VIP. MRI magnetic resonance imaging. Keywords: PHITS, gel dosimeter, three dimensional dose distribution, LET dependence, dose verification MRI 2)

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Roger Preston Kelly
5 years ago
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1 ,,,,,,, We study the radiological characteristics of VIP polymer gel dosimeters under 135 AMeV and 290 AMeV carbon beam irradiation. We examine the transverse (or spin-spin) relaxation rate R 2 of the dosimeters quantified by magnetic resonance imaging (MRI) as a function of linear energy transfer (LET) with the help of the particle transport simulation code PHITS. Our results reveal that the response decreases with increasing dose-averaged LET and that the precise dependence varies with incident carbon beam energy. One can explain the energy dependence by taking account of the contribution from fragmentation products. Furthermore, as an application of the measured response- LET relation, we compare experimental and simulated R 2 distribution in a VIP gel formed by a heterogeneous irradiated field, and obtain agreement in overall distribution and range within 10 % and 1 mm 2 mm accuracy, respectively. Keywords: PHITS, gel dosimeter, three dimensional dose distribution, LET dependence, dose verification 1 X γ Linear Energy Transfer Dependence of VIP Polymer Gel Dosimeters under Carbon Beam Irradiation Takuya Maeyama, Nobuhisa Fukunishi, Kenichi Ishikawa, Takuya Furuta, Kazuaki Fukasaku,ShuTakagi, Shigeho Noda and Ryutaro Himeno (RIKEN), Shigekazu Fukuda (NIRS), TEL: , FAX: , maeyama@riken.jp MRI magnetic resonance imaging X γ 1) MRI 2) 3) LET LET 2 VIP LET VIP N-vinylpyrrolidone based polymer gel 98 (2014) 11

2 ,,,,,,,, Figure 1. R 2 distribution of VIP gel after irradiation of 12 C AMeV. 2, 4) 4%(w/v) N,N - 8 %(w/v) N % (w/v) % (w/v) II % (w/v) PET 135 AMeV 135 MeVRIBF: RI-beam factory, RIKEN 290 AMeV HIMAC: Heavy ion beam accelerator in Chiba, NIRS) VIP 1.5 T MRI Philips MR R 2 = 1/T 2 turbo mixed TR = 3000 ms TE 1 = 800 ms TE 2 = 20 ms TI = 200 ms ETL = 86 pixel spacing = 0.78 mm Figure 1 12 C AMeV VIP R 2 0Gy 5Gy 10 Gy 15 Gy 20 Gy R 2 R 2 R 2 Fig. 2 PHITS LET Figure 2. Comparison of sensitivity for entrance surface dose of VIP gel (symbol-dotted line) and calculated dose distribution in VIP gel (solid line) with irradiation of 12 C AMeV and 12 C AMeV. LET R 2 12 C AMeV PHITS Fig AMeV LET 12 C AMeV 12 C AMeV R 2 LET LET ave Fig. 3 LET ave LET Eq. 1 LET LET Dose(LET)dLET LET ave = (1) Dose(LET)dLET Figure 3 LET ave LET 135 AMeV 290 AMeV LET 10 % 12

3 [ Δ Figure 3. Comparison of VIP gel dosimeter responsivity obtained at different incident energies. Symbols represent experimental results, continuous lines represent dose-averaged responsivity calculated with the PHITS code, and dotted line represents the fitting function obtained from Eq. 4. LET ave LET LET PHITS LET Fig. 4 Figures 4(a) (b) LET ave 135 AMeV 290 AMeV LET LET LET LET LET LET mono Eq. 2 LET Res ave Eq. 3 Assumed Response = Res(LET mono ) (2) Figure 4. LET distribution as a stacked histogram at the same dose-averaged LET (47 ev/nm) for (a) 135 AMeV and (b) 290 AMeV. Open and solid bars correspond to secondary (left axis) and primary (right axis), respectively. Bin width is Δ(log LET) = Res ave = Res(LETmono ) Dose(LET mono )LET Dose(LETmono )dlet LET 135 AMeV LET LET 4) 0.09 s 1 Gy 1 LET 0 Eq. 4 Eq. 4 y LET Assumued Response x LET mono a 0 y AMeV a 1 = t 1 = t 2 = ( y = a 1 exp x t 1 ) + (a 0 a 1 ) exp (3) ( xt2 ) + y 0 (4) Res ave Fig AMeV 290 AMeV LET Res ave Res ave LET 98 (2014) 13

,,,,,,,, Figure 6. Photographs of VIP gel after 20 Gy irradiation with 12 C 6+ 135 AMeV. X, Y, Z axes show the same direction of Fig. 5. Figure 5. Geometrical setting for non-uniform irradiation.

The distance between the gel dosimeter and the phantom was 8 cm. The gel dosimeter was sealed into a PET container with a size of 6.8cm 6.8cm 11 cm with a wall thickness of 1.2 mm.

4 ,,,,,,,, Figure 6. Photographs of VIP gel after 20 Gy irradiation with 12 C AMeV. X, Y, Z axes show the same direction of Fig. 5. Figure 5. Geometrical setting for non-uniform irradiation. The each placed phantoms are the compact bone phantoms (2 mm, 3 mm and 5 mm) and a water-equivalent phantom (10 mm) in front of the VIP gel dosimeter. The distance between the gel dosimeter and the phantom was 8 cm. The gel dosimeter was sealed into a PET container with a size of 6.8cm 6.8cm 11 cm with a wall thickness of 1.2 mm. LET LET LET LET R 2 3 Fig. 5 VIP RIBF 135 AMeV 20 Gy R 2 MRI 135 AMeV 20 Gy Fig. 6 Figure 7 MRI R 2 PHITS Figure 7. R 2 distribution of a VIP gel dosimeter for a 12 C AMeV with non-uniform irradiation, compared with PHITS simulation results: (a) measured R 2 distribution, (b) calculated physical dose distribution, (c) calculated R 2 distribution, and (d) experimental values divided by calculated values. Figure 7 (a) (b) (c) R 2 (d) / R 2 R 2 Eq. 5 14

5 R 0 VIP R R 2 = Relative dose(x,y) Res ave (x,y) 20 Gy + R 0 (5) Fig. 7(d) ±10 % 1 1 mm 2 mm LET X 4 MRI PHITS LET LET LET ±10 % 1 mm 2 mm 1) N. MacDougall, W. G. Pitchford, M. A. Smith, Phys. Med. Biol., 47 (2002) R107. 2) I. Kantemiris, L. Petrokokkinos, A. Angelopoulos, N. Bassler, I. Seimenis, P. Karaiskos, J. Phys. Conf. Ser., 164 (2009) ) T. Maeyama, N. Fukunishi, K. L. Ishikawa, T. Furuta, K. Fukasaku, S. Takagi, S. Noda, R. Himeno, S. Fukuda, Radiat. Phys. Chem., 96 (2014) 92. 4) L. Petrokokkinos, M. Kozicki, E. Pantelis, C. Antypas, J. Fijuth, P. Karaiskos, L. Sakelliou, I. Seimenis, J. Instrum., 4 (2009) P (2014) 15

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