tomography (CBCT) images have
inaccurate CT numbers because of scattered photons. Thus, quantitative analysis
of scattered photons that affect an electron density (ED) curve and calculated
doses may be effective information to achieve CBCT-based radiation treatment
planning. We quantitatively evaluated the effect of scattered photons on the
accuracy of dose calculations from a lung image. The Monte Carlo method was
used to calculate CBCT projection data, and we made two calibration curves for
conditions with or without scattered photons. Moreover, we applied cupping
artifact correction and evaluated the effects on image uniformity and dose
calculation accuracy. Dose deviations were compared with those of conventional CT in conventional and volumetric intensity
modulated arc therapy (VMAT) planning by using γ analysis and dose volume histogram (DVH) analysis. We found that
cupping artifacts contaminated the scattered photons, and the γ analysis showed that the dose
distribution was most decreasedfor a scattered photon ratio of 40%. Cupping artifact correction
significantly improved image uniformity; therefore, ED curves were near ideal,
and the pass rate results were significantly higher than those associated with
the scattered photon effect in 65.1% and 78.4% without correction, 99.5% and
97.7% with correction, in conventional and VMAT planning, respectively. In the
DVH analysis, all organ dose indexes were reduced in the scattered photon
images, but dose index error rates with cupping artifact correction were
improved within approximately 10%. CBCT image quality was strongly affected by
scattered photons, and the dose calculation accuracy based on the CBCT image
was improved by removing cupping artifacts caused by the scattered photons.
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