Author: Panagiotis Iliopoulos, Marios Myronakis, Kyriaki Theodorou 👨🔬
Affiliation: Medical Physics Department, Medical School, University of Thessaly 🌍
Purpose: To estimate individualized absorbed dose at Organs at Risk (OARs) from kV Cone Beam CT (CBCT) imaging in the pelvic area, prior radiotherapy, and assess excess risk for secondary malignancies.
Methods: Anonymized DICOM datasets of 20 male patients undergoing radiotherapy in the pelvic area were retrospectively used to evaluate individualized organ dose from kV-CBCT imaging with the Varian On-Board Imager (OBI). Phase-space files for the 125 kV energy spectrum with half-fan bowtie filter were generated using the BEAMnrc Monte Carlo application and 3×109 particles. Each DICOM image set was used to construct anatomically accurate individualized patient phantoms at the time of the treatment. Isocenter location was extracted from the DICOM metadata to set up the simulated kV-CBCT acquisition. The DOSXYZnrc application was used to simulate the acquisition with 5 degrees angular sampling. The generated three-dimensional (3D) dose distributions were processed to calculate absorbed dose at OARs.
Results: The average number of imaging sessions per patient was 53 (±12). The average accumulated dose in bladder, rectum, right and left femur, and bowel was 710mGy, 586mGy, 587mGy, 547 and 221mGy respectively. Bladder and rectum were entirely within the imaging field. There was small inter-patient variability in organ dose per imaging session. There was greater variation in the accumulated organ dose with differences attributed to number of sessions per patient, anatomy, and isocenter alignment. The highest dose recorded was 1.4 Gy in bladder and right femur for the same patient.
Conclusion: This study utilized established Monte Carlo software to estimate individual patient dose to OARs. Depending on imaging fractionation, the accumulated organ dose can exceed 1 Gy. The simulated x-ray source phase space files and 3D dose distributions will be further expanded with currently ongoing simulations to provide an accurate, comprehensive, and extensive set of data as ground-truth for future studies.