Commissioning and Assessment of Radiation Field and Dose Inhomogeneity for a Dual X-Ray Tube Cabinet Irradiator: To Ensure Accurate Dosimetry in Radiation Biology Experiments

Abstract

Standardization of X-ray cabinet irradiators calibration and dose reporting is an ongoing process. Multi-tube designs have been introduced and provide theoretical benefit but have not been assessed for pre-clinical irradiation conditions. This study was to report our experience commissioning a dual x-ray tube cabinet irradiator and assess dose distribution for various conditions. Homogeneity and percent depth dose (PDD) of various tube designs in single-tube setting have been previously reported, however multi-tube designs can overcome field asymmetry caused by the heel effect and improve dose fall-off at depth. Profile measurements and output calibration were performed with a calibrated ion chamber. We measured the percent-depth-dose (PDD) with film for each tube individually (PDDind) and both tubes simultaneously (PDDsim). To replicate conditions similar to petri dishes, we evaluated the range of attenuation and scatter under variable solid water thickness up to 9 mm with 3 mm build-up below (i.e., 1.2 cm total backscatter) under several geometries. The films were placed at a fixed position with variable amount of solid water placed above the film. We measured the output with variable water volumes with a 6 cm petri dish to simulate variable solution volumes that are frequently used during in vitro experiments. Film was also placed in a 3D printed mouse phantom with dimensions of (9.5 × 3.6 × 3.0 cm) and constructed to be roughly cylindrical. The dose in the 20 × 20 cm2 tray was within 8.4% of the central axis dose with both tubes. Dose rates decreased at the furthest points on the tray, but remained within 5% within the 15 × 15 cm2 central area. PDDsim with total 7.9 cm backscatter shows that the output is within 95% of the maximum dose at 4 cm depth. PDDsim with 1.2 cm backscatter is also within 95% of the maximum. The PDDind was less than 90% at 1.8 cm, which is similar to other single-tube systems. Slight increase in dose was seen with increasing solution; however, the doses were within 7% of the measured max dose at 6.3 mm depth. This agrees well with the solid water measurements. The results for both solid water and petri dish with variable medium is shown in Table 1. The 3D printed mouse phantom profiles for each plane showed results consistent with the ion chamber measurements. X-ray cell culture irradiation with dual-tube cabinet irradiation is robust when utilizing established dosimetric tools to confirm output and homogeneity. The conditions assumed for calibrations are often not maintained during experiments. We have confirmed that inhomogeneities are present for single tube use, however they are reduced with simultaneous tube use. Additional monitoring should be performed for each unique irradiation setup.