Monte Carlo-Calculated Patient Dose Distributions From KV-Cone Beam CT for IGRT
K Hioki*, F Araki, T Ono, Y Nakaguchi, Y Tomiyama, Kumamoto University, Kumamoto, KumamotoTU-G-103-8 Tuesday 4:30PM - 6:00PM Room: 103
Purpose: To calculate patient dose distributions from kV-cone beam CT (CBCT) for image-guided radiation therapy (IGRT) with the Monte Carlo (MC) method and to evaluate quantitatively organ doses from dose-volume histograms (DVHs).
Methods:The Varian On-Board Imager (OBI) and the Elekta X-ray Volumetric Imager (XVI) systems were modeled using the EGSnrc/BEAMnrc cord system. MC-calculated doses for both kV-CBCT were calibrated by converting ion chamber readings measured using a CT water phantom into absolute doses. The chamber measurements were performed by a Farmer chamber with 60Co absorbed dose-to-water calibration factor. Then MC-dose distributions for kV-CBCT were calculated using patient CT images. In this study, the organ doses of pelvis and head were evaluated from DVHs obtained by the MC-dose distributions. The beam setting for the pelvis with full scan of 360o was 125 kV and 680 mAs with a half-bowtie filter for OBI, and 120 kV and 1056 mAs with a full-bowtie filter for XVI. For the head with half scan of 200o, the beam setting was 100 kV and 145 mAs with a full-bowtie filter for OBI, and 100 kV and 73.2 mAs without the bowtie filter for XVI.
Results:The calculated mean doses for prostate, rectum, and bladder for OBI and XVI were 2.6 and 0.9 cGy, 2.6 and 0.9 cGy, 3.1 and 1.0 cGy, respectively. The absorbed dose in pelvis for OBI was three times higher than XVI. Similarly, mean doses for eye lens, brain stem, and spinal code for OBI and XVI were 0.08 and 0.02 cGy, 0.34 and 0.12 cGy, 0.19 and 0.15 cGy, respectively. The dose for bone increased up to three or four times compared to that of soft tissue.
Conclusion:Monte Carlo-calculated dose distributions are useful to evaluate quantitatively patient doses from kV-CBCT for IGRT.