Preliminary research on the dose response of polymer gel dosimeter with carbon beam irradiation

This paper is to investigate dose response of polymer gel dosimeter after carbon irradiation. We employed polymer gel dosimeter, MAGAT-f to measure the dose response curve, single spot shape and Bragg peak range of carbon beam. Irradiation energy was 80.55 MeV/u. In order to characterize the dose response, R2-dose curve and absorbance-dose curve were built up. An approximate linearity is found for R2-dose relationship. Saturation is found for the absorbance of the gel after 10 Gy. The results illustrate the feasibility of utilizing polymer gel dosimeter for Bragg peak and spot shape measurement of carbon ion beam.


Introduction
Carbon ion beams have the advantage of dose conformity compared with X-rays radiotherapy.Polymer gel dosimeter is a promising tool for patient-specific QA due to its advantage of 3D dose measurement with high resolution.Kantemiris et al [1] investigated carbon beam dosimetry using VIP polymer gel.Maeyama et al [2] studied the radiological characteristics of VIP polymer gel under carbon beam irradiation.Mizukami et al [3] studied a nanocomposite Fricke gel dosimeter under carbon ion beams irradiation.Furuta et al [4] compared Monto Carlo simulation and measurement with PAGAT gel in biological samples for carbon ion beam.There is little exploration in the dose response of MAGAT-f polymer gel dosimeter with carbon beam irradiation.The purpose of this paper is to investigate the dose response of MAGAT-f gel and to utilize the gel to measure the Bragg peak and spot shape of carbon ion beam.

Gel preparation
The gel was fabricated according to the procedures in [5].All the ingredients were manufactured by Sigma-Aldrich and mass concentration of each component is in Table 1.After preparation, the gel was poured into ten plastic cuvettes for dose response measurement.For Bragg peak and spot shape measurement, the gel was poured into a large volume container.

Irradiation
The Heavy Ion Research Facility in Lanzhou (HIRFL) can accelerate ions from hydrogen to uranium to high energies.It consists of Electron Cyclotron Resonances (ECR) ion sources, the Sector Focusing Cyclotron (SFC), the Separated Sector Cyclotron (SSC) and the Radioactive Ion Beam Line in Lanzhou (RIBLL) etc.In our experiments, the cuvette was positioned at ISO-center and irradiated with carbon ion beam with dose ranging from 0 to 18 Gy.The irradiation energy was 80.55 MeV/u.For Bragg peak and spot shape measurement, the irradiation dose was 10 Gy and dose rate 2 Gy/min.

Imaging
The gel was scanned by a 3.0 T MRI (Phillips Achieva TX, Best, Netherlands) with a head coil.The spin-to-spin relaxation rate (R2=1/T2) was determined using a 16-echo multi-spin echo (MSE) sequence.The scanning condition was echo spacing =22.5 ms; maximum echo time (TE) =360 ms; repetition time (TR) =3000 ms; field of view (FOV) =180 mm; matrix size (MS) =512*512 pixels; slice thickness =3 mm; number of excitations (NEX) =1.In addition, the cuvette was scanned by a spectrometer (GE, Boston, the USA) to investigate the relationship between the absorbance and dose.The wavelength ranged from 380 nm to 900 nm.

Dose response
The dose response curve (Fig. 1) illustrates linear relationship for MAGAT-f gel dosimeter.The absorbance decreases with the scanned wavelength.The integration of absorbance with the wavelength increases with the dose up to 10 Gy and an obvious saturation can be seen in Fig. 2

Spot shape
We measured the spot shape at different depths ranging from 0.3125 mm to 12.5 mm.The lateral dose distribution can be converted with R2-dose curve.Approximate Gaussian distribution is found for the lateral dose distribution at different depths.

Discussion
We study the preliminary dose response of MAGAT-f gel after carbon irradiation.The irradiation energy is 80.55 MeV/u.The dependence of energy on the response curve is not included in this paper.
A comprehensive study on more energies should be conducted.The lateral dose distribution at different depths is analysed.For further research, the polymer gel will be used for patient dose measurement.Furthermore, the effect of dose rate and fraction on the response curve should be investigated for carbon ion beam.All the gel preparation was conducted carefully.However, the reproducibility of the dose response is not investigated.This paper illustrates the preliminary feasibility of MAGAT-f gel dosimeter with carbon ion beam irradiation.The Bragg peak region demonstrates less response (saturation phenomenon).Proper correction method need be investigated for the saturation of Bragg peak.

Conclusion
Dose response curve of polymer gel dosimeter, MAGAT-f was investigated.R 2 and integration of absorbance as a function of dose are employed to characterize the dose response.An approximate linearity is found for the R2-dose relationship, while an obvious saturation is found for the absorbancedose curve after 10 Gy.The results illustrate the feasibility of utilizing polymer gel dosimeter for Bragg peak and spot shape measurement of carbon ion beam.

Figure 4 .
(a) Single spot measured with MAGAT-f gel, (b) T2-weight map (c), and lateral dose distribution at different depths

Table 1
This work was jointly supported by the National Key Research and Development Program (No.2016YFC0105406) and National Natural Science Foundation of China (No.61571262, No.11575095, No.61171115).The authors appreciate Prof. Qiang Li of Institute of Modern Physics, Chinese Academy of Sciences for the experiments support and HIRFL for the irradiation experiments.