[The Study for Standardization of Dose Evaluation Method in Cone-beam CT].

PURPOSE: This study aimed to compare the dose evaluation methods by constructing simulation models using the Monte Carlo calculation code and propose an evaluation method for cone beam CT (CBCT) that ensures accuracy and practicality. METHODS: The Particle and Heavy Ion Transport code System (PHITS) ver. 3.26 was used as the Monte Carlo calculation code. CBCT doses were measured by CB dose index (CBDI) and American Association of Physicists in Medicine task group 111 (TG111) methods. The CBDI was compared with the equilibrium doses obtained by the TG111 method. RESULTS: Although CBDI was lower than equilibrium doses obtained by the TG111 method, its practicality was ensured because it can be measured using the dosimeter and phantom that are commonly used. In contrast, the TG111 method guarantees accuracy, but it is difficult to prepare a long phantom to obtain the equilibrium dose. The TG111 method with a phantom length of 15 cm underestimated the equilibrium dose by 20% compared to that with a phantom length of 45 cm that satisfies the dose equilibrium. Therefore, the equilibrium dose obtained by the TG111 method with a phantom length of 15 cm is multiplied by 1.20 to obtain the equilibrium dose equivalent to that with a phantom length of 45 cm. CONCLUSION: This study has proposed the dose evaluation method that combines guarantees accuracy and practicality in CBCT.

A preliminary report on retrospective dose assessment by FISH translocation assay in FDNPP Nuclear Emergency Worker Study (NEWS).

In Japan, a national project of longitudinal health care and epidemiological research (NEWS) was developed in 2014 to analyse the effects of radiation on human health for workers who responded to the Fukushima Dai-ichi nuclear emergency in 2011. In 2018, peripheral blood for chromosome translocation analysis was collected from 62 workers. Retrospective dose assessment was performed with fluorescence in situ hybridisation translocation (FISH-Tr) assay. The range of estimated doses by FISH-Tr assay was 0-635 mGy, in which 22 workers had estimated doses of more than 189 mGy. Biological dose estimates were five times higher in workers with physically measured total exposure recordings above 70 mGy. It is likely that smoking and medical exposure caused the discrepancy between estimated biological and physical total exposure doses. Thus, there is a possibility that retrospective biodosimetry assessment might over-estimate occupational exposures to workers exposed to chronic radiation during nuclear emergency work.