Radiation field and dose inhomogeneities using an X-ray cabinet in radiation biology research

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

Purpose

Standardization of x-ray cabinet irradiator dose, geometry, and calibration reporting is an ongoing process. Multi-tube designs have been introduced into the preclinical market and give a theoretical benefit but have not been widely assessed for use in preclinical irradiation conditions. The aim of this study was to report our experience commissioning a dual x-ray source cabinet irradiator (CIXD, Xstrahl Limited, United Kingdom) and assess the dose distribution for various experimental conditions.

Methods and Materials

Half-value layer (HVL) measurement, profile measurements, and output calibration were performed using a calibrated ion chamber. Constancy measurements were performed twice daily over 2 weeks to assess output fluctuations. Film measurements were completed using solid water to assess percent depth dose and homogeneity within the field and within variable thicknesses of solid water and phosphate-buffered saline solution. Film measurements were repeated for various arrangements of petri dishes filled with phosphate-buffered saline or water and in a 3D-printed mouse phantom.

Results

The x-ray tubes had a measured in-air output of 1.27 Gy/min. The HVL was 1.7 mm Cu. The upper and lower tubes both exhibited the heel effect, but when operated simultaneously, the effect was reduced. Ion chamber measurements revealed a 15% dose inhomogeneity within the tray area (18 × 18 cm2). Film measurements in the petri dishes indicated minor nonuniformities in the arrangements of the experimental apparatus. Measurements from the mouse phantom with film agreed with ion chamber measurements for various phantom placements and orientations.

Conclusions

X-ray cell culture and animal irradiation with dual tube cabinet irradiation is efficient and robust when using 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 dosimetric monitoring should be performed for each unique irradiation setup.

Dose Variations Using an X-Ray Cabinet to Establish in vitro Dose-Response Curves for Biological Dosimetry Assays

In biological dosimetry, dose-response curves are essential for reliable retrospective dose estimation of individual exposure in case of a radiation accident. Therefore, blood samples are irradiated in vitro and evaluated based on the applied assay. Accurate physical dosimetry of the irradiation performance is a critical part of the experimental procedure and is influenced by the experimental setup, especially when X-ray cabinets are used. The aim of this study was to investigate variations and pitfalls associated with the experimental setups used to establish calibration curves in biological dosimetry with X-ray cabinets. In this study, irradiation was performed with an X-ray source (195 kV, 10 mA, 0.5 mm Cu filter, dose rate 0.52 Gy/min, 1st and 2nd half-value layer = 1.01 and 1.76 mm Cu, respectively, average energy 86.9 keV). Blood collection tubes were irradiated with a dose of 1 Gy in vertical or horizontal orientation in the center of the beam area with or without usage of an additional fan heater. To evaluate the influence of the setups, physical dose measurements using thermoluminescence dosimeters, electron paramagnetic resonance dosimetry and ionization chamber as well as biological effects, quantified by dicentric chromosomes and micronuclei, were compared. This study revealed that the orientation of the sample tubes (vertical vs. horizontal) had a significant effect on the radiation dose with a variation of -41% up to +49% and contributed to a dose gradient of up to 870 mGy inside the vertical tubes due to the size of the sample tubes and the associated differences in the distance to the focal point of the tube. The number of dicentric chromosomes and micronuclei differed by ~30% between both orientations. An additional fan heater had no consistent impact. Therefore, dosimetric monitoring of experimental irradiation setups is mandatory prior to the establishment of calibration curves in biological dosimetry. Careful consideration of the experimental setup in collaboration with physicists is required to ensure traceability and reproducibility of irradiation conditions, to correlate the radiation dose and the number of aberrations correctly and to avoid systematical bias influencing the dose estimation in the frame of biological dosimetry.