Imaging Dose Evaluation of the Varian Probeam CBCT Using Radiophotoluminescent Glass Dosimeters
Abstract
Purpose
Accurate patient positioning is critical in proton therapy, where range uncertainties increase sensitivity to setup errors; therefore, cone beam computed tomography (CBCT) is routinely used for image guidance. However, it also increases radiation dose to normal tissues. This study evaluated imaging doses from clinical CBCT protocols from the Varian ProBeam system.
Methods
Radiophotoluminescent glass dosimeters (RPLGD), model GD-352M, were calibrated for air kerma using 70-kVp x-ray beam (HVL = 2.9 mm Al), with beam quality dependence accounted for 70–140 kVp (HVLs = 2.90–9.02 mm Al). Measurements were performed using RPLGD GD-352M placed in an anthropomorphic phantom representing head and neck, thorax, and pelvis treatment sites. CBCT scans used Varian ProBEAM preset protocols: Head (100 kVp/811 mAs), Thorax (125 kVp/1525 mAs), Pelvis (125 kVp/1646 mAs), and Image Gently (80 kVp/548 mAs). Protocols were selected based on clinical practice, with Image Gently applied to the head and neck and pelvis regions for pediatric imaging.
Results
Organ doses from the Head protocol were 1.9±0.4, 1.8±0.2, 1.4±0.1, 1.3±0.1, 1.2±0.4, 1.2±0.1, 1.1±0.5, and 0.8±0.2 mGy for submandibular gland, thyroid, oral cavity, brainstem, optic nerve, chiasm, brain, and eye lens, respectively. For the Thorax protocol, organ doses were 4.7±0.2, 4.7±0.3, 4.6±0.3, 4.4±0.1, 4.4±0.3, and 3.9±0.2 mGy for the thyroid, breasts, heart, thymus, lungs, and esophagus, respectively. For the Pelvis protocol, organ doses were 14.8±0.8, 14.0±1.3, 12.5±0.5, 11.3±0.5, and 11.2±0.5 mGy for the bladder, bowel, rectum, uterus, and ovary, respectively. Higher organ doses were observed for protocols with higher mAs. The Image Gently protocol reduced organ doses by 70% compared with the Head protocol and by 98% compared with the Pelvis protocol.
Conclusion
This study provides organ doses for clinical Varian ProBeam CBCT protocols across multiple anatomical regions, enabling estimation of cumulative imaging dose over a treatment course.