Ⅰ. HIGHLIGHTS OF THE YEAR
RIKEN
Accel. Prog. Rep. 56 (2023)

RIKEN Accel. Prog. Rep. 56 (2023)

Radio-ﬂuorogenic nanoclay gel dosimeters with reduced linear energy
transfer dependence for carbon-ion beam radiotherapy†
T. Maeyama,∗1,∗2 A. Mochizuki,∗1 K. Yoshida,∗1 N. Fukunishi,∗2 K. L. Ishikawa,∗3 and S. Fukuda∗4
The complex therapeutic dose distributions associated with new heavy-ion irradiation techniques require the development of 3D dosimeters; however, theprecise measurement of dose distributions when using
high linear energy transfer (LET) radiation remainsa challenge. Speciﬁcally, the dosimeter sensitivity is
typically decreased, and signal saturation occurs as
a result of recombination reactions induced by highdensity ionization and excitation. Thus, heavy-ion
beam dosimetry necessitates a sensitivity correction
for the LET being used. Consequently, we reported
a magnetic resonance imaging-based 3D chemical gel
ion-beam dosimeter for high LET irradiation that does
not require the correction.1) However, several hundred
Gy is required to perform measurements.
For improving the sensitivity, we examined the development of a new gel dosimeter based on ﬂuorometry, which is the most sensitive detection method forradiation-induced products. Gel dosimeters compriseof a radiation-sensitive compound, a gelling agent, andwater. In the case of low LET radiation, several typesof radio-ﬂuorogenic gel dosimeters have been reported
to date, and we have prepared highly sensitive nanoclay radio-ﬂuorogenic gel (NC-RFG)2,3) dosimetersfrom dihydrorhodamine 123 (DHR123) and nano-sizedclay. The non-ﬂuorescent DHR123 was oxidized toﬂuorescent rhodamine 123 (RD123) via exposure toionizing radiation. The spatial information that describes the absorbed dose distribution was retained in
the nanoclay gel matrix. Consequently, scanning spatial dosimetry can be performed using a ﬂuorescent gel
such as this because the ﬂuorescence intensity increases
linearly with increasing absorbed dose.
In the NC-RFG using DHR123, the dose-responseor sensitivity of this gel dosimeters decreases with increasing LET, because RD123 is produced by oxidation reactions, primarily with OH radicals. Therefore, this study developed a method for reducing theeﬀect of LET on NC-RFG dosimeters. Through theuse of a new and optimal Fe3+ catalyst together withpyridine (Py) as a dispersant, more than 20 samples of NC-RFG with diﬀerent preparation conditions
were investigated. They were irradiated with 135
and 290 MeV/nucleon 12 C6+ ions accelerated by theRIKEN Ring Cyclotron and the Heavy Ion Medical†

∗1
∗2
∗3
∗4

Condensed from the article in Med. Phys. 50, 1073 (2023)
Department of Chemistry, Kitasato University
RIKEN Nishina Center
Department of Nuclear Engineering and Management, University of Tokyo
QST Hospital, National Institutes for Quantum Science andTechnology- S41 -Accelerator in Chiba, respectively. In this paper, only
the preparation conditions after optimization are presented.
The prepared NC-RFG was deaerated with 1 mM
Fe3+ , 100 mM Py, 100 µM DHR123, and 2 wt% Clay.
The dose response of ﬂuorescence intensity (IFL ) of irradiated samples was obtained via a ﬂuorescence gel
scanner. Figure 1(a) shows the IFL distributions obtained from the Fe3+ -Py-DHR123 NC-RFG using a290 MeV/nucleon 12 C6+ ion beam for various entrance
surface doses (ESDs). The distributions in (b) weredetermined by subtracting the pre-exposure distribution and dividing by the ESD. These plots demonstrate
good agreement with one another, and the correlation coeﬃcient (R2 ) of IFL versus dose was greaterthan 0.995 even near the Bragg peak, as shown in(c). Figure 1(d) presents a comparison of the relativeδIFL /ESD and absorbed dose distributions as evaluated through a standard method of dosimetry using
an ionization camber (IC). These data conﬁrm thatthe relative δIFL /ESD distribution closely matched thedose distribution. Although the LET value increased
to 266 eV/nm at the Bragg peak, the sensitivity wasalmost constant.4)

Fig. 1. Radiological properties of NC-RFG after irradiation
with a carbon-ion beam (12 C6+ 290 MeV/nucleon).4)

NC-RFG resulted in a signiﬁcant reduction in theunwanted eﬀect of the LET. The promotion of Fentonlike reactions decomposed H2 O2 (the main product of
the high-LET ion radiolysis of water) and thus en-

Ⅰ. HIGHLIGHTS OF THE YEARRIKEN Accel. Prog. Rep. 56 (2023)

hanced the oxidation reaction of DHR123. For the ﬁrst
time, this proposed dosimeter allows an evaluation of
the depth dose proﬁle when using a carbon-ion beam
at a typical therapeutic dose level of several Gy without the need to perform sensitivity corrections basedon LET.4)
References
1) T. Maeyama et al., J. Phys. Chem. B 121, 4238 (2017).
2) T. Maeyama et al., Sens. Actuators A Phys. 298,111435 (2019).
3) T. Maeyama et al., J. Photochem. Photobiol. A 418,113423 (2021).
4) T. Maeyama et al., Med. Phys. 50, 1073 (2023).

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