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Basic Radiobiology
Published in Kwan Hoong Ng, Ngie Min Ung, Robin Hill, Problems and Solutions in Medical Physics, 2023
Kwan Hoong Ng, Ngie Min Ung, Robin Hill
A cell survival curve describes the relationship between the absorbed doses of radiation. Based on the ‘multi-target theory’, what two parameters can be used as estimates of the size of the shoulder region of a cell survival curve? Illustrate the parameters on a cell survival curve.The linear quadratic (LQ) equation is often used to describe the cell survival curve following a single fraction of radiotherapy. Explain what is meant by α component and β component of a linear quadratic model?How can the values of α, β and α/β ratio be obtained?Sketch the cell survival curves for neutron and 250 keV X-ray on the same graph. Give the basis on the difference between the two curves.
Normal Tissue Damage Following Photodynamic Therapy: Are There Biological Advantages?
Published in Barbara W. Henderson, Thomas J. Dougherty, Photodynamic Therapy, 2020
Mang and Wieman [22] found that photobleaching in pancreatic tumor was three times greater than in normal pancreas when PII was used as photosensitizer. Photobleaching can occur through reaction with singlet oxygen and other active oxygen species, so they postulated that a singlet oxygen scavenger was present in normal pancreas, preventing photosensitizer destruction and damage to normal pancreas. Possible scavenging agents are glutathione or other intracellular thiols. The level of intracellular glutathione has been shown to affect the response of the cell to PDT [27]. In particular, if the cells were depleted of glutathione or deficient in it, the cell survival curve following PDT showed a major reduction in the width of the shoulder. This indicates that the threshold to produce damage was markedly reduced. We examined the acid-soluble—SH (glutathione) [28] content of normal pancreas before and after photodynamic therapy with AlPcSn. Normal pancreas contained thiol levels similar to other tissues that were responsive to PDT. Unfortunately, it was not possible to measure the thiol content of pancreatic tumors. We have been unable to identify a consistent fall in thiol levels following PDT. The level in untreated pancreas was 1.02 μmol/g, and in pancreas treated with PDT (5 mg/kg AlPcSn, 100 J) it was 0.68 μmol/g. This was not a significant fall, although further experiments are required to study the effect in more detail. The precise mechanism that spares normal pancreas from photodynamic damage remains unclear.
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Published in Harald Paganetti, Proton Therapy Physics, 2018
LET describes the energy deposition per path length and not the energy deposited in a cellular volume. Nevertheless, LET-based models are a valuable approximation because they can be based on few parameters. They follow the linear-quadratic dose–response curve and parameterize the change in α and β with dose-averaged LETd relative to the reference radiation according to Equations 22.13 and 22.14, with parameters given in Table 22.2. Each of the model is fitted to a subset of the available in vitro experimental data from the literature. The largest data set was used by McNamara et al. [47]. By knowing α and β from photon data and as a function of LET, one can thus predict the cell survival curve and the RBE. For mixed radiation fields, e.g., considering proton energy distribution, dose-averaged means of α and √β have to be applied.
Thermal damage during ablation of biological tissues
Published in Numerical Heat Transfer, Part A: Applications, 2018
Bruna R. Loiola, Helcio R. B. Orlande, George S. Dulikravich
Feng et al. [29] developed a two-state model for the PC3 (human prostate cancer) and RWPE (normal) cell surviving curves, for hyperthermia in-vitro experiments in the range of 44 °C to 60 °C and exposure durations of 1–30 min [29]. The cell survival curve fit given by with α = 0.00493 s−1, β = 215.64, and γ = 70,031 K provided good correlation of the experimental data for the PC3 cells. Similarly, the authors correlated their experimental results in terms of the Arrhenius model with the following parameters: A = 1.19 × 1035 s−1 and Ea = 2.318 × 105 J mol−1. The models and their associated parameters used in this work are shown in Table 1.