Design Collimator and Dosimetry of in Vitro and in Vivo Test Using MCNP-X Code

Studies were carried out to collimator modelling and dosimetry BNCT of in vitro and in vivo test using MCNP-X code. Collimator modelling performed to obtain neutron beam as required by the International Atomic Energy Agency (IAEA). Dosimetry calculations performed to obtain the results of the dose calculation (dosimetry) in the application of BNCT. Collimator modelling and dosimetry simulations performed with MCNPX program. Neutron sources used for simulation, namely cyclotrons HM-30, energy 30 MeV, the current is 1.1 mA. Collimator modelling utilizes to program MCNPX covers cells target as beryllium, collimator wall (reflector), moderate, filter, gamma-ray shielding, and aperture. The simulation results of the modelling are Φ epi 1.02241x10 n/cm2 s, D f /Φ epi 2.36487x10 Gy-cm/n, D γ /Φ epi 4.68416x10 Gy-cm/n, Φ th /Φ epi 3.76285x10, J/Φ epi 8.37678x10. Based on the calculation of the dose rate that has been done, the result that the optimal dose rate at a depth of 1cm. Keyword: BNCT, MCNPX, collimator, dosimetry


INTRODUCTION
Cancer is a group of diseases that cause the cells in the body to change and grow out of control.Most types of cancer cells eventually form a lump or mass called a tumour, and are named accordingly part of the body where the tumour originated [1].Breast cancer is one type of cancer.Breast cancer cells either spread through the bloodstream or the lymphatic system or have a predilection to metastasize to certain parts of the body such as the lymph nodes, bones, liver, lungs, and brain [2].Breast cancer is the most common cause of cancer death among women (522,000 deaths in 2012) and the most frequently diagnosed cancer among women in 140 of 184 countries worldwide [3].Treatment of breast cancer, which until now often used are surgery and is often combined with other treatments, such as radiation therapy, chemotherapy, systemic therapy, hormone therapy, and / or targeted therapy [1].Another treatment method for breast cancer is brachytherapy.Several methods of treatment turned out to have side effects for the patient.
Boron Neutron Capture Therapy (BNCT) has for many decades been advocated as an innovative form of radiotherapy, BNCT has the potential to be the ideal form of treatment for many types cancer [4].BNCT is based on the nuclear capture and fission reactions that occur when the nuclide Boron-10 non-radioactive irradiated with thermal neutrons (0.025 eV) [5]. 10 B absorbs neutrons and produce two particles have the Linear Energy Transfer (LET) high, α particles ( 4 He) and lithium ( 7 Li) [6], [5] for particles of α ≈ 150 keVμm-1 and 7 Li ≈ 175keVμm-1 [4].Results of the reaction 10 B (n, α) 7 Li has a long-range 6-10μm [7].Neutrons are used in BNCT treatment is thermal neutrons and epithermal neutrons.Neutron collimator is needed to collimate neutron beam [8].Collimator in BNCT therapy is an important be used to cells target that Beryllium ( 9 Be) and lithium ( 7 Li).Materials are selected in collimator design under which Beryllium with 5 cm thick.Beryllium has the advantage of high melting point, high thermal conductivity and the ability to directly cool the water.

Moderator
Moderator required in BNCT therapy should have a high scattering cross section for fast neutrons and a low absorption cross section for epithermal neutrons.The material used as a moderator, Al, Al 2 O 3 , AlF 3 , MgF 2 , PbF 2 , alloy of Al / AlF 3 (30% Al, 70% AlF 3 ) and fluent (30% Al, 69% AlF 3 , 1% LiF) (Kasesaz, et al., 2014).The material used in the collimator, under which Al (Aluminium) and Al 2 O 3 (Aluminium Oxide) with a thickness of 2 cm each.

Filter
Filter the collimator serves to reduce the fast neutron component.Cadmium (Cd) is a material used as a filter on the design of the collimator below 2 cm thick.

Gamma-ray Shielding
Gamma-ray shielding serves to reduce the intensity of the gamma rays produced in the collimator.Materials such as Pb, Bi and Ni can be used as a shield gamma.Nickel (Ni) thickness of 5 cm is used in the collimator design below.

Aperture
Aperture is the tip of a conical collimator shaped to focus the radiation beam.The size of the aperture in the collimator design under that thick 5 cm and 6 cm radius.

Wall Collimator (Reflector)
Materials suitable for the reflector are a material that has a high scattering cross section and has a higher atomic mass.The material used part, because the collimator has a function to collimate neutrons that come out of the neutron source, so that the neutron beam to the patient in accordance with predetermined parameters.Besides collimator, dosimetry also important in BNCT.Dosimetry is the study of various magnitudes and unit dose [9].In the BNCT treatment, the dose delivered to the healthy and tumour tissues depends on the interaction of neutrons with many different nuclei and the gamma ray back-ground [10].Modelling collimator and dose calculation is very important to test the in vitro and in vivo BNCT therapy.Modelling collimator and dose calculations can be done using one of the methods of computing, namely Monte Carlo.Monte Carlo N-Particle (MCNP) code is developed and maintained by the Los Alamos National Laboratory [11].

MATERIALS AND METHODS Collimator Modelling
Collimator modelling done by using MCNP-X.Figure 3.1 shows the geometry used in the simulation.Neutron sources used are 30 MeV cyclotron, which has a specification as in table 2.1.The modelling was conducted on the part of the target, moderator, filter, shield gamma, aperture, and a reflector.

Cells Target
Cyclotron is one of neutron source as accelerator.Cyclotron change proton particle into neutron.Proton into a neutron particle changes occur on the target.Materials that can as a reflector in collimator design is graphite with a thickness below 20 cm.

Dosimetry
The first stage is define the geometry dosimetry calculations used for simulation.Geometry used is spherical with a mathematical equation as follows: ( ) (1) Geometry as the sphere approaches the cancerous lump form.Lump is assumed to have a shape like a sphere.
The next stage is to determine the material making up the tissues, such as H, C, N, O, P, Na, P, S, Cl, and K. Tally used to calculate the dose is tally f4 by adding a coefficient of Kerma.Thus, obtained dose calculation results.The final stage in the dosimetry calculations is to calculate the total dose rate on each tissue.The equation used as follows:

RESULTS AND DISCUSSION Collimator Design
Collimator design simulation results using MCNPX program as shown below: Based on the results obtained, the value of epithermal flux for 1.02241x10 10 n/cm 2 -s IAEA has fulfilled recommendation that >1.0x10 9 n/ cm 2 -s.Meanwhile, the simulation result for fast neutron dose per epithermal flux (Df/Φepi) still exceeds the limit recommended values.This is possible because the number of fast neutrons in the collimator many passes or the number  The simulation results for gamma dose per epithermal flux (D γ /Φ epi ) still exceeding the standards because of the possible amount of radiation in a gamma ray collimator is still quite high.The possible numbers of gamma rays that pass from gamma shield quite a lot, so it is necessary election material and thickness settings in the appropriate gamma shield.The amount of thermal flux per flux epithermal (Φ th / Φ epi ) still exceeding the standards for the amount of thermal neutrons released by collimator fewer than epithermal neutron.Simulation results for current ratio per epithermal flux (J/ Φ epi ) are very far from the value recommended by IAEA, possibilities for the direction of the neutrons in the collimator is random or not mono direction.is a measure of how much the cancer visible or provable extent or location of the cancer growth.Clinical Target Volume (CTV) is a tissue that contains GTV has proven malignant disease microscopic or sub-clinical to be eliminated.Planning Target Volume (PTV) is defined as the geometric concepts to radiating volume appropriate cancer, in order to ensure that the prescribed dose to kill the cancer is absorbed.

Dosimetry
Phantom modelling is used to perform the calculation of the dose rate.Components of the dose in BNCT therapy is dose of neutrons scattering, gamma dose, Boron dose (interaction of 10 B with thermal neutrons), and recoil proton dose.Dose of neutrons scattering and gamma dose as a result of output collimator can be obtained through simulations on MCNPX program.While, for the gamma dose, boron dose, and the recoil proton dose as results of interactions thermal neutron with hydrogen in the tissues is done by manual calculation.Dose calculation results are presented in Figure 3. 3 The third chart above shows the relationship between the depth of the dose rate (dose scattering of neutrons, gamma dose, dose boron, and the recoil proton dose).Based on the results presented in the figure above, all components of the dose fluctuation.All the components of the dose began to decline when it reaches a depth of 1cm of skin.The optimal dose rate only at a depth of 1 cm, after it experienced a significant decline.This is possible because the probability of the number of neutrons reaction with the boron in tissues is less than optimal.Thus, the dose rate is only optimal when it reaches the organ PTV, not yet to the cancer cells.The amount of boron concentration and irradiation time is very important in BNCT treatment, it is necessary to optimize between them.Absorbed dose recommended for BNCT therapy was 50 Gy.The result of dosimetry calculations can not be used for in vitro and in vivo in application of BNCT.

CONCLUSION AND REMARKS CONCLUSION
Modelling collimator using cyclotron 30 MeV neutron source.The modelling includes parts of the target cell, a moderator, filter, shield gamma, reflectors, and aperture.MCNPX simulation results show that the epithermal neutron flux issued by the collimator has met standard IAEA is equal 1.02241x10 10 n/cm 2 -s.Dose rate calculation performed on the four components of the therapeutic dose BNCT.Boron concentrations used in the simulation are 20 μg/g, 25 μg/g, and 30 μg/g.Based on calculations, the optimal dose rate at a depth of 1 cm.

Remarks
Need to do further research relate to modelling collimator and dosimetry calculations, in order to obtain results in accordance with the parameters.
rate of Boron (Gy/s) p D : Dose rate of recoil proton (Gy/s) n D : Dose rate of recoil proton (Gy/s) g D : Dose rate of gamma (Gy/s) w B : radiation weighting factor of alpha, 3.8 for the tumour tissue and 1.35 for normal tissue w p : weighting factor of the proton radiation is 3.2 w n : weighting factor of neutron scattering radiation is 3.2 w γ : .

Figure 3 . 1
Figure 3.1 Modeling Simulation Results Collimator with MCNPX Program Yohannes Sardjono, and Bilalodin, Design Collimator and Dosimetry of in Vitro and in Vivo Test Using MCNP-X Code of fast neutrons in the collimator is still high.

Table 3 .
1 Neutron Beam Collimator Modelling Results with MCNPX Program