Ⅲ-3. Radiochemistry & Nuclear Chemistry

RIKEN Accel. Prog. Rep. 53

Preliminary investigation of nanoclay-gel-based fluorescent gel
dosimeters under carbon-ion beam
T. Maeyama,∗1,∗2 A. Mochizuki,∗1,∗2 N. Fukunishi,∗2 and K. L. Ishikawa∗2,∗3
High-precision radiotherapy that focuses high doses ofradiation on tumors and decreases the amount of damage to healthy tissues is becoming popular. To validate
the complex dose distribution, tissue-equivalent dosimetry with high spatial resolution in three dimensions isrequired. Gel dosimetry has attracted attention for doseverification in advanced radiotherapy. A gel dosimeteris prepared from a radiation-sensitive compound and agelling agent. After a radiation-induced chemical reaction, the gel retains the same spatial distribution, andthe 3D dose distribution can be evaluated by 3D gel scanning. Gel dosimeters are categorized according to thecharacterization method used the nuclear magnetic resonance (NMR) measurement method,1–5) absorptiometry(X-ray6) or visible light7,8) ), and fluorometry. The sensitivity of detection by fluorometry9–11) is approximately1,000 times greater than that by visible-light absorptiometry because fluorescence can be detected against
a low background, isolated from excitation light. In contrast, absorptiometry measures transmitted light relative
to high incident light levels at the same wavelength. The
measurement methods using NMR and X-ray computed
tomography (CT) also show limited detection sensitivity,
and the dose of irradiation products must be at least several Gy for successful detection. In contrast, mGy-level
dose can be detected by fluorometry. Thus, gel dosimeters employing fluorometry are expected to be the mosthighly sensitive type of gel dosimeter.
In a recent study, we reported a nanoclay-gel-based
dosimeter that uses fluorescent dyes for X-ray irradiation. Although most gel dosimeters are prepared usingorganic gelling agents, such as gelatin, sensitivity can beenhanced by introducing inorganic nanosized clay as thegelling agent.9–11) There are two types of such agents.
The first is nanoclay-based radio-fluorogenic gel (NCRFG), in which a fluorescent dye is produced when anon-fluorescent dye, such as coumarin-3-carboxylic acid(CCA), reacts with radicals formed from the radiolysisof water. The second type of gel is nanoclay-based radio degradation fluorescence gel (NC-RDG), in which
a non-fluorescent dye is produced when a fluorescentdye, such as rhodamine 6G (R6G) or 7-diethylamino-4methylcoumarin (7D4MC), reacts with radicals. In thisstudy, nanoclay-gel-based fluorescent gel dosimeters wereapplied to heavy-ion-beam irradiation for preliminary investigation.
Fluorescent gels were prepared using nanoclay as thegel, deionized water, and a fluorescence dye or probe.
Two types of gel dosimeters prepared for this study. Onewas prepared from 2 µM 7D4MC and 2.5-wt% nanoclay
as follows. First, clay dispersion stock was prepared by∗1∗2∗3Kitasato Univ
RIKEN Nishina Center
Univ. of TokyoFig. 1. Scanned image of fluorescent gel after irradiation with135 MeV/nucleon 12 C6+ (entrance surface dose = 12 Gy).

mixing 23.35 g of ultra-pure water (Simplicity UV, Millipore Corp.) with 0.75 g of clay in a glass beaker atroom temperature. A fluorescence probe stock solution(500 mL) was prepared by mixing 1.14 mg of 7D4MCwith water. The clay dispersion stock (24.1 g) and fluorescence probe stock solution (5.9 mL) were then introduced to a 50 mL screw-capped glass vial and mixedusing a rotation/revolution vacuum mixer (V-mini300,EME Corp., Japan) for 10 min at 1200/600 rpm and−90 hPa gauge pressure to obtain a uniform dispersion.
The other was prepared from 100 µM dihydrorhodamine 123 (DHR123) and 2.5-wt% nanoclay using arotation/revolution vacuum mixer with the same procedure. Finally, prepared gels were enclosed in a 100×100×10 mm plate. The prepared samples were irradiated using 12 C6 -ion beams (135 MeV/nucleon) accelerated by
the RI Beam Factory (RIBF) at RIKEN. The irradiatedsamples were scanned using a fluorescent gel scanner.
The irradiation changes in fluorescence intensity withirradiation were observed, as shown in Fig. 1. The left
image shows the entrance surface. In 7D4MC fluorescentgel, the fluorescence intensity decreases with increasing
dose, and in DHR123 fluorescent gel, the fluorescenceintensity increases with increasing dose. The detailed
results are currently being analyzed, and some results
will be submitted.
References

1) T. Maeyama et al., J. Radiat. Res. 57, 318 (2016).
2) T. Maeyama et al., J. Phys. Chem. B 121, 4238 (2017).
3) T. Maeyama et al., Radiat. Phys. Chem. 96, 92 (2014).
4) T. Maeyama et al., Radiat. Phys. Chem. 107, 7 (2015).
5) T. Maeyama et al., Radiat. Phys. Chem. 146, 121 (2018).
6) E. Maynard et al., Phys. Med. Biol. 63, 075014 (2018).
7) K. I. Penev et al., Med. Phys. 44, 1948 (2017).
8) I. Kawamura et al., Jpn. J. Appl. Phys. 58, 022003 (2019).
9) T. Maeyama et al., J. Phys. Conf. Ser. 1305, 012045 (2019).
10) T. Maeyama et al., Sens. Actuators A 298, 111435 (2019).
11) T. Maeyama et al., Radiat. Phys. Chem. 151, 42 (2018).

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