Dose Verification and Monte Carlo Modeling of an Image-Guided Small Animal Radiotherapy Irradiator & Investigation of Occupational Radiation Exposure to Interventional Radiologists from Use of Fluoroscopic Imaging

Abstract

Project 1 (Chapter 2): Dose Verification andMonte Carlo Modeling of an Image-GuidedSmall Animal Radiotherapy IrradiatorPurpose: Preclinical trials play a crucial role in advancing the understanding of cancer biology and developing effective therapeutic interventions. The purpose of this project is to simulate and validate beam output of a Small Animal Radiotherapy Research Platform (SARRP, xStrahl) with both physical dosimetry and Monte Carlo simulation models.Materials and Methods: The SARRP console was set up to deliver an intended dose of 8Gy (4 Gy anterior-posteriorly (AP), 4 Gy posterior-anteriorly (PA)) in 142 seconds to a flat mouse phantom. The x-ray irradiation parameters were set to 13 mA, 220 kVp, with a 33.725 cm source to surface distance. Beam filtration included 0.8 mm Be (inherent) and 0.15 mm Cu (added), with collimation set to 40x30 mm. Dose verification was conducted through two methods: utilizing an energy-calibrated MOSFET dosimeter and employing Monte Carlo Simulations using Monte Carlo N- Particle Transport (MCNP). MOSFET Calibration encompassed four setups to ensure precision. The first two involved calibrating the MOSFET with an ion chamber in air at 0 degrees (Setup 1) and 180 degrees (Setup 2). The subsequent two setups calibrated the MOSFET positioned inside the phantom (Setups 3 and 4) with an ion chamber in air. After the calibrations, the MOSFET, placed inside the phantom, received the intended 4 Gy dose for verification. The MCNP simulation comprised two stages: a point source simulation and a simulation of the x-ray tube. For the point source, the SARRP geometry was replicated, with the x-ray tube modeled as a collimated point source. The x-ray tube simulation entailed modeling components of the xray tube. Validation methods included comparing energy spectra, Half Value Layer (HVL) testing, and film analysis of the anode heel effect.Results: In the dose verification, Setup 1 exceeds the intended 4 Gy dose by 7.72%, while Setup 2 underdoses by 2.96%, resulting in a cumulative overdose of 2.38% for Setups 1 and 2. Setup 3 aligns with the intended 4 Gy dose, underdosing slightly by 0.14%, while Setup 4 underdoses by 2.31%. The cumulative dose for Setups 3 and 4 totals 7.90 Gy, indicating a 1.27% underdose. The two calibration techniques demonstrate a difference of 3.6%. Calibration in air is the preferred method due to the ionization chamber also being present in air. Point Source Simulation yielded doses of (4.27 ± 0.02) Gy (AP) and(3.77 ± 0.02) Gy (PA). X-ray Tube Simulation resulted in (3.95 ± 0.02) Gy (AP). Energy spectrum of the MCNP model showed good agreement with the manufacturer model in key spectral characteristics (peaks, mean energies). HVL comparison showed good agreement with only a 0.5% difference between simulated and experimental half value thicknesses. The anode-heel effect analysis was inconclusive.Conclusions: The dose verification processes establish the SARRP’s efficacy in delivering the intended radiation dose. The integration of advanced measurement techniques set a benchmark for small animal dosimetry and ultimately strengthens the reliability of radiation doses in preclinical studies.Project 2 (Chapter 3): Investigation of Occupational Radiation Exposure to InterventionalRadiologists from Use of Fluoroscopic ImagingPurpose: The purpose of this project is to investigate radiation exposure among Interventional Radiology (IR) physicians using fluoroscopic imaging through experimental data collection and retrospective analysis, with objectives to understand Automatic Exposure Rate Control mechanisms, assess exposure rates to operators, and identify trends amongIR physicians.Materials and Methods: Three interventional fluoroscopes were investigated: Philips AlluraClarity Xper FD 20/15, Philips Allura Xper FD20, and GE Discovery IGS 740. Two phantoms were employed to replicate patient and operator. The “patient” phantom was comprised of water-equivalent slabs (5cm thick, 30cm x 30cm). The “operator” phantom (Atom Dosimetry Labs Adult Male phantom) was placed beside the table and covered with a 0.25 mm lead apron. A Ludlum 9DP pressurized ion chamber was positioned at collar level of the operator phantom. Parameters varied included patient thickness (20cm-40cm), collimation, and fluoroscopy and acquisition modes. Exposure (mR) to “operator”were measured and normalized to number of x-ray pulses. Retrospective analysis used Radiation Dose Structured Report data for an 8-month period. Physician caseload, averageCumulative Air Kerma (CAK) by physician, and total CAK by physician were determined.Results: Based on measured data, acquisition mode exhibits longer pulse widths (7.20x-31.3x) and higher tube current (1.48x-7.58x) compared to fluoroscopy for all units. Tube voltage increases with phantom thickness in both fluoroscopy and acquisition mode for all units. Increasing phantom size and collimated field size elevate the operator exposure rate for all C-arms. Larger field sizes contribute to higher exposure rates compared to small field sizes (4.91x-6.27x fluoroscopy, 5.51x-8.23x acquisition). Additionally, acquisition mode contributes to higher exposure rates (23.4x–107.1x) than fluoroscopy. Analysis of proceduraldata identified trends in case distribution and dose to patients across physicians.Conclusions: Overall, patient size, collimation settings, and fluoroscopy vs. acquisition mode were identified as significant contributors to operator exposure rates. Outliers among IR physicians highlighted the need for targeted interventions to mitigate excessive radiation exposure.