<p>Protontherapy is a rapidly expanding radiotherapy modality where accelerated proton beams are used to precisely deliver the dose to the tumor target but is generally considered ineffective against radioresistant tumors. Proton-Boron Capture Therapy (PBCT) is a novel approach aimed at enhancing proton biological effectiveness. PBCT exploits a nuclear fusion reaction between low-energy protons and B atoms, i.e. p+ B→ 3α (p-B), which is supposed to produce highly-DNA damaging α-particles exclusively across the tumor-conformed Spread-Out Bragg Peak (SOBP), without harming healthy tissues in the beam entrance channel. To confirm previous work on PBCT, here we report new in-vitro data obtained at the 62-MeV ocular melanoma-dedicated proton beamline of the INFN-Laboratori Nazionali del Sud (LNS), Catania, Italy. For the first time, we also tested PBCT at the 250-MeV proton beamline used for deep-seated cancers at the Centro Nazionale di Adroterapia Oncologica (CNAO), Pavia, Italy. We used Sodium Mercaptododecaborate (BSH) as B carrier, DU145 prostate cancer cells to assess cell killing and non-cancer epithelial breast MCF-10A cells for quantifying chromosome aberrations (CAs) by FISH painting and DNA repair pathway protein expression by western blotting. Cells were exposed at various depths along the two clinical SOBPs. Compared to exposure in the absence of boron, proton irradiation in the presence of BSH significantly reduced DU145 clonogenic survival and increased both frequency and complexity of CAs in MCF-10A cells at the mid- and distal SOBP positions, but not at the beam entrance. BSH-mediated enhancement of DNA damage response was also found at mid-SOBP. These results corroborate PBCT as a strategy to render protontherapy amenable towards radiotherapy-resilient tumor. If coupled with emerging proton FLASH radiotherapy modalities, PBCT could thus widen the protontherapy therapeutic index.</p>

Digital object identifier (DOI): 10.3389/fonc.2021.682647

Use of ionizing radiation (IR) in various industrial, medical and other applications can potentially increase the risk of medical, occupational or accidental human exposure. Additionally, in the event of a radiological or nuclear (R/N) incident, several tens of hundreds and thousands of people are likely to be exposed to IR. IR causes serious health effects including mortality from acute radiation syndrome and therefore it is imperative to determine the absorbed radiation dose, which will enable physicians in making an appropriate clinical 'life-saving' decision. The 'Dicentric Chromosome Assay (DCA)' is the gold standard for estimating the absorbed radiation dose but its performance is time consuming and laborious. Further, timely evaluation of dicentric chromosomes (DCs) for dose estimation in a large number of samples provides a bottleneck because of a limited number of trained personnel and a prolonged time for manual analysis. To circumvent some of these technical issues, we developed and optimized a miniaturized high throughput version of DCA (mini-DCA) in a 96-microtube matrix with bar-coded 1.4 ml tubes to enable the processing of a large number of samples. To increase the speed of DC analysis for radiation dose estimation, a semi-automated scoring was optimized using the Metafer DCScore algorithm. The accuracy of mini-DCA in dose estimation was verified and validated though comparison with conventional DCA performed in 15 ml conical tubes. The mini-DCA considerably reduced the sample processing time by a factor of 4 when compared to the conventional DCA. Further, the radiation doses estimated by mini-DCA using the triage mode of scoring (50 cells or 30 DCs) were similar to that of conventional DCA using 300-500 cells. The mini-DCA coupled with semi-automated DC scoring not only reduced the sample processing and analysis times by a factor of 4 but also enabled the processing of a large number of samples at once. Our mini-DCA method, once automated for high throughput robotic platforms, will be an effective radiological triage tool for mass casualty incidents.

Digital object identifier (DOI): 10.1093/rpd/ncy127

Protontherapy is hadrontherapy’s fastest-growing modality and a pillar in the battle against cancer. Hadrontherapy’s superiority lies in its inverted depth-dose profile, hence tumour-confined irradiation. Protons, however, lack distinct radiobiological advantages over photons or electrons. Higher LET (Linear Energy Transfer) 12C-ions can overcome cancer radioresistance: DNA lesion complexity increases with LET, resulting in efficient cell killing, i.e. higher Relative Biological Effectiveness (RBE). However, economic and radiobiological issues hamper 12C-ion clinical amenability. Thus, enhancing proton RBE is desirable. To this end, we exploited the p + 11B → 3α reaction to generate high-LET alpha particles with a clinical proton beam. To maximize the reaction rate, we used sodium borocaptate (BSH) with natural boron content. Boron-Neutron Capture Therapy (BNCT) uses 10B-enriched BSH for neutron irradiation-triggered alpha particles. We recorded significantly increased cellular lethality and chromosome aberration complexity. A strategy combining protontherapy’s ballistic precision with the higher RBE promised by BNCT and 12C-ion therapy is thus demonstrated.

The mechanisms behind the transmission of chromosomal aberrations (CA) remain unclear, despite a large body of work and major technological advances in chromosome identification. We reevaluated the transmission of CA to second- and third-division cells by telomere and centromere (TC) staining followed by M-FISH. We scored CA in lymphocytes of healthy donors after in vitro irradiation and those of cancer patients treated by radiation therapy more than 12 years before. Our data demonstrate, for the first time, that dicentric chromosomes (DCs) decreased by approximately 50% per division. DCs with two centromeres in close proximity were more efficiently transmitted, representing 70% of persistent DCs in ≥M3 cells. Only 1/3 of acentric chromosomes (ACs), ACs with four telomeres, and interstitial ACs, were paired in M2 cells and associated with specific DCs configurations. In lymphocytes of cancer patients, 82% of detected DCs were characterized by these specific configurations. Our findings demonstrate the high stability of DCs with two centromeres in close proximity during cell division. The frequency of telomere deletion increased during cell cycle progression playing an important role in chromosomal instability. These findings could be exploited in the follow-up of exposed populations.

Digital object identifier (DOI): 10.1038/s41598-017-03198-7

Canonical non-homologous end joining (c-NHEJ) repairs DNA double-strand breaks (DSBs) in G1 cells with biphasic kinetics. We show that DSBs repaired with slow kinetics, including those localizing to heterochromatic regions or harboring additional lesions at the DSB site, undergo resection prior to repair by c-NHEJ and not alt-NHEJ. Resection-dependent c-NHEJ represents an inducible process during which Plk3 phosphorylates CtIP, mediating its interaction with Brca1 and promoting the initiation of resection. Mre11 exonuclease, EXD2, and Exo1 execute resection, and Artemis endonuclease functions to complete the process. If resection does not commence, then repair can ensue by c-NHEJ, but when executed, Artemis is essential to complete resection-dependent c-NHEJ. Additionally, Mre11 endonuclease activity is dispensable for resection in G1. Thus, resection in G1 differs from the process in G2 that leads to homologous recombination. Resection-dependent c-NHEJ significantly contributes to the formation of deletions and translocations in G1, which represent important initiating events in carcinogenesis.

Digital object identifier (DOI): 10.1016/j.molcel.2016.12.016

<p>The ability of parabens to promote the appearance of multiple cancer hallmarks in breast epithelium cells provides grounds for regulatory review of the implication of the presence of parabens in human breast tissue. It is well documented that telomere dysfunction plays a significant role in the initiation of genomic instability during carcinogenesis in human breast cancer. In the present study, we evaluated the genotoxic effect of ethyl 4-hydroxybenzoate (ethyl-paraben), with and without metabolic activation (S9), in studies following OECD guidelines. We observed a significant increase in genotoxic damage using the Mouse Lymphoma Assay and in vitro micronucleus (MN) tests in the L5178Y cell line in the presence of S9 only after a short exposure. A high frequency of MN was observed in the TK6 cells after a short exposure (3 h) in the presence of S9 and a long exposure (26 h) without S9. We found significant increases in the MN frequency and induced chromosomal aberrations in the lymphocytes of only one donor after ethyl-paraben exposure in the presence of S9 after a short exposure. Cytogenetic characterization of the paraben-treated cells demonstrated telomere shortening associated with telomere loss and telomere deletions in L5178Y and TK6 cells and lymphocytes of the paraben sensitive-donor. In a control cohort of 68 human lymphocytes, telomere length and telomere aberrations were age-dependent and showed high inter-individual variation. This study is the first to link telomere shortening and the genotoxic effect of ethyl paraben in the presence of S9 and raises the possibility that telomere shortening may be a proxy for underlying inter-individual sensitivity to ethyl-paraben. Copyright © 2016 John Wiley &amp; Sons, Ltd.</p>

Digital object identifier (DOI): 10.1002/jat.3425

We recently reported an increase in dicentric chromosome (DIC) formation after a single computed tomography (CT) scan (5.78-60.27 mSv: mean 24.24 mSv) and we recommended analysis of 2000 metaphase cells stained with Giemsa and centromere-FISH for dicentric chromosome assay (DCA) in cases of low-dose radiation exposure. In the present study, we analyzed the frequency of chromosome translocations using stored Carnoy's-fixed lymphocyte specimens from the previous study; these specimens were from 12 patients who were subject to chromosome painting of Chromosomes 1, 2 and 4. Chromosomes 1, 2 and 4 were analyzed in ∼5000 cells, which is equivalent to the whole-genome analysis of almost 2000 cells. The frequency of chromosome translocation was higher than the number of DICs formed, both before and after CT scanning. The frequency of chromosome translocations tended to be higher, but not significantly higher, in patients with a treatment history compared with patients without such a history. However, in contrast to the results for DIC formation, the frequency of translocations detected before and after the CT scan did not differ significantly. Therefore, analysis of chromosome translocation may not be a suitable assay for detecting chromosome aberrations in cases of low-dose radiation exposure from a CT scan. A significant increase in the frequency of chromosome translocations was not likely to be detected due to the high baseline before the CT scan; the high and variable frequency of translocations was probably due to multiple confounding factors in adults.

Digital object identifier (DOI): 10.1093/jrr/rrv090

Chromothripsis is the massive but highly localized chromosomal rearrangement in response to a one-step catastrophic event, rather than an accumulation of a series of subsequent and random alterations. Chromothripsis occurs commonly in various human cancers and is thought to be associated with increased malignancy and carcinogenesis. However, the causes and consequences of chromothripsis remain unclear. Therefore, to identify the mechanism underlying the generation of chromothripsis, we investigated whether chromothripsis could be artificially induced by ionizing radiation. We first elicited DNA double-strand breaks in an oral squamous cell carcinoma cell line HOC313-P and its highly metastatic subline HOC313-LM, using Single Particle Irradiation system to Cell (SPICE), a focused vertical microbeam system designed to irradiate a spot within the nuclei of adhesive cells, and then established irradiated monoclonal sublines from them, respectively. SNP array analysis detected a number of chromosomal copy number alterations (CNAs) in these sublines, and one HOC313-LM-derived monoclonal subline irradiated with 200 protons by the microbeam displayed multiple CNAs involved locally in chromosome 7. Multi-color FISH showed a complex translocation of chromosome 7 involving chromosomes 11 and 12. Furthermore, whole genome sequencing analysis revealed multiple de novo complex chromosomal rearrangements localized in chromosomes 2, 5, 7, and 20, resembling chromothripsis. These findings suggested that localized ionizing irradiation within the nucleus may induce chromothripsis-like complex chromosomal alterations via local DNA damage in the nucleus.

Digital object identifier (DOI): 10.18632/oncotarget.7186

XRCC4-like factor (XLF) functions in classical non-homologous end-joining (cNHEJ) but is dispensable for the repair of DNA double-strand breaks (DSBs) generated during V(D)J recombination. A long-standing hypothesis proposes that, in addition to its canonical nuclease activity, the RAG1/2 proteins participate in the DNA repair phase of V(D)J recombination. Here we show that in the context of RAG2 lacking the C-terminus domain (Rag2(c/c) mice), XLF deficiency leads to a profound lymphopenia associated with a severe defect in V(D)J recombination and, in the absence of p53, increased genomic instability at V(D)J sites. In addition, Rag2(c/c) XLF(-/-) p53(-/-) mice develop aggressive pro-B cell lymphomas bearing complex chromosomal translocations and gene amplifications involving Igh and c-myc/pvt1 loci. Our results reveal an unanticipated functional interplay between the RAG complex and XLF in repairing RAG-induced DSBs and maintaining genome integrity during antigen receptor gene assembly.

Digital object identifier (DOI): 10.1038/ncomms10529

In general, it was assumed that the chromosome aberration induced by ionizing radiation is proportional to the chromosome size. From this viewpoint, the higher chromosome size, the more resistant to radiation. However, different opinions, in which chromosomes are particularly sensitive or resistant to radiation, are also still followed until now. Here in this research, we compared the chromosome sensitivity between chromosomes number 1, 2, and 4 using the FISH (fluorescence in situ hybridization) technique. From this research, we expect that the information obtained could show clearly whether a longer chromosome is more frequently involved in translocations and also more resistant to radiation than a shorter one. The type of chromosome aberration considered was limited only to translocation and we used one sample donor in order to avoid donor variability. The whole blood from a healthy female was irradiated with γ-rays with doses of 1, 3 and 5 Gy, respectively. Isolated lymphocytes from the whole blood were then cultured for 48 hours. After the culture process was completed, preparations of harvest and metaphase chromosomes were carried out. Chromosomes 1, 2, and 4 were stained with different fluorochromes. The translocation of each chromosome at each dose point was subsequently evaluated from 50 images obtained from an automated metaphase finder and capturing system. An additional analysis was performed to identify which chromosome arm was more frequently involved in translocation. Further analyses were also conducted with the aim of determining which chromosome band had a higher frequency of radiation-induced breakage. The experimental results showed that chromosome number 4 was more frequently involved in translocations compared to chromosomes 1 and 2 at 5 Gy. In contrast, at doses of 1 and 3 Gy translocations involving chromosomes number 1 and 2 were more numerous compared to the ones involving chromosome 4. However, if the number of translocation was accumulated for all the doses applied, the chromosome number 4 was the chromosome most frequently involved in translocations. Breakpoint analysis revealed that in chromosome 1, chromosome 2, and chromosome 4, the highest chromosome bands as break position were in band q32, p13, and q21, respectively. It can be concluded that chromosome 4 is more sensitive to radiation in all doses point, despite having less DNA content than chromosomes 1 and 2. Thus, it was showed that our research cannot support the general assumption about chromosome aberration induced by radiation being proportional to DNA content.

Human tumors using the alternative lengthening of telomeres (ALT) exert high rates of telomere dysfunction. Numerical chromosomal aberrations are very frequent, and structural rearrangements are widely scattered among the genome. This challenging context allows the study of telomere dysfunction-driven chromosomal instability in neoplasia (CIN) in a massive scale. We used molecular cytogenetics to achieve detailed karyotyping in 10 human ALT neoplastic cell lines. We identified 518 clonal recombinant chromosomes affected by 649 structural rearrangements. While all human chromosomes were involved in random or clonal, terminal, or pericentromeric rearrangements and were capable to undergo telomere healing at broken ends, a differential recombinatorial propensity of specific genomic regions was noted. We show that ALT cells undergo epigenetic modifications rendering polycentric chromosomes functionally monocentric, and because of increased terminal recombinogenicity, they generate clonal recombinant chromosomes with interstitial telomeric repeats. Losses of chromosomes 13, X, and 22, gains of 2, 3, 5, and 20, and translocation/deletion events involving several common chromosomal fragile sites (CFSs) were recurrent. Long-term reconstitution of telomerase activity in ALT cells reduced significantly the rates of random ongoing telomeric and pericentromeric CIN. However, the contribution of CFS in overall CIN remained unaffected, suggesting that in ALT cells whole-genome replication stress is not suppressed by telomerase activation. Our results provide novel insights into ALT-driven CIN, unveiling in parallel specific genomic sites that may harbor genes critical for ALT cancerous cell growth.

Multicolour fluorescence in situ hybridisation (M-FISH) and multicolour banding (M-BAND) are advanced chromosome painting techniques combining multiple chromosome- or region-specific paints in one step. M-FISH identifies all chromosomes or chromosome arms at once, whereas M-BAND identifies the different regions of a single chromosome. The use of either or both can improve the accuracy of karyotyping and help identify cryptic chromosome rearrangements. These probes are prepared by pooling multiple chromosome- or chromosome region-specific DNA libraries, each labelled with a unique combination of fluorochromes. Commercial probes are available, avoiding the need for probe preparation. In the protocol described here, a commercial probe is used. Well-spread metaphases are prepared according to standard techniques, followed by alkaline denaturation and application of the denatured probe. After an incubation period, the slides are washed. A fluorescence microscope with filter sets specific to the fluorescent labels is used for analysis, together with specialised image analysis software. The software interprets the combination of fluorochromes to identify each chromosome and produce a false colour image specific for each chromosome or region. The single colour galleries - which show the hybridisation patterns of the individual fluorochromes - are useful to help interpret and confirm the false colour images produced by the software, including ambiguous signals.

<p>The purpose of the present study was to investigate as to what extent differences in the linear energy transfer (LET) are reflected at the chromosomal level. For this study human lymphocytes were exposed to 9.5 MeV/u C-ions (1 or 2 Gy, LET=175 keV/microm) or X-rays (1-6 Gy), harvested at 48, 72 or 96 h post-irradiation and aberrations were scored in first cycle metaphases using 24 color fluorescence in situ hybridization (mFISH). Additionally, in selected samples aberrations were measured in prematurely condensed G2-phase cells. Analysis of the time-course of aberrations in first cycle metaphases showed a stable yield of simple and complex exchanges after X-ray irradiation. In contrast, after C-ion exposure the yields profoundly increased with harvesting time complicating the estimation of the frequency of aberrations produced by high LET particles within the entire cell population. This is especially true for the yield of complex exchanges. Complex aberrations dominate the aberration spectrum produced by C-ions. Their fraction was about 50\% for the two measured doses. In contrast, isodoses of X-rays induced smaller proportions of complex aberrations (i.e. 5% and 15%, respectively). For both radiation qualities the fraction of complexes did not change with harvesting time. As expected from the different dose deposition of high and low LET radiation, complex exchanges produced by high LET C-ions involved more breaks and more chromosomes than those induced by isodoses of X-rays. Noteworthy, C-ions but not X-rays induced a small number of complex chromatid-isochromatid exchanges that are not expected for cells exposed in the G0-phase. The results obtained so far for cells arrested in G2-phase confirm these patterns. Altogether our data show that the increased effectiveness of C-ions for the induction of aberrations in first cycle cells is determined by complex exchanges, whereas for simple exchanges the relative biological effectiveness is about one.</p>

Papillary thyroid carcinoma (PTC) is a known radiation-induced tumor. Rearrangements in human chromosome 10 and in particular intrachromosomal exchanges are often associated with PTC formation. In this study we measured intrachromosomal exchanges in human thyroid follicular cells exposed to sparsely or densely ionizing radiation. Assuming that inversions in chromosome 10 are a biomarker of PTC risk, we estimated the relative biological effectiveness (RBE) of heavy ions using a molecular marker in vitro. The analysis of chromosomal aberrations was performed with the mBAND technique, which allows detection of both inter- and intrachromosomal exchanges. Our results do not show any significant increase in the yield of intrachanges in samples exposed to heavy ions compared to X rays. Within the constraints imposed by the experimental model we used, we conclude that heavy ions would not necessarily be more effective than X rays in the induction of thyroid cancer.

Human risks from chronic exposures to both low- and high-LET radiation are of intensive research interest in recent years. In the present study, human epithelial cells were exposed in vitro to gamma-rays at a dose rate of 17mGy/h or secondary neutrons of 25mGy/h. The secondary neutrons have a broad energy spectrum that simulates the Earth's atmosphere at high altitude, as well as the environment inside spacecrafts like the Russian MIR station and the International Space Station (ISS). Chromosome aberrations in the exposed cells were analyzed using the multicolor banding in situ hybridization (mBAND) technique with chromosome 3 painted in 23 colored bands that allows identification of both inter- and intrachromosome exchanges including inversions. Comparison of present dose responses between gamma-rays and neutron irradiations for the fraction of cells with damaged chromosome 3 yielded a relative biological effectiveness (RBE) value of 26+/-4 for the secondary neutrons. Our results also revealed that secondary neutrons of low dose rate induced a higher fraction of intrachromosome exchanges than gamma-rays, but the fractions of inversions observed between these two radiation types were indistinguishable. Similar to the previous findings after acute radiation exposures, most of the inversions observed in the present study were accompanied by other aberrations. The fractions of complex type aberrations and of unrejoined chromosomal breakages were also found to be higher in the neutron-exposed cells than after gamma-rays. We further analyzed the location of the breaks involved in chromosome aberrations along chromosome 3, and observed hot spots after gamma-ray, but not neutron, exposures.

BACKGROUND AND PURPOSE: To investigate the cytogenetic damage in blood lymphocytes of patients treated for prostate cancer with different radiation qualities and target volumes. MATERIALS AND METHODS: Twenty patients receiving carbon-ion boost irradiation followed by IMRT or IMRT alone for the treatment of prostate cancer entered the study. Cytogenetic damage induced in peripheral blood lymphocytes of these patients was investigated at different times during the radiotherapy course using Giemsa staining and mFISH. A blood sample from each patient was taken before initiation of radiation therapy and irradiated in vitro to test for individual radiosensitivity. In addition, in vitro dose-effect curves for the induction of chromosomal exchanges by X-rays and carbon ions of different energies were measured. RESULTS: The yield of chromosome aberrations increased during the therapy course, and the frequency was lower in patients irradiated with carbon ions as compared to patients treated with IMRT with similar target volumes. A higher frequency of aberrations was measured by increasing the target volume. In vitro, high-LET carbon ions were more effective than X-rays in inducing aberrations and yielded a higher fraction of complex exchanges. The yield of complex aberrations observed in vivo was very low. CONCLUSION: The investigation showed no higher aberration yield induced by treatment with a carbon-ion boost. In contrast, the reduced integral dose to the normal tissue is reflected in a lower chromosomal aberration yield when a carbon-ion boost is used instead of IMRT alone. No cytogenetic #signature# of exposure to densely ionizing carbon ions could be detected in vivo.

To characterise the radiation response of human hematopoietic stem and progenitor cells (HSPC) with respect to X and carbon ion irradiation.HSPC from peripheral blood of healthy donors treated with granulocyte-colony stimulating factor (G-CSF) were enriched for the transmembrane glycoprotein CD34 (cluster of differentiation) and irradiated with X rays or carbon ions (29 keV/microm monoenergetic beam and 60-85 keV/microm spread-out Bragg peak), mimicking radiotherapy conditions. Apoptotic cell death, cell cycle progression and the frequency of chromosomal aberrations were determined.After radiation exposure no inhibition in the progression of the cell cycle was detected. However, an enhanced frequency of apoptotic cells and an increase in aberrant cells were observed, both effects being more pronounced for carbon ions than X rays, resulting in a relative biological effectiveness (RBE) of 1.4-1.7. The fraction of complex-type aberrations was higher following carbon ion exposure.RBE values of carbon ions are low, as expected for radiosensitive cells. The observed frequencies of apoptotic cells and chromosome aberrations in HSPC are similar to those reported for human peripheral blood lymphocytes suggesting that at least with respect to apoptosis and chromosomal aberrations mature lymphocytes reflect the respective radiation responses of their proliferating progenitors.

In 1957/58 the British Government conducted a series of nuclear tests in the mid-Pacific codenamed Operation Grapple, which involved several naval vessels from Britain and New Zealand. Two New Zealand frigates with 551 personnel onboard were stationed at various distances between 20 and 150 nautical miles from ground zero. In the present study we applied the cytomolecular technique mFISH (multicolour fluorescent in situ hybridisation) to investigate a potential link between chromosome abnormalities and possible past radiation exposure in New Zealand nuclear test veterans who participated in Operation Grapple. Compared to age matched controls, the veterans showed significantly higher (P < 0.0001) frequencies of chromosomal abnormalities (275 translocations and 12 dicentrics in 9,360 cells vs. 96 translocations and 1 dicentric in 9,548 cells in the controls), in addition to a significant excess of CCRs (complex chromosomal rearrangements) in the veterans. A Kolmogorov-Smirnoff test showed that the distributions of translocations for the two groups were significantly different.

We measured residual cytogenetic damage in the progeny of human peripheral blood lymphocytes exposed to 1 GeV/ nucleon iron ions or gamma rays. Arm-specific DNA probes for chromosome 1 were used to detect aberrations as a function of dose in cells harvested 144 h after exposure. In addition, arm-specific mFISH was applied to samples exposed to a single dose of 2 Gy. These methods allowed the detection of interarm intrachanges (pericentric inversions) in addition to interchanges. The ratio of these types of aberrations (F ratio) has been proposed as a fingerprint of exposure to densely ionizing radiation. The fractions of aberrant cells in the progeny of cells exposed to iron ions were similar to those in the population exposed to gamma rays, possibly because many rearrangements induced by heavy ions ultimately lead to cell death. Simple inter- and intrachanges were also similar, but more complex rearrangements were found in cells that survived after exposure to iron ions. We did not find a significant difference in the ratio of simple interchanges to simple intrachanges for the two radiation types. However, iron ions induced a much higher frequency of events involving both inter- and intrachanges. We conclude that these complex rearrangements represent a hallmark of exposure to heavy ions and may be responsible of the decrease of the F ratio with increasing LET reported in the literature in some in vitro and in vivo experiments.

Chromosome aberration analysis in astronauts has been used to provide direct, biologically motivated estimates of equivalent doses and risk associated to cosmic radiation exposure during space flight. However, the past studies concentrated on measurements of dicentrics and translocations, while chromosome intrachanges (inversions) have never been measured in astronauts' samples. Recent data reported in the literature suggest that densely ionizing radiation can induce a large fraction of intrachanges, thus leading to the suspicion that interchanges grossly underestimate the cosmic radiation-induced cytogenetic damage in astronauts. We have analyzed peripheral blood lymphocytes from 11 astronauts involved in short- or long-term space flights in low-Earth orbit using high-resolution multicolor banding to assess the frequency of intrachromosomal exchanges in both pre- and post-flight samples. We did not detect any inversions in chromosome 5 from a total of 2800 cells in astronauts' blood. In addition, no complex type exchanges were found in a total of 3590 astronauts' lymphocytes analyzed by multifluor fluorescence in situ hybridisation. We conclude that, within the statistical power of this study, the analysis of interchanges for biological dosimetry in astronauts does not significantly underestimate the space radiation-induced cytogenetic damage, and complex-type exchanges or intrachanges have limited practical use for biodosimetry at very low doses.