Abstract
A calibration method was proposed in the present work to determine the medium-thickness-dependent proton doses absorbed in cellular components (i.e., cellular cytoplasm and nucleus) in radiobiological experiments. Consideration of the dependency on medium thickness was crucial as the linear energy transfer (LET) of protons could rise to a sharp peak (known as the Bragg peak) towards the end of their ranges. Relationships between the calibration coefficient R vs medium-layer thickness were obtained for incident proton energies of 10, 15, 20, 25, 30 and 35 MeV, and for various medium thicknesses up to 5000 μm, where R was defined as the ratio DA/DE, DA was the absorbed proton dose in cellular components, and DE was the absorbed proton dose in a separate radiation detector. In the present work, DA and DE were determined using the MCNPX (Monte Carlo N-Particle eXtended) code version 2.4.0. For lower incident proton energies (i.e., 10, 15 and 20 MeV), formation of Bragg-peak-like features were noticed in their R-vs-medium-layer-thickness relationships, and large R values of >7 and >6 were obtained for cytoplasm and nucleus of cells, respectively, which highlighted the importance of careful consideration of the medium thickness in radiobiological experiments.
Highlights
Many experimental data were accumulated in literature with the ultimate goal of revealing the biological effects of protons with various energies for different types of cells
Dhanesar et al.[14] pointed out that measurements of the percentage depth dose (PDD) for proton beams were mostly accomplished by a water tank dosimetry system with ionization chamber
The doses absorbed in the studied cells were commonly surrogated by the doses recorded using an external radiation detector, which would require an accurate conversion coefficient R (=DA/DE), where DA was the dose absorbed in exposed cells or cellular components while DE was the dose recorded by the external detector
Summary
Many experimental data were accumulated in literature with the ultimate goal of revealing the biological effects of protons with various energies for different types of cells. The biological effects of protons with energies from 5 to 35 MeV were studied on in-vitro irradiated cell lines of human origin[7], beagle eyes[9] and on whole body of the primates[10] For such radiobiological experiments, accurate dosimetry for these protons in the cells or the cellular components (such as the cell nuclei) would be crucial for establishing realistic dose-response relationships and for meaningful comparisons among different types of radiations (e.g., proton and photon beams). The doses absorbed in the studied cells were commonly surrogated by the doses recorded using an external radiation detector (e.g., ionization chamber), which would require an accurate conversion coefficient R (=DA/DE), where DA was the dose absorbed in exposed cells or cellular components while DE was the dose recorded by the external detector Determination of this conversion coefficient R was the focus of our recent studies on the calibration for realistic neutron[29] and photon[30] dosimetry in radiobiological experiments. Based on these previous works, the present task was to enable quantification of the absorbed dose (DA) in exposed cells or cellular components due to protons through the dose (DE) through the conversion coefficient R, the value of which depended on the medium thickness
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