Dose rate determination for preclinical total body irradiation

Abstract

The accuracy of delivered radiation dose and the reproducibility of employed radiotherapy methods are key factors for preclinical radiobiology applications and research studies. In this work, ionization chamber (IC) measurements and Monte Carlo (MC) simulations were used to accurately determine the dose rate for total body irradiation (TBI), a classic radiobiologic and immunologic experimental method. Several phantom configurations, including large solid water slab, small water box and rodentomorphic mouse and rat phantoms were simulated and measured for TBI setup utilizing a preclinical irradiator XRad320. The irradiator calibration and the phantom measurements were performed using an ADCL calibrated IC N31010 following the AAPM TG-61 protocol. The MC simulations were carried out using Geant4/GATE to compute absorbed dose distributions for all phantom configurations. All simulated and measured geometries had favorable agreement. On average, the relative dose rate difference was 2.3%. However, the study indicated large dose rate deviations, if calibration conditions are assumed for a given experimental setup as commonly done for a quick determination of irradiation times utilizing lookup tables and hand calculations. In a TBI setting, the reference calibration geometry at an extended source-to-surface distance and a large reference field size is likely to overestimate true photon scatter. Consequently, the measured and hand calculated dose rates, for TBI geometries in this study, had large discrepancies: 16% for a large solid water slab, 27% for a small water box, and 31%, 36%, and 30% for mouse phantom, rat phantom, and mouse phantom in a pie cage, respectively. Small changes in TBI experimental setup could result in large dose rate variations. MC simulations and the corresponding measurements specific to a designed experimental setup are vital for accurate preclinical dosimetry and reproducibility of radiobiological findings. This study supports the well-recognized need for physics consultation for all radiobiological investigations.

Fig. 1.

The configurations used for dose measurements. a) In-air calibration setup; b) the solid water phantom; c) the small water box phantom; d) the silicon rubber mouse phantom; e) the silicon rubber rat phantom; and f) the mouse pie cage. Fully opened collimator with the maximum irradiation field, 26.4 × 26.4 cm², projected on the steel plate at a 65 cm focal spot distance.

Fig. 2.

(a1-a2) Custom made small water tank with the sidewall opening for ionization chamber midplane dose measurements (a1) and a vertically inserted film for depth dose measurements (a2). (b) and (c) are photos of mouse and rat phantoms, respectively.

Fig.3.

Comparison of the measured (red) and MC simulated (blue) PDD curves.