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Analysis of Dose Calculation Accuracy in Cone Beam Computed Tomography with Various Amount of Scattered Photon Contamination

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Document pages: 19 pages

Abstract: Cone-beam computed tomography (CBCT) images haveinaccurate CT numbers because of scattered photons. Thus, quantitative analysisof scattered photons that affect an electron density (ED) curve and calculateddoses may be effective information to achieve CBCT-based radiation treatmentplanning. We quantitatively evaluated the effect of scattered photons on theaccuracy of dose calculations from a lung image. The Monte Carlo method wasused to calculate CBCT projection data, and we made two calibration curves forconditions with or without scattered photons. Moreover, we applied cuppingartifact correction and evaluated the effects on image uniformity and dosecalculation accuracy. Dose deviations were compared with those of conventional CT in conventional and volumetric intensitymodulated arc therapy (VMAT) planning by using γ analysis and dose volume histogram (DVH) analysis. We found thatcupping artifacts contaminated the scattered photons, and the γ analysis showed that the dosedistribution was most decreased for a scattered photon ratio of 40 . Cupping artifact correctionsignificantly improved image uniformity; therefore, ED curves were near ideal,and the pass rate results were significantly higher than those associated withthe scattered photon effect in 65.1 and 78.4 without correction, 99.5 and97.7 with correction, in conventional and VMAT planning, respectively. In theDVH analysis, all organ dose indexes were reduced in the scattered photonimages, but dose index error rates with cupping artifact correction wereimproved within approximately 10 . CBCT image quality was strongly affected byscattered photons, and the dose calculation accuracy based on the CBCT imagewas improved by removing cupping artifacts caused by the scattered photons.

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