No abstract

Synchronous V79 Chinese hamster cells were exposed in G1 to either X-rays or 4.6 MeV/u Ar-ions (LET = 1840 keV/micrometer) and the induction of chromosomal damage was measured at five sampling times ranging from 14 to 30 h after treatment. To distinguish between cells in the first and second post-irradiation cycle the fluorescence-plus-Giemsa technique was applied. The experiment showed that the time-course of the appearance of damaged cells was markedly influenced by radiation-induced cell cycle delays and depended on both radiation quality and dose. The yield of aberrant metaphases and the number of aberrations per metaphase was found to increase with sampling time, but this increase was more pronounced for Ar ions. These differences in yield-time profiles of X-ray and Ar ion induced chromosomal damage are particularly important for an accurate determination of the RBE for particles. Our data clearly indicate that meaningful RBEs can only be obtained if chromosomal damage is analysed at several post-irradiation sampling times and the complete time-course of the expression of chromosomal damage is taken into account. Besides these quantitative differences, differences in the spectrum of chromosomal lesions were observed for X-rays and Ar ions. Following particle exposure more breaks and less exchange-type aberrations were formed compared with X-irradiation and, despite irradiation in G(1), a significant number of chromatid-type aberrations occurred in Ar-irradiated samples. The experimental results are interpreted on the basis of the different pattern of energy deposition by sparsely and densely ionizing radiation. In addition, a statistical analysis based on the Neyman type A distribution is performed, which takes into account the specific stochastic properties of particle irradiation.

The time-course of Fe-ion (200 MeV/u, 440 keV/microm) and X-ray induced chromosomal damage was investigated in human lymphocytes. After cells were exposed in G0 and stimulated to grow, aberrations were measured in first-cycle metaphases harvested 48, 60 and 72 h post-irradiation. Additionally, lesions were analysed in G2 and mitotic (M) cells collected at 48 h using calyculin A-induced premature chromosome condensation (PCC). Following X-irradiation, similar aberration yields were found in all of the samples scored. In contrast, after Fe-ion exposure a drastic increase in the aberration frequency with sampling time was observed, i.e. cells arriving late at the first mitosis carried more aberrations than those arriving at earlier times. The PCC data indicate that the delayed entry of heavily damaged cells into mitosis observed after Fe-ion irradiation resulted from a prolonged arrest in G2. Altogether these experiments provide further evidence that in the case of high-LET exposure cell-cycle delays of severely damaged cells have to be taken into account for any meaningful quantification of chromosomal damage and, consequently, for an accurate estimate of the RBE.

The observation that cell-cycle progression is related to the amount of aberrations harboured by a cell demonstrates that the routinely applied method to measure aberration frequencies in metaphase cells at only one post-irradiation sampling time will unavoidably result in an under- or overestimation of the cytogenetic effects of particles. Consequently, for a meaningful quantification of chromosomal damage, multiple fixation regimes should be used so that the complete time-course of aberrations can be taken into account. Moreover, to avoid bias, all aberration types should be recorded and included in the analysis since the aberration spectrum changes with LET.

The relationship between heavy-ion-induced cell cycle delay and the time-course of aberrations in first-cycle metaphases or prematurely condensed G(2)-cells (G(2)-PCC) was investigated. Lymphocytes of the same donor were irradiated with X-rays or various charged particles (carbon, iron, xenon, and chromium) covering an LET range of 2-3,160 keV/μm. Chromosome aberrations were measured in samples collected at 48, 60, 72, and 84 h postirradiation. Linear-quadratic functions were fitted to the data, and the fit parameters α and β were determined. At any sampling time, α values derived from G(2)-cells were higher than those from metaphases. The α value derived from metaphase analysis at 48 h increased with LET, reached a maximum around 155 keV/μm, and decreased with a further rise in LET. At the later time-points, higher α values were estimated for particles with LET > 30 keV/μm. Estimates of α values from G(2)-cells showed a similar LET dependence, yet the time-dependent increase was less pronounced. Altogether, our data demonstrate that heavily damaged lymphocytes suffer a prolonged G(2)-arrest that is clearly LET dependent. For this very reason, the standard analysis of aberrations in metaphase cells 48 h postirradiation will considerably underestimate the effectiveness of high-LET radiation. Scoring of aberrations in G(2)-PCC at 48 h as suggested by several authors will result in higher aberration yields. However, when particles with a very high-LET value (LET > 150 keV/μm) are applied, still a fraction of multiple damaged cells escape detection by G(2)-analysis 48 h postirradiation.

In tomographic optoacoustic imaging, multiple parameters related to both light and ultrasound propagation characteristics of the medium need to be adequately selected in order to accurately recover maps of local optical absorbance. Speed of sound in the imaged object and surrounding medium is a key parameter conventionally assumed to be uniform. Mismatch between the actual and predicted speed of sound values may lead to image distortions but can be mitigated by manual or automatic optimization based on metrics of image sharpness. Although some simple approaches based on metrics of image sharpness may readily mitigate distortions in the presence of highly contrasting and sharp image features, they may not provide an adequate performance for smooth signal variations as commonly present in realistic whole-body optoacoustic images from small animals. Thus, three new hybrid methods are suggested in this work, which are shown to outperform well-established autofocusing algorithms in mouse experiments in vivo.

No abstract

No abstract