Characterizing Dose Rate Variability of a Mark I Cesium-137 Irradiator
Sessoms, Julian Patrick
The Mark I irradiator is commonly used by institutions of higher learning and other facilities to administer a given radiation dose to numerous materials. Growing concern regarding highly variable mouse survival data observed post-irradiation necessitated an investigation of the research irradiator’s dose delivery variability. Dosimetry was performed to determine if dose rate variability of the Mark I irradiator, which uses a cesium-137 gamma ray (662 keV) source, was significant enough to explain variable biologic data, such as inconsistent survival parameters in animal models. Due to the anticipated relatively even distribution of dose, it was hypothesized that dose rate variability would not exceed ±5%, and ultimately not serve as a significant contributor to the decrease in animal survivability or increase in disease susceptibility. Thermoluminescent dosimeters (TLDs), which absorb radiation and subsequently emit light possessing intensity proportional to the energy of the absorbed dose, were placed in four locations of the rodent irradiation pie-shaped cage to simulate the presence of mice within the chamber. A decay-corrected nominal dose of 8 gray (Gy) was administered to the TLDs, corresponding to 5.53 minutes (1.45 Gy/min) within the irradiator. A second method of measurement utilized an ion chamber to determine the number of ionizations within the irradiator chamber; after which, a conversion factor was applied to quantify the dose. A two-way analysis of variance (ANOVA) was used to compare means of dose corresponding to dosimeter positions and trials. It was determined that the mean dose associated with position 1 was statistically significantly different from the mean dose associated with positions 2-4, which was expected. Additionally, the percent error, in comparison to the isodose gradient zone-corrected nominal dose, was determined for the positions of most importance (i.e., positions 2 and 3) and found to be -6.5% and -5.6% for positions 2 and 3, respectively. Though these values were within close proximity of the manufacturer’s accuracy claim of ±5%, they fell just outside of the expected range. Ion chamber results suggested a statistically significant difference between the measured dose and nominal dose; however this was most likely due to limitations associated with the methodology.The results of this experiment failed to support the hypothesis; however measurements suggest a reduced dose rather than an overdose. This further implies that in regard to the animal model results, the measured dose would not explain increased incidence of morbidity (which would be associated with overdose). Therefore, dose rate variability can be eliminated as a potential source of the effects observed in animal models. Additionally, research studies demonstrate that experimental data for animal models could be influenced significantly by variables other than dose rate (e.g., chronic stress, diet, presence of an overly aggressive animal, and genetic make-up, etc.). This is the most likely alternative explanation for the variability of radiation responses in mice.
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