  Cone beam CT (CBCT) has been widely used for patient setup in image guided
radiation therapy (IGRT). Radiation dose from CBCT scans has become a clinical
concern. The purposes of this study are 1) to commission a GPU-based Monte
Carlo (MC) dose calculation package gCTD for Varian On-Board Imaging (OBI)
system and test the calculation accuracy, and 2) to quantitatively evaluate
CBCT dose from the OBI system in typical IGRT scan protocols. We first
conducted dose measurements in a water phantom. X-ray source model parameters
used in gCTD are obtained through a commissioning process. gCTD accuracy is
demonstrated by comparing calculations with measurements in water and in CTDI
phantoms. 25 brain cancer patients are used to study dose in a standard-dose
head protocol, and 25 prostate cancer patients are used to study dose in pelvis
protocol and pelvis spotlight protocol. Mean dose to each organ is calculated.
Mean dose to 2% voxels that have the highest dose is also computed to quantify
the maximum dose. It is found that the mean dose value to an organ varies
largely among patients. Moreover, dose distribution is highly non-homogeneous
inside an organ. The maximum dose is found to be 1~3 times higher than the mean
dose depending on the organ, and is up to 8 times higher for the entire body
due to the very high dose region in bony structures. High computational
efficiency has also been observed in our studies, such that MC dose calculation
time is less than 5 min for a typical case.
