Comparative microdosimetry of photoelectrons and Compton electrons: an analysis in terms of generalized proximity functions

Research on the proximity functions of microdosimetry of low energy electrons in liquid water based on different Monte Carlo codes

PURPOSE

The proximity function is an important index in microdosimetry for describing the spatial distribution of energy, which is closely related to the biological effects of organs or tissues in the target area. In this work, the impact of parameters, such as physic models, cut-off energy, and initial energy, on the proximity function are quantitated and compared.

METHODS

According to the track structure (TS) and condensed history (CH) low-energy electromagnetic models, this paper chooses a variety of Monte Carlo (Monte Carlo, MC) codes (Geant4-DNA, PHITS, and Penelope) to simulate the track structure of low-energy electrons in liquid water and evaluates the influence of the electron initial energy, cut-off energy, energy spectrum, and physical model factors on the differential proximity function.

RESULTS

The results show that the initial energy of electrons in the low-energy part (especially less than 1 keV) has a greater impact on the differential proximity function, and the choice of cut-off energy has a greater impact on the differential proximity function corresponding to small radius sites (generally less than 10 nm). The difference in the electronic energy spectrum has little effect on the result, and the proximity functions of different physics models show better consistency under large radius sites.

CONCLUSIONS

This work comprehensively compares the differential proximity functions under different codes by setting a variety of simulation conditions and has basic guiding significance for helping users simulate and analyze the deposition characteristics of microscale electrons according to the selection of an appropriate methodology and cut-off energy.

On the difference between reference radiations used in radiobiology

PURPOSE

To add further support to the designation of hard gamma-rays, such as those emitted by (60)Co or (137)Cs, as the common reference radiation, as recommended by the International Commission on Radiological Protection (ICRP) Task Group 37 in the recent ICRP Publication 92.

MATERIALS AND METHODS

The study examined the microdosimetric properties of seven commonly used reference radiations and quantified differences in radiation quality among them. The principal tool is the proximity function, which is derived from the spatial distributions of energy deposits calculated by Monte Carlo simulations.

RESULTS

The microdosimetric properties of the seven commonly used reference radiations were significantly different. However, there were no significant differences in the spectra of energy deposition in microscopic regions between hard gamma-rays over a wide range of distances of biological interest.

CONCLUSIONS

From a microdosimetric point of view, gamma-rays from (60)Co or (137)Cs are the most suitable reference radiations for specification of the relative biological effectiveness.

Adaptive response and human benefit: Part I. A microdosimetry dose-dependent model

PURPOSE

It is important to evaluate how adaptive response may be of human benefit from the risks of ionizing radiation. The purpose of this work is to develop and apply a microdosimetric dose response model capable of explicitly determining, for broad beam exposures, the threshold and progressive activation of natural spontaneous and radiation damage protective mechanisms associated with adaptive response and other cellular negative response behavior.

MATERIALS AND METHODS

A biophysical model was developed quantifying the accumulation of Poisson distributed microdose specific energy hits to cell critical nucleus volumes. The model was applied to the adaptive response data of Wiencke et al., Redpath et al., Azzam et al. and Pohl-Ruling et al. The model was also applied to non-adaptive response data showing dose response reductions below the zero dose natural spontaneous level and to data exhibiting mid-range non-monotonic dose response plateaus.

RESULTS

We find good fits of the model to all data. For adaptive response, a significant result is, that only one or two specific energy hits of low linear energy transfer (LET) radiation in the cell nucleus activates the protective mechanisms for both the natural spontaneous and radiation damage. Several data support a dose plateau for radon progeny alpha production of chromosome aberrations in human lymphocytes. Using the model, a bystander factor of about 30 is obtained with the model for high dose rate, in vitro alpha particle data. For low dose rate in vivo, the bystander effect is minimal suggesting for alphas that the bystander effect may be dose rate dependent. There is no evidence of bystander effects in the low LET adaptive response data analysis.

CONCLUSIONS

The microdosimetry model allows concise determinations of specific energy hits within the cell critical nucleus volume to activate both protective and damage mechanisms. One or two low LET hits can result in reduction of both zero dose natural spontaneous and radiation-induced, carcinogenic causing damage. The model should be useful in comparing in vitro and in vivo broad beam to single track microbeam exposure data. The model is capable of determining, to an accuracy of +/- one specific energy hit, the minimum threshold for induction of radioprotective mechanisms--crucial to assessing the potential human benefit of adaptive response and other negative dose response behavior.

The microdosimetry of low-energy photons in radiotherapy

Low energy photons are more and more in use in clinical practice, for treatment in radiotherapy as well as for imaging purposes. Their relative biological effectiveness is however still debated. In this paper, some microdosimetric parameters have been calculated for different sources: (125)I, (103)Pd, (131)Cs, an electronic brachytherapy source and various clinical mammography X-ray qualities. These parameters have been used to deduce the quality factors as defined in ICRU 40.

Measurement of the Initial Levels of DNA Damage in Human Lymphocytes Induced by 29 kV X Rays (Mammography X Rays) Relative to 220 kV X Rays and γ Rays

Experiments using the alkaline comet assay, which measures all single-strand breaks regardless of their origin, were performed to evaluate the biological effectiveness of photons with different energies in causing these breaks. The aim was to measure human lymphocytes directly for DNA damage and subsequent repair kinetics induced by mammography 29 kV X rays relative to 220 kV X rays, 137Cs gamma rays and 60Co gamma rays. The level of DNA damage, predominantly due to single-strand breaks, was computed as the Olive tail moment or percentage DNA in the tail for different air kerma doses (0.5, 0.75, 1, 1.5, 2 and 3 Gy). Fifty cells were analyzed per slide with a semiautomatic imaging system. Data from five independent experiments were transformed to natural logarithms and fitted using a multiple linear regression analysis. Irradiations with the different photon energies were performed simultaneously for each experiment to minimize interexperimental variation. Blood from only one male and one female was used. The interexperimental variation and the influence of donor gender were negligible. In addition, repair kinetics and residual DNA damage after exposure to a dose of 3 Gy were evaluated in three independent experiments for different repair times (10, 20, 30 and 60 min). Data for the fraction of remaining damage were fitted to the simple function F(d) = A/(t + A), where F(d) is the fraction of remaining damage, t is the time allowed for repair, and A (the only fit parameter) is the repair half-time. It was found that the comet assay data did not indicate any difference in the initial radiation damage produced by 29 kV X rays relative to the reference radiation types, 220 kV X rays and the gamma rays of 137Cs and 60Co, either for the total dose range or in the low-dose range. These results are, with some restrictions, consistent with physical examinations and predictions concerning, for example, the assessment of the possible difference in effectiveness in causing strand breaks between mammography X rays and conventional (150-250 kV) X rays, indicating that differences in biological effects must arise through downstream processing of the damage.

Microdosimetric Analysis of Various Mammography Spectra: Lineal Energy Distributions and Ionization Cluster Analysis

In view of recent recommendations on the frequency and the starting age of mammography screening in healthy women, it is desirable to quantify the enhanced relative biological effectiveness (RBE) of mammography X rays compared to hard X rays. While there is little doubt that the former are more potent in inducing biological damage than the latter, the magnitude of the effect is still hotly debated in the literature. We used Monte Carlo simulations and track structure analysis in micrometer and nanometer volumes to investigate differences in distributions of lineal energy and ionization clusters for a range of mammography X-ray qualities. Dose-averaged lineal energies, (yD), in breast tissue for various mammography qualities were found to result in quality factors about 40% higher than unity. Among the various mammography qualities studied, the popular molybdenum/molybdenum target/filter combination was found to have the highest (yD) in 1-microm spheres (about 5.0 keV/microm near the entrance surface of breast tissue). In 10-nm radius spheres, the mean ionization cluster order was found to be about 35% higher in mammography X rays compared to 300 keV electrons (roughly representing 60Co or 192Ir photon radiation). In even smaller spheres (2 nm radius), no significant differences were observed for the mean ionization cluster order between mammography X rays and 300 keV electrons. We conclude that the potential of mammography X rays to induce biological damage is probably not much higher than a factor of two compared to hard X rays.

Comparison of Microdosimetric Simulations Using PENELOPE and PITS for a 25 keV Electron Microbeam in Water

The calculations presented compared the performances of two Monte Carlo codes used for the estimation of microdosimetric quantities: Positive Ion Track Structure code (PITS) and a main user code based on the PENetration and Energy Loss of Positrons and Electrons code (PENELOPE-2000). Event-by-event track structure codes like PITS are believed to be superior for microdosimetric applications, and they are written for this purpose. PITS tracks electrons in water down to 10 eV. PENELOPE is one of the few general-purpose codes that can simulate random electron-photon showers in any material for energies from 100 eV to 1 GeV. The model used in the comparison is a water cylinder with an internal scoring geometry made of spheres 1 microm in diameter where the scoring quantities are calculated. The source is a 25 keV electron pencil beam impinging normally on the sphere surface. This work shows only the lineal energy y and spectra graphical presentation as a function of y since for microdosimetry and biology applications, and for discussion of radiation quality in general, these results are more appropriate. The computed PENELOPE results are in agreement with those obtained with the PITS code and published previously in this journal. This paper demonstrates PENELOPE's usefulness at low energies and for small geometries. What is still needed are experimental results to confirm these analyses.