Speaker
Description
Introduction
This study aimed to quantify the cumulative absorbed dose delivered to pelvic organs at risk (OARs) from kilovoltage cone-beam computed tomography (kV-CBCT) during image-guided radiation therapy (IGRT) for prostate cancer.
Methods and Materials
A retrospective analysis was conducted on a 105-patient cohort treated with a Varian OBI system. To model the 125 kV source (equipped with a half-fan bowtie filter), we utilized the EGSnrc/BEAMnrc Monte Carlo toolkit, generating a phase space file of 3×109 particles. Patient 3D phantom geometries were generated directly from each patient's planning CT. We used EGSnrc/DOSXYZnrc to simulate image acquisition and generate 3D dose distributions to quantify the absorbed doses in OARs.
Results
Mean cumulative absorbed doses were 4238.40 mGy (right femoral head), 4092.16 mGy (left femoral head), 3720.23 mGy (bladder), 3197.2 mGy (rectum), 2156.30 mGy (penile bulb), and 2122.29 mGy (bowel). Bladder and rectum doses were high due to their complete inclusion within the imaging field. The high femoral head doses occurred because of the predominant photoelectric effect in bone tissue at kV energy levels. The total number of CBCT scans per patient was the most critical factor driving cumulative dose variation. Cumulative imaging doses exceeded the AAPM TG-180 recommended 5% therapeutic threshold in 68% and 64% of cases for the right and left femoral heads, respectively, as well as 51% for the bladder and 11–28.5% for other OARs. These contributions should be integrated into treatment planning.
Key Words
CBCT, Monte Carlo, Cumulative Imaging Dose, Prostate IGRT