Masatoshi Suzuki

Radioactive materials are released into the environment and then transferred to animals and humans whenever atmospheric nuclear tests or nuclear disasters occur (Suzuki et al. 2019). Radioactive pa...

Following the Fukushima Daiichi Nuclear Power Plant (FNPP) accident, water-insoluble silicon dioxide microparticles adsorbing radioactive cesium (CsMPs) were found in the environment. CsMPs are classified into two categories, designated as type A and type B, based on their size and radioactivity. Inhalation of CsMPs is thought to affect surrounding cells at the deposition sites, both through exposure to ionizing radiation and contact with silica particles. In this study, to understand the cellular effects of CsMPs, we performed a comparative analysis of gene expression changes in epithelial cells co-incubated with a type B CsMP or with a control nonradioactive (mock) particle using DNA microarray analysis. Gene expression was defined as upregulated when it increased by 1.5-fold or more, and downregulated when it decreased by 1.5-fold or lower after co-incubation with the particle. Confluent normal human retinal epithelial hTERT-RPE1 cells were co-incubated with either of the particles in a 6.4 mm diameter well for 24 h. Co-incubation with the CsMP revealed that the number of upregulated genes was greater than that of downregulated genes. Gene Ontology Biological Process (GO-BP) analysis revealed that five pathways were enriched for the upregulated gene set. Three of these were related to viral infection, while the other two were related to interferon. The CsMP upregulated several interferon-stimulated genes (ISGs), whose basal expression is reported to protect cells from stress. STAT1 is known to up-regulate the expression of its downstream ISGs. These suggest that type B CsMP, which has high radioactivity and is not phagocytosed by cells, activates the STAT1-ISG pathway in cells surrounding its deposition. As a result, it is surprising that the particle nature of CsMPs cannot be ruled out as a factor that may mitigate the biological effects on normal cells.

Occupational radiation exposure in nuclear medicine presents complex spatial and temporal patterns due to the use of unsealed radiopharmaceuticals and prolonged proximity to patients. Traditional passive dosimetry provides only cumulative dose values, limiting its usefulness in identifying task-specific exposures or capturing momentary fluctuations. This study applied a real-time dosimetry system capable of second-by-second measurements, combined with time-series analysis, to evaluate staff exposure during myocardial perfusion imaging using technetium-99m. Dosimeters were placed on the left and right sides of the neck and head of two radiological technologists. Dose rates were continuously recorded throughout the injection and imaging phases. The right side of the neck received the highest cumulative and peak dose rates among all sites. Although no significant difference in total dose was observed between the injection and imaging phases, specific high-exposure events were detected. Notably, ECG lead placement and post-injection handling produced dose spikes. A positive correlation was found between administered activity and dose rate at neck-level sites but not at head-level sites. These findings demonstrate the value of real-time dosimetry in identifying procedural actions associated with elevated exposure. Time-series analysis further contextualized these peaks, supporting improved task-specific protective strategies beyond the capabilities of conventional dosimetry.

The International Commission on Radiological Protection has reduced the occupational eye lens dose limit to 20 mSv/year. Interventional radiology (IR) physicians are frequently exposed to high levels of scattered X-rays, and radiation protection tools, including lead glasses, are used for minimizing exposure. However, the effectiveness of lead eyewear for radiation workers who wear prescription glasses (e.g. over-glasses-type eyewear) has not been comprehensively investigated. This study evaluated the shielding effectiveness of over-glasses-type eyewear worn over plastic prescription glasses through phantom experiments. Furthermore, the dose reduction provided by the plastic lenses and front frame of the prescription glasses (excluding lead) was assessed. The over-glasses-type eyewear demonstrated high shielding effectiveness, with average shielding rates of 0.72–0.74 and 0.74–0.80 for the left and right eyes, respectively, against radiation incident from the lower left oblique, as typically encountered in clinical settings. In contrast, the plastic lenses of prescription glasses provided <9% dose reduction when it irradiated with 80- and 120-kV X-rays, indicating minimal contribution to eye protection. The plastic front frame offered a 14% reduction at 80 kV and 11% at 120 kV. These findings suggest that the front frame of the prescription glasses contributes to photon attenuation and improves overall shielding. However, the improper fit of over-glasses-type eyewear may lead to gaps, compromising its shielding effectiveness. To minimize gaps and optimize protection, IR physicians should select over-glasses-type eyewear that fits securely over prescription glasses. Additionally, combining protective tools is essential for effective eye protection.

To apply electron spin resonance (ESR) dosimetry to wild Japanese macaques captured in the ex-evacuation area during the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident, the improvement of the detection limit is crucial. In this study, we optimized the microwave power in ESR measurements to suppress the noise of native radicals and enhanced the signals of radiation-induced carbonate radicals. Tooth enamels of a Japanese macaque captured in a control area were prepared and irradiated with gamma rays from ⁶⁰Co source. The ESR spectra of the enamel samples with different absorbed doses ranging from 0 to 1800 mGy were measured with varying microwave power. The ESR spectra were analyzed by an in-house multi-component decomposition code using a simulated annealing method. Intensities of both components originating from carbonate and native radicals saturated and decayed as the microwave power increased. The intensity ratio of carbonate radicals to native radicals, i.e., signal to noise ratio, increased monotonically at microwave powers below 30 mW. We also examined the linearity of the intensity of carbonate radicals against the absorbed doses and recommended a microwave power range of 5-25 mW. In this study, we showed that optimizing the microwave power is an effective way to improve the quantitation accuracy of carbonate radicals in samples with low absorbed doses. The improved measurement conditions will expand the applicable range of ESR dosimetry for research on the effects of radiation on wild animals related to the FDNPP accident.

We developed a new computer program for the application of electron spin resonance (ESR) to dosimetry of wild animals related to the Fukushima Daiichi Nuclear Power Plant accident. The ESR spectra of carbonate radicals and other inorganic radicals are calculated by the complete elliptic integral. A simulated annealing method is implemented for the parameter optimization. A cost function is designed to include the second derivative form of the microwave absorption spectrum to improve the fitting accuracy. As a testing ground for the developed code, we prepared tooth enamel samples from a Japanese macaque captured in a control area. The developed code well reproduced the measured ESR spectrum. With a test spectrum, we demonstrated that the cost function that includes the second derivative form of the microwave absorption spectrum is helpful for the precise analysis of the low-dose enamel samples. The smoothness of the ESR spectrum plays an important role in utilizing this feature. The developed computer code can be used to analyze the ESR spectrum of tooth enamels of Japanese macaques. A precise analysis is essential to lower the detection limit and expand the applicability of ESR dosimetry. The code is independent of the computer operating system and is available publicly.

This study investigated the induction of DNA double-strand breaks (DSBs) in the hTERT-immortalized normal human diploid epithelial cells (RPE1-hTERT) continuously exposed to 6000 Bq/ml of tritiated water (HTO) and organically bound tritium (OBT). The relationship of the DSBs induction with the intracellular amount as well as the localization of tritium was also examined. Tritium-labeled thymidine (3H-Thy) and palmitic acid (3H-PA) were used as OBT. The average number of DSBs, which were indicated as co-localized foci of 53BP1 with phosphorylated H2AX, per cell was higher in the order of treatments with 3H-Thy, 3H-PA, and HTO. This order was consistent with that of tritium localization in the insoluble nuclear fraction but not with the intracellular amount of tritium. In addition to the intracellular amount of tritium, we showed that the subcellular localization of tritium is an important factor in cellular effects stimulated by DSBs.

Senescence-like growth arrest (SLGA), which is a radiation-induced cell death pathway, is induced in immortalized normal human epithelial cell (hTERT-RPE1) by the daily fractionated X-irradiation with 1.5 Gy within 30 times. We here demonstrate that pre-treatment induces acquired radioresistance (ARR) that can survive from the lethal fractionated radiation. The parent cells were daily fractionated with 1.5 Gy for 5 d and then incubated for 7 d without fractionated radiation. After this, the daily fractionated radiation with 1.5 Gy was restarted. A small population of surviving cells appeared after 30 times of the daily fractionated radiation was completed and they were continuously growing up to 120 times of the daily fractionated radiation (RPE1–1.5Fr). We confirmed a higher basal expression level of p53, which functions in the activation of the SLGA pathway but fails to further accumulate after 1.5 Gy of single irradiation in RPE1–1.5Fr. It is the first report to induce ARR phenotype for fractionated radiation in normal human cells.

Brachytherapy is a practical, effective procedure for the local treatment of cancer; it delivers a high radiation dose to a limited tissue volume while sparing the surrounding normal tissues. Although the clinical benefit of brachytherapy is clear, there have been very few studies on the radiation dose received by physicians during the procedure. Furthermore, no study has investigated the eye radiation dose received by physicians performing ¹⁹⁸Au grain (seed) brachytherapy, using an eye dosimeter. Recently, the International Commission on Radiological Protection (ICRP) recommended significantly reducing the occupational lens dose limit, from 150 to 20 mSv/yr (100 mSv/5 years). Therefore, it has become essential to evaluate the eye radiation doses of medical workers. We evaluated the eye radiation dose of a brachytherapy physician performing ¹⁹⁸Au permanent grain implantation for tongue cancer; this is the first study on this topic. The maximum eye dose was ~0.1 mSv/procedure, suggesting that it is unlikely to exceed the ICRP limit (20 mSv/yr) for the lens, unless many procedures are performed with inappropriate radiation protection. To reduce the dose of radiation received by ¹⁹⁸Au grain brachytherapy physicians, it is necessary to use additional lead shielding equipment when preparing the treatment needles, i.e., when loading the grains. This study provides useful information on radiation exposure of physicians conducting ¹⁹⁸Au permanent grain brachytherapy.

After the Fukushima nuclear power plant accident in 2011, many types of survey meters were used, including Geiger–Müller (GM) survey meters, which have long been used to measure β-rays. Recently, however, a novel radiation survey meter that uses a plastic-scintillation sensor has been developed. Although manufacturers’ catalog data are available for these survey meters, there have been no user reports on performance. In addition, the performance of commercial plastic-scintillation survey meters has not been evaluated. In this study, we experimentally compared the performance of a plastic-scintillation survey meter with that of a GM survey meter. The results show that the two instruments performed very similarly in most respects. The GM survey meter exhibited count losses when the radiation count rate was high, whereas the plastic-scintillation survey meter remained accurate under such circumstances, with almost no count loss at high radiation rates. For measurements at background rates (i.e., low counting rates), the counting rates of the plastic-scintillation and GM survey meters were similar. Therefore, an advantage of plastic-scintillation survey meters is that they are less affected by count loss than GM survey meters. We conclude that the plastic-scintillation survey meter is a useful β-ray measuring/monitoring instrument.