A GATE Monte Carlo dose analysis from Varian XI conebeam computed tomography
Thesis DisciplineMedical Physics
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Cone-beam computed tomography (CBCT) is a commonly used imaging technique in radiation therapy. Images are acquired prior to radiation therapy treatment to assess changes in patient anatomy and correct any errors in patient positioning. CBCT is an x-ray modality and its use for imaging includes a dose to the patient. This study aims to calculate the dose of ionizing radiation received during conebeam computed tomography (CBCT) imaging. Monte Carlo (MC) methods were used to perform these dose calculations. The CBCT scanner examined in this study was the Varian Truebeam X-ray Imager (XI). The present study is comprised of two main parts:
• Construction and validation of a MC model of a CBCT imager. The scanner was modelled using the GATE MC simulation toolkit. The results simulated by the model were compared with experimental measurements. Precise dose measurements were then made to convert the simulated dose into experimental dose for a given x-ray tube potential.
• Estimation of organ doses and effective dose in a human phantom. The GATE CBCT model was used to perform MC dose calculations in a population-based computational phantom. The absorbed dose calculations were converted into effective dose to evaluate the risk of developing secondary cancers due to CBCT imaging. Patient organ doses were analysed by dose-volume histograms (DVHs) and expressed using the minimum dose delivered to 50% and 10% of the organ volume, D50% and D10%.
For the latter part of the study simulated and measured percentage depth doses (PDD) and profiles were compared. PDDs matched within 2% while profiles matched within 10%. Experimental measurements were performed in an anthropomorphic phantom using thermoluminiscent dosimeters (TLDs). Simulated and experimental results from the anthropomorphic phantom were compared to calculate an energy-specific MC calibration factor. The average difference between simulated and experimental results was 13.7% for pelvis CBCT and 6.4% for head CBCT.
Effective doses were measured for the three main CBCT scan protocols: head and neck, thorax, and pelvis. The pelvis effective dose was the highest at 3.91 mSv ± 0.11 mSv. Thorax CBCT imaging, which uses an identical beam energy and geometry to pelvis imaging but lower mAs, had an effective dose of 1.72 mSv ± 0.07 mSv. Head CBCT imaging uses a lower beam energy and varying beam arrangement to the other sites. The effective dose for head CBCT was 0.289 mSv ± 0.020 mSv.
DVHs were evaluated for six different sites using the three main CBCT imaging protocols. The D50% and D10% were calculated for radiosensitive OARs. For pelvis CBCT imaging, centred on the prostate, the dose at the centre of the scan was 16.9 mGy. For selected radiosensitive organs the bladder (D50%=18.7 mGy, D10%=22.4 mGy), rectum (D50%=17.8 mGy, D10%=18.7 mGy) received the highest absorbed dose. For thorax CBCT imaging, centred in the mid-lung region, the centre dose was 5.1 mGy. For selected radiosensitive organs the heart (D50%=3.58 mGy, D10%=4.85 mGy) and lungs (right: D50%=3.89 mGy, D10%=5.27 ; left:D50%=2.22 mGy, D10%=3.59 mGy) received the highest absorbed dose. For head CBCT imaging, centred on the brain, the highest doses were received when the partial gantry rotation was through the anterior structures of the head. DVH analysis of the eyes showed D50% was around 6 mGy while the D50% of the brain and spinal cord were 2.45 mGy and 1.87 mGy.
The intention of this study was to provide dose detriment and treatment planning data for CBCT imaging using the XI scanner. The DVH data is intended to provide estimates to be used during radiation therapy treatment planning.