THE DOSE ANALYSIS OF BORON NEUTRON CAPTURE THERAPY ( BNCT ) TO THE BRAIN CANCER ( GLIOBLASTOMA MULTIFORM ) USING MCNPX-CODE WITH NEUTRON SOURCE FROM COLLIMATED THERMAL COLUMN KARTINI RESEARCH NUCLEAR

This research was aimed at discovering the optimum concentration of Boron-10 in concentrations range 20 μgram/gram until 35 μgram/gram with Boron Neutron Capture Therapy (BNCT) methods and the shortest time irradiation for cancer therapy. The research about dose analysis of Boron Neutron Capture Therapy (BNCT) to the brain cancer (Glioblastoma Multiform) using MCNPX-Code with a neutron source from Collimated Thermal Column Kartini Research Nuclear has been conducted. This research was a simulation-based experiment using MCNPX, and the data was arranged on a graph using OriginPro 8. The modelling was performed with the brain that contains cancer tissue as a target and the reactor as a radiation source. The variations of Boron concentrations in this research was on 20, 25, 30 and 35 μg/gram tumours. The outputs of MCNP were neutron scattering dose, gamma ray dose and neutron flux from the reactor. Neutron flux was used to calculate the doses of alpha, proton and gamma rays produced by the interaction of tissue material and thermal neutrons. Based on the calculations, the optimum concentration of Boron-10 in tumour tissue was for a 30 μg/gram tumour with the radiation dose in skin at less than 3 Gy. The irradiation times required were 2.79 hours for concentration 20 μg/gram ; 2.78 hours for concentration 25 μg/gram ; 2.77 hours for concentration 30 μg/gram ; 2.8 hours for concentration 35 μg/gram.


INTRODUCTION
Cancer is the disease which gets special attention because of its lethal and benign characteristics (American Cancer Society, 2014).Some treatment methods for cancer are radiotherapy, surgery and chemotherapy.The surgery method is that of taking cancer out of the human body.This method is usually used for the first stage of cancer where the cancer is benign and it does not spread to another of the body's tissues (Cember, Johnson, & Alaei, 2009;Sudhakar, 2009).Chemotherapy methods can be combined with surgery, utilizing drugs to kill the cancerous tissue (Cember et al., 2009).
The new technology of radiotherapy that may be used for cancer treatment is Boron Neutron Capture Therapy (BNCT).BNCT is a cancer therapy technique that uses the physics principles of nuclear reactions which occur when Boron-10 is irradiated by low energy neutrons (thermal neutrons).This reaction between boron and thermal neutrons produces Boron-11 (t1/2=10 -23 second).Boron-11m decays by emitting alpha particle, Lithium-7 particle and gamma ray.In BNCT, Boron-10 and its carrier drug are administered to the patient.This carrier will take the compound to the location of the cancer tissue where Boron-10 needs to be accumulated.Then the cancer area is irradiated by neutron source (Bakirdere, Orenay, & Korkmaz, 2010).Boron-10 in the cancer tissue will capture the thermal neutron resulting from a prompt nuclear reaction Boron-10(n,α)Lithium-7.Both γ-particle and Lithium-7 have relatively high Linear Energy Transfer (LET) values (175 keVmn and above) and short path lengths (approximately 4.5 to 10 µm), hence the energy deposition is locally limited around the cancer tissue (W.Sauerwein, 1993).Lithium has a dose limit in the human body amounting to 900-1800 mg/day or 1.5-2.5 meQ/L.Lithium concentrations exceeding 3.5 meQ/L are fatal for humans.The ratio of Boron-10 concentrations in cancer tissue and health tissue was 10:1 so that the research is selective because the neutron reaction probability with Boron-10 is greater in cancer tissue(W.A. G. Sauerwein & Moss, 2009).

MATERIAL AND METHODS
This study was a simulation-based experiment using MCNPX.In the first step, it was necessary to make a model of the reactor since it would be used as the neutron source.Several parts of the reactor were modelled, such as core, the radial reflector, rotary specimen rack, and piercing beam port.The thermal column was also built since it would become the point of interest.Simulation was done to make sure that the criticality value was approximately 1, and the thermal neutron flux in the ring B was approximately (12.45 ± 0.23) x 10 11 n. −2 . −1 (Fauziah Nina, Widiharto, & Sardjono, 2015).The result should be recorded for the next collimator conceptual design process.In an MCNP input file, tallies are the information that a user wants to obtain by Monte Carlo calculation.According to the beam criteria in Table 1, the tallies were needed for resulting fluxes and current data.Neutron and gamma fluxes were calculated by using F4 tally and corresponding dose values were determined using fluence to kerma conversion factors reported in ICRU 63.Moreover, neutron current was calculated using F1 tally.Normalization factors for each tally were calculated for normalizing the tallies reactor within 100 kW thermal power.
Secondly, the collimator was designed.A rough collimator design was made by using MCNPX codes: 95 cm length of collimator, 54 cm of outer collimator diameter, 3 cm thick of beam delimiter, and 3 cm sized aperture.The beam delimiter was made of 6Li2CO3polyethylene.In designing the collimator, one should start with the varied sizes of the collimator wall.The material used was Ni.The best thickness would be that which provided the highest epithermal neutron flux.Then, moderator, Al, was varied until the fast neutron component decrease was no longer significant.In this point,  60 , which served as a fast neutron absorber, also absorbed the thermal neutrons, and started to be used and varied until the fast and thermal neutron components were reached.
The next step was to employ Bi as gammaray shielding into the collimator and alter its thickness until the desired gamma-ray component was gained.The last parameter of beam quality was the examination of the direction of the collimator.If the direction of the collimator was less than desired, more beam delimiter would be added.The last step conducted was setting the aperture or the beam cross section size to find out the performance of the collimator design.
The modelling of brain geometry was ellipsoid with diameters x axis was 21.2 cm.The cancer location in a lobus frontalis with 3.3 cm of deepness from skin surface.In this simulation, we used a 0.6 cm thickness of scalp, a 0.6 cm thickness of cranium, a 0.6 cm thickness of cranium, a16.7 cm thickness of soft tissue, a 0.3 cm thickness of Planning Tumor Volume (PTV), a 0.5 cm thickness of Clinical Tumor Volume (CTV), and a 4 cm diameter of Gross Tumor Volume (GTV) or cancer).

RESULT AND DISCUSSION
Thermal

Table 1 .
neutron source was produced by the model of Collimated Thermal Column Kartini Research Nuclear Reactor.The neutron criteria of thermal column outputs was shown at Table 1 (IAEA, 2014).Neutron criteria of collimated thermal column outputs.

Table 2 .
The outputs of collimated thermal column.

Table 3 .
Dose rate and time irradiation on the cancer tissue.

Table 5 .
The estimated deteriministic effect on the skin