Ice Alloys

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Ice Alloys A Materials Science Lab for High School Students Richard Hendrick Nazareth Academy Rochester, New York

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ARTICLE Effects of Secondary Neutron Beam Generated in Radiotherapy on Electronic Medical Devices 1,* 11 1 Tomonori ISOBE, Hiroaki KUMADA, Kenta TAKADA, Takayuki HASHIMOTO, 1 11 1 Haruko HASHII, Koichi SHIDA, Kiichi TADANO, Toshioh FUJIBUCHI, 2 11 Masahiro HANMURA, Hideyuki SAKURAIand Takeji SAKAE 1 Graduate School of Comprehensive Human Sciences, University of Tsukuba, 111 Tennodai, Tsukuba, Ibaraki, 3058575 Japan 2 Division of Radiation Oncology, Shizuoka Cancer Center Hospital, 1007 Shimonagakubo, Nagaizumicho, Suntogun, Shizuoka, 4118777 Japan In recent years, the administration of radiotherapy to patients bearing an implantable electronic device, such as a pacemaker, has been increasing. Although the effects of primary radiation on electronic devices have been studied, research on the effects of the secondary radiation beam is limited. Because semiconductors constitute the main parts of an electronic device, the effects of the secondary neutrons in radiation should be considered. We performed a Monte Carlo simulation of neutron generation during radiotherapy, and considered the influence of neutrons on the electronic medical devices. Monte Carlo simulations were performed using the Particle and Heavy Ion Transport code System (PHITS), developed by the Japan Atomic Energy Agency. Nuclear data was taken from ENDF/BVII. We irradiated a water phantom with a 10 MV Xray beam and a 200 MeV proton beam. Then, the neutron dose was evaluated by using PHITS. Moreover, we also measured the dose with a CR39 dosimeter. In the case of the 10 MV Xray beam, although fast neutrons (> 100 keV) were generated in the upstream part of the beam, thermal neutrons were scarcely generated. Inside the water phantom, which simulated a human body, the generation of fast neutrons decreased rapidly as a result of thermalization. In the case of the 200 MeV proton beam, fast neutrons were distributed along the beam axis inside the water phantom, and were generated inside the water phantom. Thermal neutrons (< 0.5 eV) were uniformly distributed throughout the whole phantom. In the evaluation of secondary neutrons generated during radiotherapy, we consider that using Monte Carlo calcu lations in addition to experimental measurements will be useful for clinical applications. KEYWORDS: neutron, radiotherapy, electronic medical device, Monte Carlo simulation, PHITS, CR39, dosi metry, proton, Xray * I. Introductionas well as to assess the risk and accuracy of radiotherapy. Moreover, radiationinduced cancer is also associated with In recent years, the number of patients who undergo radi6) secondary neutron exposure. otherapy has increased; accordingly, radiation exposure not It has already been confirmed in previous research that only to the patient but also to electronic medical devices im electronic medical devices are damaged by direct irradiation planted the human body has become an important topic in3,4,7) 1,2)The effects of scatteredwith Xray and proton beams. radiation biology.Radiotherapy is already known to have Xrays on electronic medical devices have also been re a negative effect on electronic medical devices in the human8) ported. However,the effect of secondary neutrons body, owing to electromagnetic interference and other ef 3,4)generated in radiotherapy on such devices has not yet been fects of ionizing radiation.clarified. In the past, Xray beams of 6 MV or less have been used The purpose of the present research is to evaluate the ef for radiotherapy. For these Xray beam energies, the genera fects of secondary neutrons on electronic medical devices tion of neutrons through photonuclear reactions does not 5)through experimental measurements using CR39 and need to be taken into acount.However, current radiothera through Monte Carlo simulations. py uses Xray beams with energy of 10MV or more. For this reason, neutron generation via photonuclear reactions II. Materials and Methods cannot be ignored. Furthermore, the use of particle radiothe rapy is also increasing. The effects of neutrons on medical1. Measurement with CR39devices such as pacemakers, implantable cardioverter defiFigure 1shows the experimental setup. A water phantom brillators and ventriculoperitoneal shunts should bewith 30mmthick polyethylene walls and dimensions of investigated. It is important to evaluate secondary neutrons,24 cm 24 cmwas used to simulate the human 24 cm body. The water phantom was placed on the treatment bed. * Corresponding author, Email:tiso@md.tsukuba.ac.jp

Fig. 1of beam delivery system. (a) Crosssectional image of Varian linear accelerator and (b) Side view of Schematic the proton beam line at the Proton Medical Research Center, University of Tsukuba.

For the measurements, a gantry angle of 90° and Xray and proton beam energies of 10 MV and 200 MeV, respectively, were used. The Xray beam was generated by a linear acce lerator (Varian, Palo Alto, CA) (Fig. 1(a)). The proton beam was generated by a synchrotron (Hitachi, Tokyo, Japan) at the Proton Medical Research Center, University of Tsukuba. Figure 1(b) shows the setup of the proton beam line at our institution. The field size of the Xray beam was set to 10 cm10 cm with an isocenter depth of 12 cm, which was at the center of the phantom. The field size of the proton beam was set to 10 cm10 cm; the proton beam was passed through a collimator made of brass. The spreadout Bragg peak (SOBP) was 6cm. The isocenter was at a depth of 17 cm,which was the center of SOBP. We used CR39 (Luxel badge; Nagase Landauer, Ibaraki, Japan) to measure the secondary neutron beam. CR39 has been widely used as a personal neutron dosimeter that records the track of re 9,10) coiled protonsand α particlewas arranged ats. CR39 equal intervals on the phantom away from a beam axis and in parallel, in order to avoid direct radiation (Fig. 2). The measurement error in detection of neutrons with CR39 de

pends on the number of neutrons. In order to minimize the measurement error, we predicted the dose of generated neu trons in advance, and then determined the exposure. The dosage was set to 300Gy for the 10MV Xray beam and 10 Gy for the 200 MeV proton beam. Values measured with CR39 were standardized to the track density per unit dose 2 1 (Track・cm・Gy ). The measurement energy range of CR39 is below 0.5eV for thermal neutrons and more than 100 keV for fast neutrons. 2. Monte Carlo Simulation To estimate the dose of secondary neutrons, the conditions of the experiment using CR39 were modeled in a numerical simulation, and then fluxes and the distribution of secondary radiation in the phantom were determined by Monte Carlo calculations. The Monte Carlo transport code PHITS (Japan Atomic Energy Agency: JAEA, Japan) was used to create a detailed model of an accelerator head, the geometry and composition of which were based on detailed technical drawings provided by the manufacturer. In performing transport calculations for photons, electrons, and neutrons

Fig. 2 Schematicof measurement setup (gantry angle: 90°).

2 1 Fig. 3 Measuredneutron dose (etch pit density) per unit dose (Tracks∙cm∙to a water phantom as a function ofGy ) distance from the beam incidence plane for two radiotherapy beams: (a) 10 MV Xray beam and (b) 200 MeV proton beam.

with energy below 20MeV in PHITS code, nuclear data is needed. Using PHITS, we can calculate not only the dose from proton beam exposure but also the transport of secondary neutrons. The nuclear data required for the Monte Carlo calculations was taken from ENDF/BVII. For the 8 Xray beam, we simulated the collision of 10electrons with a copper target. Then, the photonuclear reaction that oc curred was analyzed. In the simulation of proton irradiation, the particle history number was set 6.5 million NPS, and both the therapeutic dose of protons and incidental dose of secondary radiation were determined. In the calculation, the energy of a neutron was set to coincide with the measure ment energy range of CR39 (thermal neutrons: <0.5 eV; fast neutrons: > 100 keV). III. Results

Figure 3 showsthe distribution of the secondary neutron dose to the water phantom for various distances from the

beam incidence plane. The neutron dose here is the number of etch pits, which is the number of cracks in CR39 formed by recoiled protons and α particles. For theXray10 MV beam (Fig.3(a)), fast neutrons decreased rapidly with in creasing distance downstream from the beam. The dose of thermal neutrons was almost zero at the center of the water phantom. In the case of the proton beam (Fig. 3(b)), attenua tion of the thermal neutrons and fast neutrons was slight, and they penetrated deep into the phantom. In comparison with the 10 MV Xray beam, the fast neutron dose resulting from the proton beam was 14 fold higher at position A, 30 fold higher at position B and 45 fold higher at position C. In terms of thermal neutrons, the dose was 48 times higher at position A, 430 fold higher at position B, and 366 fold high er at position C.

Figure 4 showsthe analysis results of the Monte Carlo simulation for the secondary neutron beam generated by ir radiation with the 10 MV Xray beam. In the upstream part

Fig. 4of secondary neutrons generated by 10 MV Xray beam in Monte Carlo simulations: (a) upstream Distribution part of the beam and (b) interior of the water phantom.

of the beam (Fig. 4(a)), thermal neutrons were not generated, while fast neutrons were generated. As shown in Fig. 4(b), the tracks of the fast neutrons passed through the phantom. Thermal neutrons were also generated in the water phantom. Figure 5 showsthe analysis output of the Monte Carlo simulation of the secondary neutron beam generated when the water phantom was irradiated with the 200MeV proton beam. Fast neutrons were distributed along the beam axis. On the other hand, thermal neutrons were uniformly distri buted throughout the entire phantom. IV. Discussion In order to clarify the effects of the secondary neutrons generated by radiotherapy on implanted medical electronic devices, we evaluated the secondary neutrons in a compari son between CR39 measurements and Monte Carlo simulations using PHITS. From the result of the Monte Car lo simulation, we found that thermal neutrons were clearly not generated in the upstream part of the beam during irradi ation with the 10MV Xray beam, whereas fast neutrons were generated. The fast neutrons were generated at the reaction of the upstream part of the beam and the primary 11) beam.

The fast neutrons generated in the upstream part of the beam reached the water phantom. In the case of the 10 MV Xray beam, the distribution of fast neutrons inside the water phantom was not clear in the Monte Carlo results. This is a limitation of the PHITS code used in this research. We con sider that the photonuclear reaction cannot be correctly calculated using PHITS. In the future, we plan to use the latest version of PHITS, which has a calculation function for the photonuclear reaction; then, we will compare the expe rimental results with the Monte Carlo simulation results. Using CR39, we measured the secondary neutrons generat ed by the 10MV Xray beam inside the water phantom, where the quantity of the fast neutrons decreased rapidly as they moved downstream. This is a phenomenon that accom panies thermalization of fast neutrons. Although we expected that the thermal neutrons would increase in the depth direction due to thermalization of fast neutrons, the thermal neutrons actually decreased. This finding can be explained by the fact that the part of CR39 that detects thermal neutrons is also sensitive to fast neutrons. CR39 consists of two neutron detectors, a polyethylene radiator and a boron nitride converter. Although CR39 detects the thermal neutrons via the reaction with the boron nitride con verter, fast neutrons also react with the boron nitride

Fig. 5of secondary neutrons generated by 200 MeV proton beam inside the water phantom. Distribution

converter, and thus a fast neutron component is detected as thermal neutrons. In regard to this problem, we consider that thermal neutrons can be measured correctly by using activa tion of gold foils or CR39 that is wrapped around its circumference with cadmium. In the case of proton beam exposure, the Monte Carlo si mulation inside a phantom was possible. The fast neutrons were distributed along the beam axis. On the other hand, the thermal neutrons were uniformly distributed over the whole phantom. Fast neutrons travel less than 10cm in water be 12) cause of thermalization.Therefore, we expected that the value of fast neutrons measured with CR39 at position B of Fig. 2 became zero. However, the value was hardly different from the value of the position of A. The distribution ob tained from the Monte Carlo simulation (Fig. 5) agreed well with the measured distribution. These results show that the fast neutrons were generated inside the water phantom by the

reaction between protons and water, and the neutrons then advanced into the phantom. V. Conclusions

We evaluated the secondary neutrons generated during ra diotherapy through measurements with CR39 and through Monte Carlo simulations. In the case of the 10 MV Xray beam, although fast neu trons were generated in the upstream part of the beam, thermal neutrons were scarcely generated. Inside the water phantom, which simulated the human body, the quantity of the fast neutrons decreased rapidly as a result of thermaliza tion. In the case of the 200MeV proton beam, the fast neutrons were distributed along the beam axis inside the wa ter phantom. Moreover, fast neutrons were generated inside the water phantom. The thermal neutrons were uniformly distributed throughout the whole phantom.

Lastly, we will briefly discuss two areas that remain for4)C. W. Hurkmans, E. Scheepers, B. G. Springorum, H. Uiter waal, “Influence of radiotherapy on the latest generation of future study. In CR39, thermal neutrons cannot be measured implantable cardioverterdefibrillators,”Int. J. Radiat. Onco.l correctly. Therefore, the first area of future research is to Biol. Phys.,63[1], 282289 (2005). measure the thermal neutrons using another technique. In 5)National Council on Radiation Protection and Measurements, this study, the distribution inside the water phantom of fast “Neutron contamination from medical electron accelerators,” neutrons generated by the 10 MV Xray beam could not be NCRP Report 79 (1984). clearly simulated using PHITS. Therefore, the second area of 6)U. Schneider, A. Lomax, N. Lombriser, “Comparative risk as future research is developing a more precise method for cal sessment of secondary cancer incidence after treatment of culation of photonuclear reactions with PHITS. Hodgkin’s disease with photon and proton radiation,”Radiat. Since the electron device consists of semiconductors, it isRes.154[4], 382388 (2000). greatly influenced by a neutron. Therefore, in the evaluation7)Y. Oshiro, S.Sugahara, M. Noma, M. Sato, Y. Sakakibara, T. of secondary neutrons generated during radiotherapy, weSakae, Y. Hayashi, H. Nakayama, K. Tsuboi, N. Fukumitsu, A. Kanemoto, T. Hashimoto, K. Tokuuye,“Proton beam therapy consider that using Monte Carlo calculations in addition to interference with implanted cardiac pacemakers,”Int. J. Radiat. experimental measurements will be useful for clinical appli Oncol. Biol. Phys.,72[3], 723727 (2008). cations. 8)S. Kapa, L. Fong, C. R. Blackwell, M. G. Herman, P. J. Schomberg, D. L. Hayes, “Effects of scatter radiation on ICD Acknowledgement and CRT function,”Pacing Clin. Electrophysiol.,31[6], 727732 (2008). This work was supported in part by Nagase Landauer 9)K. Oda, M. Ito, Y. Miyake, J. Yamamoto, T. Tsuruta, (Ibaraki, Japan). “Doseequivalent response CR39 track detector for personal neutron dosimetry,”Nucl. Instr. Meth. Phys. Res.,B61, 302308 (1991). References 10)R. J. Tanner, D. T. Bartlett, L.G. Hager, “Operational and do 1)R. M. Howell, N. E. Hertel, Z. Wang, J. Hutchinson, G. D.simetric characteristics ofetchedtrack neutron detectors in Fullerton, “Calculation of effective dose from measurementsroutine neutron radiation protection dosimetry,”Radiat. Meas., of secondary neutron spectra and scattered photon dose from40[26], 549559 (2005). dynamic MLC IMRT,”Med. Phys.,33[2], 360368 (2006).11)U. Titt, O. N. Vassiliev, F. Pönisch, S. F. Kry, R. Mohan, 2)G. Wilde, J. Sjostrand, “A clinical study of radiation cataract“Monte Carlo study of backscatter in a flattening filter free formation in adult life following gamma irradiation of the lensclinical accelerator,”Med. Phys.,33[9], 32703273 (2006). in early childhood,”Br. J. ophthamol.,8112)[4], 261266 (1997).D. Shin, M. Yoon, J. Kwak, J. Shin, S. B. Lee, S. Y. Park, S. 3)J. R. Marbach, M. R. Sontag, J. Van Dyk, A. B. Wolbarst,Park, D. Y. Kim, K. H. Cho, "Secondary neutron doses for “Management of radiation oncology patients with implantedseveral beam configurations for proton therapy."Int. J. Radiat. cardiac pacemakers: report of AAPM Task Group No. 34.Onco.l Biol. Phys.,74[1], 260265 (2009). American Association of Physicists in Medicine,”Med. Phys.21[1], 8590 (1994).