TU‐G‐103‐06: Evaluations of Cone Beam CT Dose in Image Guided Radiation Therapy for Brain Cancer Patients Via GPU‐Based Monte Carlo Simulations

Purpose: Cone beam CT (CBCT) has been widely used for patient setup in image guided radiation therapy (IGRT). The radiation dose from CBCT scans is a clinical concern. The purpose of this study is to quantitatively evaluate CBCT dose to brain cancer patients under IGRT using a GPU‐based Monte Carlo CBCT dose calculation package gCTD. Methods: We first performed dose measurements in a water phantom scanned under Varian OBI system and the measurement data are used to commission gCTD. Eight brain cancer patients under IGRT are studied. For each patient, dose distribution under a standard head scan protocol is computed, in which a 100 kVp x‐ray source rotates around the patient head in a 200‐degree range through the posterior side. Mean dose to key organs are computed. Mean dose to 2% voxels that have the highest dose is also computed to characterize dose inhomogeneity. Results: For each CBCT scan, dose to brain, brainstem, chiasm, eyes, and optical nerves ranges in 0.24–0.28cGy, 0.22–0.31cGy, 0.13–0.17cGy, 0.036–0.13cGy, and 0.061–0.17cGy, respectively, depending on the patient head size and isocenter position. The dose is not homogeneous and the mean doses to 2% voxels with the highest dose for these organs are 0.40‐0.58cGy, 0.30–0.43cGy, 0.18–1.02cGy, 0.063– 0.28cGy, and 0.093–0.83cGy, respectively. Although mean dose inside body is 0.26–0.41cGy, the maximum dose can be up to 3.3 cGy, which locates on the bone. It takes about 1 minutes on an Nvidia C2050 GPU card to simulate 1 billion source photons, yielding an average relative uncertainty less than 1%. Conclusion: For the standard head scan protocol, while average CBCT dose to each organ is relatively low, the dose distribution is not homogeneous and the maximum dose can be substantially high. gCTD provides an efficiency and accurate way to evaluate CBCT dose in IGRT.