Abstract
It is experimentally proven that nanoparticles of high-Z materials can be used as radiosensitizers for photon beam therapy. In the authors’ opinion, data available as of today on the impact of secondary particles (electrons, photons and positrons generated in biological tissue by penetrating beam of primary photons) on the distribution of deposited dose during photon beam therapy in the presence of nanoparticles, are insufficient. Investigation of this impact constituted the main goal of this work. Two-stage simulation was performed using Geant4 platform. During the first stage a layer of biological tissue (water) was irradiated by monoenergetic photon sources with energies ranging from 10 keV to 6 MeV. As the result of this modeling spectra of electrons, photons and positrons were obtained at the depth of 5 cm. During the second stage the obtained photon spectra were used to irradiate gold, gadolinium and water nanoparticles. Radial distributions of energy deposited around nanoparticles were obtained as the result of this modeling. Radial DEF (Dose Enhancement Factor) values around nanoparticles of gold and gadolinium positioned in water at the depth of 5 cm were obtained after processing the collected data. Contributions from primary photons and secondary particles (electrons, photons and positrons generated in the layer of water with 5-cm thickness by the penetrating beam of primary photons) in the additional dose deposited around the nanoparticles were calculated as well. It was demonstrated that layer of biological tissue placed between the source of photons and nanoparticles considerably changes the initial spectrum of photons and this change is significant in the analysis of mechanism of radiosensitization of biological tissues by nanoparticles for all energies of photon sources (up to 6 MeV). It was established that interaction of electrons and positrons with nanoparticles does not lead to significant increase of additional dose in the vicinity of their surfaces and can be most likely excluded from consideration in the analysis of radiosensitization mechanism of nanoparticles.
Highlights
Methods for physical aiming of radiation on the tumor, such as three-dimensional conformal radiation therapy (3D CRT), intensity-modulated radiotherapy (IMRT), image guided radiation therapy (IGRT), stereotactic radiation surgery (SRT) ensuring maximum precision of radiation dose rendering to tumor target have reached the limits of their further refinement (Van Dyk 2005)
One nanoparticle positioned at the depth of 5 cm from flat monoenergetic photon source was simulated for investigating distribution of additional dose associated with presence of nanoparticles in the water
Sharp jumps of deposited dose in the vicinity of surfaces of nanoparticles emerge under irradiation of biological tissue with photons with energies up to 6 MeV
Summary
Methods for physical aiming of radiation on the tumor, such as three-dimensional conformal radiation therapy (3D CRT), intensity-modulated radiotherapy (IMRT), image guided radiation therapy (IGRT), stereotactic radiation surgery (SRT) ensuring maximum precision of radiation dose rendering to tumor target have reached the limits of their further refinement (Van Dyk 2005). As a rule, the chemical or pharmacological agents augmenting necrocytosis under irradiation when these agents are present in biological tissue. Important feature of radiation sensitizers is their differentiated behavior with respect to normal tissues and tumors, i.e. they must increase sensitivity of tumors higher than that of healthy tissues. One of the options of radiosensitization is the introduction in the biological medium of elements with significantly higher radiation absorption cross-section than that of the biological tissue per se. Emerging secondary short-range radiation localizes energy absorption in the vicinity of these elements and affects only adjacent biological structures
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