The objective of this work is to determine the amount of radiation doses to normal tissues from on-board Cone-beam CT (CBCT) used for daily patient positioning or adaptive radiotherapy for Head and Neck patients during the treatment course.
Materials/Methods
Varian's on-board imager (OBI) device on the Trilogy linear accelerator is investigated. The Monte Carlo code BEAMnrc/EGSnrc was used in the x-ray beam simulation and the dose calculations. The kV x-ray beams from this OBI system have been accurately simulated using the Monte Carlo technique and have been benchmarked against experiments [1]. The accurate description of patient inhomogeneities is adequately supplied by the volumetric images acquired from the CBCT acquisition. The dose to the patient geometry was calculated when the CBCT x-ray beams were rotated around the patient a full circle (360 degrees) as is an actual CBCT image acquisition.
Results
The Monte Carlo calculated dose is calibrated against experimental measurements using an ionization chamber placed at isocenter in a water phantom. The calculated results show 5 cGy to the brain stem and 7.5 cGy to lens and optical nerve. Figure A shows the point dose on a CBCT slice. The values at each point are expressed as percentage dose. A value of 100 corresponds to a dose of 25 cGy. Figure B to D presents the calculated isodose lines on three planes from the reconstructed volumetric CBCT image. The red isodose lines inside bones correspond to 100% dose lines (25 cGy). The much higher doses to the bones are due to the increased photoelectric effect at low energy x-rays.
Conclusions
The detailed Monte Carlo calculations using realistic kV x-ray beam on volumetric CBCT images have shown that there may be significant additional radiation dose to the patient critical organs such as eye and optical nerve for typical CBCT image acquisition for Head and Neck IMRT treatment verification. The CBCT dose to a patient for head and neck IMRT treatment for a total of 33 fractions can range from 165 cGy to brain stem, 250 cGy to eye and optical nerve. In the case when the dose limits to the critical organs are already reached in the patient IMRT treatment plans the additional dose from the CBCT acquisitions may have an adverse effect. The effect of additional significant kV CBCT dose should be taken into account as part of the decision making in the process of Image-Guided Radiation Therapy (IGRT).
[1]G. X. Ding, D. M. Duggan, and C. W. Coffey, “Characteristics of kilovoltage x-ray beams used for cone-beam computed tomography in radiation therapy,” Phys Med Biol, vol. 52, pp. 1595–615, Mar 21 2007.
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