Room: Room 207
Purpose: The purposes of this study are to estimate the organ equivalent dose and the effective dose for four-dimensional cone-beam computed tomography (4D-CBCT) using Monte Carlo technique, and to evaluate the excess absolute risk (EAR) of secondary cancer incidence using the organ equivalent dose.
Methods: The EGSnrc/BEAMnrc was used to simulate the on-board imager (OBI) mounted the TrueBeam. The OBI was modeled based on percent depth dose (PDD) and off-center ratio (OCR) measured using 3D water phantom and chamber for 125 kV. A calibration factor which convert relative dose to absolute dose was calculated using CT dose index phantom.For clinical cases, 10 lung and liver cancer patients were simulated in EGSnrc/DOSXYZnrc. These simulations assumed that the number of 4D-CBCT acquisitions was 4 and 30. Equivalent doses to skin, thyroid, esophagus, lungs, heart, liver, spleen, stomach, pancreas and kidneys were calculated using mean doses and weighting factors for each organ. The effective dose for 4D-CBCT was calculated by accumulating each equivalent dose. Furthermore, we calculated EARs of secondary cancer incidence for each organ based on published data.
Results: The simulated PDD and OCR agreed with measured PDD and OCR within 2%. The effective doses of 4 and 30 CBCT acquisitions were 40.1 Â± 10.1 and 300.5 Â± 75.6 mSv for lung patients; 80.7 Â± 12.8 and 605.4 Â± 96.2 mSv for liver patients. The lungâ€™s EARs of 4 and 30 acquisitions were 5.2 and 39.0 case per one million persons-year for lung patients. The liverâ€™s EARs of 4 and 30 acquisitions were 5.2 and 39.2 cases per one million persons-year for liver patients.
Conclusion: Our results showed the effective imaging dose and EAR of secondary cancer incidence increased in proportional to the number of acquisitions. For clinical implementation, it might be necessary to avoid the extra 4D-CBCT acquisitions.