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NСT methods are experimental therapies, calculations of the neutron dose in tissue are very complex, and there are no commercial computer treatment planning systems. Studies and all clinical trials are conducted in accordance with the estimates of the performed simulations using the MCNP program. Kerma (K), a close analogue of the absorbed dose, is often used to calculate the dose. The advantage of the kerma is the possibility of calculating it for both the monoenergetic neutron flux and the neutron spectrum. Kerma calculations consider all the processes that form the absorbed dose in biological tissues. Using the values of the partial components of dose estimates in soft biological tissue, it is possible to calculate the value of the total absorbed dose in GDNT depending on the concentration of natural Gd in the biological tissue. As our studies of the dependence of the kerma in natGd on the neutron energy show, the main contribution to the total kerma (i.e., the absorbed dose) is made by neutrons with En energies ≥ 10-7 MeV [1]. The neutrons flux density in the target (in the tumor) depends on the distance and on the elemental composition of the tissue that they must pass to the target. Therefore, it is necessary to conduct studies of changes in the density of the epithermal neutron flux of the special channel of the WWR-SM reactor of the Institute of Nuclear Physics of the Academy of Sciences of the Republic of Uzbekistan on various human organ phantoms. On the other hand, testing a new method in experiments with phantom objects is a necessary step in preparing the method for clinical trials. Therefore, to improve the dose calculations, calculations were carried out to determine the neutron flux density for the head phantom. The simulated calculations were performed using the MCNP-4C program. Figure 1 shows the geometry of calculating the human head phantom. For calculations, this phantom is represented in the form of a sphere with a tumor inside; the following elemental composition of human tissues was used, taken from the ICRU 46 (Goorley) data bank [2] and [3,4]. The density values used in the calculations were as follows: p (adult brain) = 1,040 g/cm3; p (skull bone) = 1,610 g/cm3; p (tumor) = 1,070 g/cm3; (eye) = 1,100 g/cm3; The human head phantom had the shape of a sphere and the following dimensions: R1 = 8 cm, R2 = 7.7 cm, R of the tumor = 1.2 cm, d of the skull = 15 cm, the distance from the exit of the external collimator to the phantom of the head l = 3 cm. At the beginning, the tumor was located in the left edge of the phantom, then this tumor was gradually moved to the right edge and changes in the neutron spectrum were studied. The calculations performed yielded the following results: From a comparison of the spectra, it was found that neutrons with energies from 0.1 to 10 MeV, despite the noticeable removal of the tumor from the left edge of the skull, hardly change, i.e. the attenuation is weak.
Fig.1.
Geometry of the human head phantom for MCNP calculations: 1-horizontal reactor channel, 2-external collimator, 3-human skull, 4-tumor, 5-human brain
The neutron flux with an energy of 1 MeV decreases markedly with increasing distance to the tumor. The neutron flux from 10-7 to 10-5 MeV changes slightly with increasing distance from the edge of the skull to the tumor, and increases by 1.5 orders of magnitude at a distance of 4 cm. Then it decreases again, and at a distance of 8 cm it increases again by 1.3 orders of magnitude, then it decreases linearly. This is probably due to the attenuation of the fast neutron flux, which becomes noticeable when these neutrons pass through a 4 cm thick layer of brain tissue. The neutron flux with an energy of 10-4 - 0.1 MeV in the tumor gradually decreases with the removal of the tumor from the left edge of the skull. The photon flux in the tumor gradually decreases as the distance of the tumor from the skull increases, but the photon flux with an energy of 0.1-0.6 MeV increases at a distance of 8 cm. As this distance increases, photons with energies between 0.01 and 0.03 MeV appear with a flux density of 106 at a distance of 10 cm, 103 at a distance of 13 cm.
These calculations show that when using an epithermal neutron beam, it is possible to form a spectrum to create the required maximum dose in head tumors with GdNRT. This requires further studies with sophisticated 3D geometry, with the introduction of Magnevist, in order to study the pharmacokinetics of the drug in tumors and ensure radiation planning taking into account the pharmacokinetics of the drug in head tumors.
Acknowledgement (Financial support): The work is financed by the state budget of the Republic of Uzbekistan.
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