|Year : 2016 | Volume
| Issue : 2 | Page : 826-829
Measurement of in-phantom neutron flux and gamma dose in Tehran research reactor boron neutron capture therapy beam line
Elham Bavarnegin1, Alireza Sadremomtaz2, Hossein Khalafi3, Yaser Kasesaz3
1 Department of Physics, University of Guilan, Rasht; Department of Reactor, Nuclear Science and Technology Research Institute, Tehran, Iran
2 Department of Physics, University of Guilan, Rasht, Iran
3 Department of Reactor, Nuclear Science and Technology Research Institute, Tehran, Iran
|Date of Web Publication||25-Jul-2016|
Department of Physics, University of Guilan, Rasht
Source of Support: None, Conflict of Interest: None
Aim: Determination of in-phantom quality factors of Tehran research reactor (TRR) boron neutron capture therapy (BNCT) beam.
Materials and Methods: The doses from thermal neutron reactions with 14N and 10B are calculated by kinetic energy released per unit mass approach, after measuring thermal neutron flux using neutron activation technique. Gamma dose is measured using TLD-700 dosimeter.
Results: Different dose components have been measured in a head phantom which has been designed and constructed for BNCT purpose in TRR. Different in-phantom beam quality factors have also been determined.
Conclusions: This study demonstrates that the TRR BNCT beam line has potential for treatment of superficial tumors.
Keywords: Activation method, boron neutron capture therapy irradiation facility, head phantom, Tehran research reactor, TLD-700 dosimeter
|How to cite this article:|
Bavarnegin E, Sadremomtaz A, Khalafi H, Kasesaz Y. Measurement of in-phantom neutron flux and gamma dose in Tehran research reactor boron neutron capture therapy beam line. J Can Res Ther 2016;12:826-9
|How to cite this URL:|
Bavarnegin E, Sadremomtaz A, Khalafi H, Kasesaz Y. Measurement of in-phantom neutron flux and gamma dose in Tehran research reactor boron neutron capture therapy beam line. J Can Res Ther [serial online] 2016 [cited 2020 Feb 28];12:826-9. Available from: http://www.cancerjournal.net/text.asp?2016/12/2/826/174541
| > Introduction|| |
Boron neutron capture therapy (BNCT) is expected to be very effective for several types of cancer such as malignant brain tumor and skin melanomas, for which no successful treatment has been, developed., This method is based on irradiation of the boron-containing tumor cells by appropriate neutron beam. Lethal dose deposited by 10 B(n, a) 6 Li reaction products causes the destruction of the tumor cells., Nuclear research reactors and accelerators are the major neutron sources for BNCT. Two types of beams are commonly used in BNCT: The thermal beams for shallow depth tumor and the epithermal beam which used for deep tumors. In both types of the beam, the dosimetry of BNCT requires the careful analysis of the different components of the radiation field.
Dosimetry of BNCT is much complicated because of the presence of different dose components including 1-boron dose (DB): The dose from 10 B(n,α) 7 Li reaction. 2-gamma dose (Dg): The dose from neutron beam and 1 H(nth,γ) 2 H reaction. 3-thermal neutron dose (Dth): The dose resulting from thermal neutron capture in nitrogen 14 N(nth, p)14 C 4-fast neutron dose (Df): The dose from the 1 H(n, n')2 H reaction. The measurement of these dose distributions in a phantom is a necessary step in BNCT. This not only enables the determination of the characterization of the radiation field. However, also prepares the data to be used for determination of beam quality factors such as therapeutic depth (TD), namely the depth at which the tumor dose falls below twice the maximum dose to normal tissue; advantage depth dose rate (ADDR) namely, the maximum dose rate to normal tissue and therapeutic time. In-phantom dose measurements have been performed in many BNCT facilities over years.,,,,
Tehran research reactor (TRR) is the only active neutron source which can be considered for BNCT in Iran. Recently, the thermal column of TRR has been modified to provide a suitable thermal neutron beam for BNCT researches. The main objectives of this paper are to measure different dose components of TRR beam along the central axis of a head phantom and to quantify the in-phantom quality factors.
| > Materials and Methods|| |
Tehran research reactor boron neutron capture therapy beam line
TRR BNCT beam line is based on the use of the thermal column. The thermal column is about 3 m in length with a wide square shape cross-section of 1.2 m × 1.2 m. It is filled with removable graphite blocks. In order to produce a proper thermal neutron beam, the configuration of graphite blocks has been rearranged in such a way that it is possible to create a 2.6 m × 0.3 m × 0.3 m empty channel, as shown in [Figure 1]. In addition, a 30 cm × 30 cm × 12 cm lead block as a gamma shield and a collimator have been constructed and installed in the beam line.
|Figure 1: Thermal column structure for thermal boron neutron capture therapy application|
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The head phantom
In order to perform dosimetry experiments in TRR, an ellipsoidal acrylic head phantom has been constructed. The phantom design is close to MIT head phantom. The theoretical model of the head phantom shell is represented by two ellipsoids as shown in the Equations (1) and (2):
Water has been chosen for the inside of the phantom as recommended by. [Figure 2] shows the head phantom.
The measurements of neutron flux and gamma dose
In order to fatally damage tumor cells with BNCT, one of the important parameter is the ratio of boron concentration in the tumor to normal tissue which should be in the range of 3–5 using a boron delivery drug like boronophenylalanine where tissue and tumor uptake is approximately 18 and 65 ppm, respectively. The head phantom has been filled with water containing 18 and 65 ppm 10 B for normal and tumor tissue, respectively, and then has been irradiated in TRR BNCT beam. Cadmium ratio (the response of bare foil to cadmium covered foil) of 440.41 has been measured at the beam exit. This shows that about 99.77% of the beam is thermal. Hence, the thermal neutron dose, boron dose, and gamma dose have been considered as three main dose components in TRR BNCT beam line. In order to measure thermal neutron dose and boron doses, a 197 Au wire and a pair of 115 In foils (bare and cadmium covered) have been used for flux distribution along the phantom center line. The activity of foils has been counted using an HPGe detector. The recorded spectra have been analyzed, and the saturated activities of the 115 In foils have been determined from the fitted gamma peak areas, the timing information, the foil masses, the Indium half-life, and the detection efficiencies. Equation (3) has been used to calculate the thermal neutron flux from the measured activities of foils:
Where AB and ACd are the activities of bare and cadmium covered foils per cm 3, respectively; Fcd is the cadmium correction factor, n is the atomic density; σth is the thermal activation cross-section; is decay constant (1/s); and t is the irradiation time (s). The time of irradiation was 4 h and time gap between irradiated foil and measurement were 8 h and 41 h for indium foil and Au wire, respectively.
Kinetic energy released per unit mass (KERMA) factors have been used to convert thermal neutron flux to Dth and DB.
In order to measure Dγ, several TLD-700 chips are positioned along the center line of the phantom. TLD's are positioned every 2 cm. TLD-700 chips (Harshaw) of dimensions of 3 mm × 3 mm × 9 mm, all belonging to the same batch are used. TLD-700 consists of LiF, enriched in 7 Li (0.007%6 Li). TLD readings have been obtained using a Harshaw 4500 TLD reader. The annealing procedure is 1 h at 400°C, followed by 2 h at 100°C. The response is the charge collected for the total glow curve.
Owing to the different relative biological effectiveness of radiations, it is necessary to determine the separate contributions to the absorbed dose of each field component. Biological weighting factors for Dγ, Dth, DB in tumor and DB in normal tissue have been considered 1, 3.2, 3.8, and 1.3, respectively.
The total biologically weighted dose of TRR thermal BNCT beam line has been measured using Equation (4):
| > Results and Discussion|| |
Following irradiation of the head phantom in TRR BNCT beam line, thermal neutron flux and Dγ distributions have been measured along the central axis of the head phantom. Measurements have been carried out at the reactor power of 3 MW. [Figure 3] shows the thermal neutron flux.
|Figure 3: Thermal neutron flux distribution in the center line of the head phantom|
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Dth and DB are also calculated using measured thermal neutron flux and KERMA factor approaches. Dth, Dγ, and DB are also shown in [Figure 4].
As shown in [Figure 3] and [Figure 4], with an increase of the depth in the phantom, thermal neutron flux and, therefore, Dth and DB are decreased. A decrease is also seen in Dγ. These decreases are because of the neutron and gamma attenuation through the water. The most significant dose arises from boron dose in the tumor. The thermal neutron dose has the minimum dose contributions. In-phantom beam quality factors have been obtained from these dose profiles. The TD is found to be 5.7 cm. [Figure 5] shows the therapeutic gain (TG) parameter inside of the head phantom. TG is the ratio between the tumor dose and the maximum dose to the normal tissue. Higher TG means lower complications of therapy. As [Figure 5] shows with an increase in the depth of the phantom, the TG parameter is decreased. For the depth below 5 cm, this ratio is more than 2. This shows that the TRR BNCT beam has potential for treatment of superficial brain tumors. This shows that the TRR thermal BNCT beam is suitable for treatment of tumors up to 5 cm depth. The maximum dose rate for the normal tissue (ADDR) is 0.039 Gy/min. The time required for destruction of the tumor at the depth of 1 cm is about 110 min by receiving 20 Gy-eq.
| > Conclusions|| |
TRR in-phantom dose distribution and beam quality factors have been measured for the reactor power of 3MW. These parameters are the main criteria for evaluation of BNCT beam. The results show TRR thermal neutron beam has potential for treatment of superficial brain tumors up to 5 cm depth. Some essential works are needed to be done for performing clinical studies in TRR reactor. Now, this beam is usable for biological studies and animal trials.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]