Neutron field analysis for a proton therapy installation

Transcription

1 Neutron field analysis for a proton therapy installation Sandri Sandro 1 ; Benassi Marcello 2 ; Ottaviano Giuseppe 1 ; Picardi Luigi 3 ; Strigari Lidia 2 1 ENEA ION-IRP Institute of Radiation Protection - Via Enrico Fermi, Frascati (Rome), ITALY 2 Laboratory of Medical Physics and Expert System Regina Elena National Cancer Institute IFO - Rome, ITALY 3 ENEA FIM Accelerator Section - Via enrico Fermi, Frascati (Rome), ITALY Abstract A proton therapy centre is planned to be sited in Rome, Italy. It will be based on a medium energy proton accelerator and should be associated to a National Health Institute. At least two treatment station should be realized: a 140 MeV area for shallow tumors therapy and the MeV full energy station for deep tumors treatment. Additional experimental areas are foresee for studying the interactions of low energy, high LET, protons with tissues and the effectiveness of proton therapy on specific pathologies. The project is now in a preliminary design phase. The accelerator is under study, the building layout has to be defined and the preliminary safety solutions are considered as well in this phase. The radiation protection approach requires that all the radiation fields are well known, even those that are not useful for the treatment itself. In this frame the neutron field due to proton interactions in solid, liquid and gaseous materials has to be analyzed during the design phase in order to reduce this component to its minimum extent and to address the radiation protection program. The injector of the proton accelerator has been installed at the ENEA Research Center in Frascati, Rome and will be used to perform the preliminary low proton energy testing and the benchmarks for the simulations needed to assess the neutron field. In the paper the simulation models and the calculation performed with Monte Carlo codes are described. The related results are presented together with the comparison with the low energy benchmarks and the data found in the literature for similar projects. Considerations about workers and patients protection are issued taking into account different technical options and related advantages and disadvantages. Introduction In Italy a new project is close to be launched with the aim of realizing an innovative proton therapy facility. The TOP-IMPLART project is based on the use of a linear accelerator for producing the beam. TOP-IMPLART are the acronym of Terapia Oncologica con Protoni (Oncological Therapy with Protons) and Intensity Modulated Proton Linear Accelerator for Therapy. The project benefits of previous studies, designs and tests done under the funding of the TOP project carried on in the years by ENEA and ISS (The National Institute of Health). The TOP-IMPLART project has recently been considered suitable for a substantial funding by the Department of Innovation of Regione Lazio. A final decision about this is expected to be taken in the very next months. The work will be carried on in collaboration between ENEA, ISS and IFO (Istituti Fisioterapici Ospedalieri, the most important oncological hospital in Rome). The base of the TOP-IMPLART project is substantially the same as the TOP 1

2 Project. The accelerator, that constitutes the main peculiar characteristic of this design is a linear accelerator, or, better to say, a sequence of linear accelerators. The low energy part is a commercial 7 MeV proton linac produced from AccSys-Hitachi, that itself is a sequence of a source, a RFQ and a DTL operating at the frequency of 425 MHz. The project is aimed to develop a proton irradiation facility that could be devoted to different applications taking advantage of the modular nature of the linear accelerators. Using a linear machine instead of a compact circular accelerator (syncrotrons and cyclotrons) permits the possibility to proceed by steps in the construction and operation process and makes it possible the combined use of different irradiation stations at various energies between the minimum (about 7 MeV) and the maximum (about 250 MeV). The sequential setup of each partial irradiation module and its clinical application also before the whole facility has been completed will match the financial support flux that will be discontinue and spread over many years. This process will provide clinical and social advantages in a shorter time than the one that should be required for the construction and operation of the whole facility. The first 7 MeV module of the accelerator, is already installed and has been tested at the ENEA Research Center in Frascati, Rome. Additional modules will be added to the injector leading proton energy to 30, 70 and 150 MeV in a step by step project. The present study is finalized to the neutron field analysis for the radiation protection of the workers involved in the testing activities of the first two modules of the linear accelerator, with a final proton energy of 30 MeV. The main irradiation model considers the proton beam hitting a cell-culture-in-water target. The study has been performed using FLUKA code [1], a powerful computer program based on the Monte Carlo method, implemented to simulate the experimental setup in order to evaluate the neutron field parameters. Material and methods The TOP LINAC concept Usually proton linear accelerators used for research purpose, operate with intense beams, run at low frequencies and must therefore have large apertures. This setup is unnecessary for tumor treatments, where very weak intensities are required (the average beam currents are only about 10 na), and therefore an high frequency technology is applicable. The idea of using 3 GHz structures with gradient of the order of 15 MeV/m is at the basis of the studies initiated in 1993 by the TERA Foundation. This machine was the first 3 GHz linear accelerator dedicated to proton therapy [2, 3]. It has been designed to produce a continuously variable energy beam, promoting a xyz scanning. This requires a pulse repetition rate as high as possible (400 Hz) to irradiate a single pixel at least twice during a 1 min treatment, and a precise control (2-3 % in a relative intensity range of ) of the dose rate of the single pulse. The fine energy variation is achieved by amplitude variation in the last module still under power, giving a pretty smooth energy variation with a small energy spread (about 0.3%) in the range above 130 MeV. Xyz scanning totally avoids to use passive systems such as absorbers, scattering foils and collimators, whose thickness and shape need to be designed and milled for each tumor shape and volume to provide the desired angular and energy distribution of the beam. 2

3 The cost-effectiveness of this facility is a particularly relevant issue. In fact, many costeffectiveness reviews suggest that at the moment no clear evidence can be drawn either in favor or against proton radiotherapy as opposed to conventional photon radiotherapy. Some authors suggest a net advantage of proton therapy for a limited number of tumor sites, such as melanomas and others ocular tumors, skull base chordomas and chondrosarcomas, medulloblastoma in pediatric patients [4, 5]. For other pathologies such as breast, prostate, head-and neck tumors, similar evidence has been reported for selected patient subgroups [6, 7, 8]. A cost reduction in building proton therapy facilities equipped with robotic systems for patient positioning instead of rotating gantries, is expected to reveal more clearly the clinical advantage of proton versus photon therapy demonstrating improved dose distribution. IMPLART Frascati test facility and related models The test facility that will be sited in Frascati will be located inside a narrow, 13 m long shielded room, also referred as IMPLART bunker. The bunker is sized to accommodate the first two accelerating sections up to 30 MeV, with an average beam current of about 10 na. Geometrical dimensions and shielding material compositions, as derived from the FLUKA database, are shown in the tables from 1 to 4. Table 1. Shielding materials. Shielding Material Thickness [cm] Ceiling Concrete 50 Wall on each side of the beam Concrete 100 Additional shielding Lead 10 Wall in front of the beam Concrete 150 Wall opposite to the beam Concrete 100 Floor Steel 2+2 Table 2. Composition of concrete, ρ = 2,34 g cm -3. Atom fraction C-Carbon (Z=6) 23,0 O-Oxygen (Z=8) 40,0 Si-Silicon (Z=14) 12,0 Ca-Calcium (Z=20) 12,0 H-Hydrogen (Z=1) 10,0 Mg-Magnesium (Z=12) 2,0 3

4 Table 3. Composition of typical stainless-steel, ρ = 8,0 g cm -3. Atomic fraction Cr-Chromium (Z=24) 18,0 Fe-Iron (Z=26) 74,0 Ni-Nickel (Z=28) 8,0 Table 4. Composition of kapton membrane, ρ = 1,42 g cm -3, thickness 80 μm. Mass fraction H-Hydrogen (Z=1) 0, C-Carbon (Z=6) 0, N-Nitrogen (Z=7) 0,07327 O-Oxygen (Z=8) 0, FLUKA Monte Carlo code has been implemented to simulate a 30 MeV proton beam hitting a cell culture in water enclosed in a Petri s capsule and positioned on the beam axis at about 2 cm from the kapton membrane located at the end of the vacuum accelerating channel particle stories were used in the Monte Carlo execution. The calculation model includes the following main sections: Final segment of the accelerating section; Petri s-capsule target; Shielding walls. The following kind of results were obtained: Neutron fluences inside the bunker, inside the shielding and outside the bunker; Neutron dose equivalent inside the bunker, inside the shielding and outside the bunker; Double differential neutron fluence spectrum inside the bunker; These results allow a preliminary assessment of the effectiveness of the existing shielding in relation to the neutron fields. The final segment of the accelerating section has been shaped like a stainless-steel hollow cylinder, with vacuum inside, end-closed by a kapton membrane. The target is a Petri s capsule: a glass or plastic cylindrical box that is the most common container for cell cultures. The simplified model consists of an hollow cylinder containing three layers of material. The hollow cylinder simulate the Petri s capsule with an air-equivalent-plastic (ICRU C-552); the two outer layers simulate the water while the inner layer simulate the cell culture with a tissue-equivalent material, as shown in figure 1. 4

5 Petri s capsule Water Cell culture Water Fig. 1. Schematic view of the target (not in scale). The tables from 5 to 7 show the FLUKA material composition of the target. Table 5. Composition of the air-equivalent-plastic (ICRU C-552), ρ = 1,76 g cm -3. Mass fraction H-Hydrogen (Z=1) 0,02468 C-Carbon (Z=6) 0,50161 O-Oxygen (Z=8) 0, F-Fluorine (Z=9) 0, Si-Silicon (Z=14) 0, Table 6. Composition of water, ρ = 1,0 g cm -3. Atom fraction H-Hydrogen (Z=1) 2,0 Oxygen (Z=8) 1,0 Table 7. Composition of tissue-equivalent material, ρ = 1,0 g cm -3. Mass fraction H-Hydrogen (Z=1) 0, C-Carbon (Z=6) 0,10002 N-Nitrogen (Z=7) 0,02964 O-Oxygen (Z=8) 0, Results FLUKA calculations provide results in term of quantity per single source proton that have to be scaled accounting for the actual particles flux in the beam. The data obtained 5

6 in the following will be than multiplied by the proton-per-hour yield of the Implart modules, that is about 2, h -1, giving the dose rate in the same time unit. In the figures from 2 to 5 the final segment of the accelerating section (thin strip) and the target are located at the right, inside the bunker. In the figures 2 and 4, the thin strip below outside the bunker represents the second steel layer. In the figures 3 and 5, the thin sector at the top, inside the bunker, represents the additional lead shielding. Figures 2 and 3 show neutron fluence due to the proton interaction on the system target as a result of the FLUKA run. Fig. 2. Spatial distribution of the neutron fluence. YZ plane (vertical section). The abscissas and the ordinates show the linear dimensions in cm, while the spatial distribution is represented by color scale in unit of part/cm 2. The result is normalized per source-particle or primary. Fig. 3. Spatial distribution of the neutron fluence. XZ plane (orizontal section). The abscissas and the ordinates show the linear dimensions in cm, while the spatial distribution is represented by color scale in unit of part/cm 2. The result is normalized per source-particle or primary. 6

7 Fig. 4. Spatial distribution of the neutron ambient dose-equivalent. YZ plane (vertical section). The abscissas and the ordinates show the linear dimensions in cm, while the spatial distribution is represented by color scale in unit of psv. The result is normalized per source-particle or primary. Fig. 5. Spatial distribution of the neutron ambient dose-equivalent. XZ plane (orizontal section). The abscissas and the ordinates show the linear dimensions in cm, while the spatial distribution is represented by color scale in unit of psv. The result is normalized per source-particle or primary. In the Figures 4 and 5 the main result of the simulation is shown. The neutron ambient dose-equivalent is normalized to the single source proton. In the chromatic graphs the walls of the bunker are reported in order to assess the related shielding effect. A general result is represented in Figure 6 showing the neutron fluence spectrum in air, at 10 cm from the target. 7

8 Fig. 6. Differential distribution of the neutron fluence. The abscissas show the energy in unit of [GeV], while the ordinates show the differential distribution of the fluence dφ/de in unit of [cm -2 GeV -1 ]. The result is normalized per source-particle or primary. Discussion The neutron field is generated by interaction of the proton beam with the nuclei of the target, the air and the shielding. It represents an inevitable and undesirable radiation field which could cause undue dose to the operators. Figure 2 shows the neutron fluence in vertical projection (YZ plane): around the target we have about 1e-07 part cm -2 ; inside the bunker it ranges from 1e-08 to 1e-09 part cm -2, depending on the distance from the target; outside the bunker it varies from 1e-11 part/[cm -2 ] beyond the concrete, to 1e-09 part/[cm -2 ] in the room below the steel layers; Figure 3 shows the situation in a horizontal projection (XZ plane). The fluence shows a general behavior similar to that shown in the vertical plane, with a slightly lower intensity. The neutron ambient dose equivalent shown in Figure 4 gives the trend of the field in vertical projection (YZ plane): around the target, is about 5e-05 psv (i.e. 11,2 msv/h, for a 10 na current); inside the bunker it varies from about 5e-06 to 1e-07 psv, depending on the distance from the target; outside the bunker it can be seen as it varies, in the points of greatest field strength, from about 5e-08 psv (i.e. 11,2 µsv/h, for a 10 na current) beyond the concrete, to 1e-06 psv (i.e. 224 µsv/h, for a 10 na current) in the room below the steel layers; Figure 5 shows the situation in the horizontal projection (XZ plane). The difference from fluence to dose-equivalent is due to the application of the conversion coefficients that for neutrons are strongly dependent from their energy. 8

9 In the case study the neutron energy has the spectral trend shown in Figure 6. The curve shows that the neutron fluence spectrum has the main peak at about 0,598 MeV, then decreases rapidly dying at about 21 MeV and also shows a tail starting from the thermal region at about 4,361 mev with a plateau from about 1e-06 MeV up to 1e-03 MeV. The thermal component could be caused by slowing of fast neutrons in concrete and subsequent backscatter inside the bunker. Conclusions The TOP-Implart project could provide important medical applications suggesting the needing of testing the first sections of the accelerator in an existing facility. The current work describes the initial analysis of the neutron field produced by the 30 MeV proton beam provided by the first two accelerating sections. The preliminary result obtained with the FLUKA simulation shows that the existing shielding walls and structures, effective in reducing the dose rate at an acceptable level for the initial module of 7 MeV protons, have to be improved to keep the dose rate at a similar low level for the upgrade to 30 MeV. In the simulation wall shielding appears to be able to slow down the dose-equivalent by a factor of about 10 only. A better attenuation seems to occur for the neutron fluence due to the neutron spectrum and the conversion coefficients distribution vs. neutron energy. Anyway the dose rate assessed in the simulation is greater than 10 µsv/h in many areas beyond the shielding walls and this would limit the system workload at less than 1000 hour per year. In conclusion the simulation model has been setup but further analysis is needed to correctly size the shielding walls or to better define distance and materials. Possible local shielding or beam dumps have to be considered too. If needed, also the accelerator workload and average beam current could be reduced during the testing phase. References [1] A. Ferrari, P. R. Sala, A. Fassò, J. Ranft. FLUKA program version 2008 [2] L. Badano, M. Benedikt, P. J. Bryant, M. Crescenti, P. Holy, P. Knaus, A. Meier, M. Pullia and S. Rossi. Proton-Ion Medical Machine Study (PIMMS). Part I, CERN/PS DI. CERN, Geneva 1999 [3] L. Badano, M. Benedikt, P. J. Bryant, M. Crescenti, P. Holy, P. Knaus, A. Meier, M. Pullia, and S. Rossi. Proton-Ion Medical Machine Study (PIMMS). Part II, CERN/PS DR. CERN, Geneva 2000 [4] M. Lodge, M. Pijls-Johannesma, L. Stirk, A. J. Munro, D. De Ruysscher, T. Jefferson. A systematic literature review of the clinical and cost-effectiveness of hadron therapy in cancer. Radiother Oncol. 2007, 83: [5] Lundkvist J, Ekman M, Ericsson SR, Jönsson B, Glimelius B.: Costeffectiveness of proton radiation in the treatment of childhood medulloblastoma. Cancer. 2005, 103: [6] Lundkvist J, Ekman M, Ericsson SR, Isacsson U, Jönsson B, Glimelius B.: Economic evaluation of proton radiation therapy in the treatment of breast cancer. Radiother Oncol. 2005, 75:

10 [7] Lundkvist J, Ekman M, Ericsson SR, Jönsson B, Glimelius B.: Proton therapy of cancer: potential clinical advantages and cost-effectiveness. Acta Oncol. 2005, 44: [8] Glimelius B, Ask A, Bjelkengren G, Björk-Eriksson T, Blomquist E, Johansson B, Karlsson M, Zackrisson B.: Number of patients potentially eligible for proton therapy. Acta Oncol. 2005, 44: