Scintillation light detection in the 6-m drift-length ProtoDUNE Dual Phase liquid argon TPC

DUNE is a dual-site experiment for long-baseline neutrino oscillation studies, neutrino astrophysics and nucleon decay searches. ProtoDUNE Dual Phase (DP) is a 6  ×  6  ×  6 m 3 liquid argon time-projection-chamber (LArTPC) that recorded cosmic-muon data at the CERN Neutrino Platform in 2019-2020 as a prototype of the DUNE Far Detector. Charged particles propagating through the LArTPC produce ionization and scintillation light. The scintillation light signal in these detectors can provide the trigger for non-beam events. In addition, it adds precise timing capabilities and improves the calorimetry measurements. In ProtoDUNE-DP, scintillation and electroluminescence light produced by cosmic muons in the LArTPC is collected by photomultiplier tubes placed up to 7 m away from the ionizing track. In this paper, the ProtoDUNE-DP photon detection system performance is evaluated with a particular focus on the different wavelength shifters, such as PEN and TPB, and the use of Xe-doped LAr, considering its future use in giant LArTPCs. The scintillation light production and propagation processes are analyzed and a comparison of simulation to data is performed, improving understanding of the liquid argon properties.

E-tool for mental health prevention: a study of the receptivity and engagement in a large-scale group of subjects

Introduction

Due to the Covid-19 effects, mental health conditions are now, more than ever, affecting our daily lives - both personally and professionally. The average delay between the onset of first symptoms of a mental health disorder and seeking suitable healthcare is 11 years. The WHO states that the only sustainable way to reduce mental healthcare burden is by acting earlier.

Objectives

The aim of this project is to assess the receptivity and engagement of a mobile app for mental health prevention, amongst a large-scale and heterogeneous group of individuals. The main hypothesis under testing is that people are receptive to actively act towards mental health prevention, despite still being a very neglected and stigmatized topic.

Methods

A mobile app for mental health improvement and disease prevention was developed through the digitalization of positive psychology strategies, such as mood tracking, journaling, breathing exercises, among others, which are personalized to the user through biofeedback. The app aims at teaching people how to autonomously cope with mental health conditions, identifying early signs and redirecting them to proper mental health professionals. The app is being released for a population of 35,000 subjects resident in Portugal.

Results

Receptivity and engagement metrics will be assessed on a weekly and monthly basis, for 3 months, segmented by different subject profiles. Mental health metrics will also be assessed, namely anxiety, depression, and burnout levels - using standard psychiatric scales.

Conclusions

We have yet to draw conclusions from the project; however, we aim to achieve first results in due time.

Disclosure

The aim of this research is to assess the receptivity of mental health prevention strategies using technology, namely a mobile app provided by a company.

The anxiety of nursing professionals during the COVID-19 pandemic period in a brazilian regional university hospital

Introduction

Nursing work requires technical, scientific competence, knowledge, skill and emotional control over practice, considering that care presents risk situations, physical and emotional stress, responsibilities with people’s lives, coping with fears and suffering. All this situation in which the professional is exposed can lead to the occurrence of psychological wear, high stress and anxiety, this is conceptualized as a vague and unpleasant feeling of fear, apprehension, with characteristics of tension or discomfort derived from anticipating danger, something unknown or strange.

Objectives

To identify the prevalence and factors associated with anxiety among nursing professionals who work coping with COVID-19 in a Brazilian regional university hospital.

Methods

Cross-sectional observational study, with sociodemographic questionnaire and anxiety measurement scale (HAD), with 88 nursing professionals. The data were analyzed using absolute and relative frequency, using the software StatisticalPackage for the Social Sciences.

Results

There was a prevalence of anxiety (48.9%), with the majority of the sample consisting of women, over 40 years old, married or in a stable relationship, white, with higher education or postgraduate education, with income above R $ 3,000.00, tendered, with a work regime of 40 hours per week and time in the hospital from 1 to 5 years.

Conclusions

The impact should be considered on Nursing Mental Health caused by COVID-19 and intervene with coping strategies to minimize anxiety.

Anesthetic Management of Successful Extracorporeal Resuscitation After Six Hours of Cardiac Arrest Due to Severe Accidental Hypothermia

Accidental hypothermia (AH) in Mediterranean countries often is underestimated. AH should be suspected in patients also in moderate climates throughout all seasons. Compared with other countries, the mortality rate due to AH in Spain is low, and hypothermia rarely is recognized and treated. The case of a patient who experienced cardiac arrest due to severe AH and was resuscitated for more than six hours using extracorporeal life support recently was published. Herein that case is reviewed, with the anesthetic management during cannulation detailed. In addition, the authors highlight how the application of extracorporeal cardiopulmonary resuscitation guidelines is different in AH patients, how in-hospital (HOPE score) triage criteria should be applied, and how the establishment of clear standard operating procedures and education strategies should be promoted.

Periacetabular osteotomy for hip dysplasia treatment through a mini-invasive technique. Our results at mid-term in 131 cases

BACKGROUND AND OBJECTIVE

Periacetabular osteotomy (PAO) is an accepted and worldwide technique recognized for residual dysplasia treatment and even in unstable hips with limited acetabular coverage. The aim of this study is to analyse the functional, radiological and complication results in patients treated with mini-invasive PAO.

MATERIAL AND METHODS

We performed a retrospective study in which we analysed 131 cases undergoing mini-invasive PAO at our centre. The degree of joint degeneration was evaluated with Tönnis scale, Wiberg angle, acetabular index (AI), anterior coverage angle (AC), joint space, complications and functional outcome with the Non-Arthritic Hip Score (NAHS) were analysed preoperatively and at the end of follow-up.

RESULTS

The average age was 32.3±9.5 (SD) years, 102 (77.9%) were female and 29 (22.1%) were male. 7.7±2.8 (SD) years follow up. The radiological parameters improved between the pre-surgical phase and the end of follow-up, Wiberg angle+18.5° (18.3° versus 36.8°, 95% CI 17.3 to 19.7), AC angle+13.5° (26.2° versus 39.7°, 95%CI 11.6 to 15.4) and the AI -11.1° (19.5° versus 8.4°; 95%CI -12.1 to -10,1). In addition, the functional results, with the NAHS scale, improved+31.3 points (60.7 pre-surgical versus 92 at the end of follow-up, 95% CI 28.7 to 33.8). The most common complication was transient lateral femoral cutaneous nerve hypoaesthesia in 10 cases (7%).

CONCLUSION

The mini-invasive PAO approach is a reproducible technique, it allows restoration of acetabular coverage and provides an improvement in functional scales as confirmed by our series.

Analysis of γ-rays induced chromosome aberrations: A fingerprint evaluation with a combination of pan-centromeric and pan-telomeric probes

PURPOSE

To evaluate the types of induced chromosome aberrations after the exposure of peripheral blood to gamma-rays by the simultaneous detection of all centromeres and telomeres; and to analyse the suitability of different radiation fingerprints for the assessment of radiation quality in cases of recent exposures.

MATERIAL AND METHODS

Peripheral blood samples were irradiated at 2, 4 and 6 Gy of gamma-rays. Cytogenetic analysis was carried out by fluorescence in situ hybridization (FISH) technique with pan-centromeric and peptide nucleic acid (PNA)-telomeric DNA probes. Cells were analysed using a Cytovision FISH workstation, chromosome aberrations and the length of the acentric fragments were recorded.

RESULTS

The total number of the incomplete chromosome elements was 276. The ratio between incomplete elements and multicentrics was 0.38. The number of acentrics was 1096, 71% were complete acentrics, 15% incomplete acentrics, and 14% interstitial fragments. The relative length of complete, incomplete and interstitial acentrics fragments were 2.70 +/- 0.04, 1.91 +/- 0.07, and 1.42 +/- 0.04 respectively. The mean value of the F-ratio was 11.5 higher than the one, 5.5, previously obtained for alpha-particles. For the G-ratio there was no difference between gamma-rays and alpha-particles, 2.8 and 2.8 respectively. The mean value of the H-ratio for gamma-rays, 0.25, was lower than for alpha-particles 0.40.

CONCLUSION

The results support that the percentage of incomplete chromosome aberrations depends on radiation type; low-linear energy transfer (LET) radiation would produces less incomplete aberrations than high-LET radiation. The F- and H-ratios seem to be good indicators of radiation quality, although a real estimation of the H-ratio is only possible using pan-telomeric probes.

Comparison of dose calculation algorithms in slab phantoms with cortical bone equivalent heterogeneities

To evaluate the dose values predicted by several calculation algorithms in two treatment planning systems, Monte Carlo (MC) simulations and measurements by means of various detectors were performed in heterogeneous layer phantoms with water- and bone-equivalent materials. Percentage depth doses (PDDs) were measured with thermoluminescent dosimeters (TLDs), metal-oxide semiconductor field-effect transistors (MOSFETs), plane parallel and cylindrical ionization chambers, and beam profiles with films. The MC code used for the simulations was the PENELOPE code. Three different field sizes (10 x 10, 5 x 5, and 2 x 2 cm2) were studied in two phantom configurations and a bone equivalent material. These two phantom configurations contained heterogeneities of 5 and 2 cm of bone, respectively. We analyzed the performance of four correction-based algorithms and one based on convolution superposition. The correction-based algorithms were the Batho, the Modified Batho, the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system (TPS), and the Helax-TMS Pencil Beam from the Helax-TMS (Nucletron) TPS. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. All the correction-based calculation algorithms underestimated the dose inside the bone-equivalent material for 18 MV compared to MC simulations. The maximum underestimation, in terms of root-mean-square (RMS), was about 15% for the Helax-TMS Pencil Beam (Helax-TMS PB) for a 2 x 2 cm2 field inside the bone-equivalent material. In contrast, the Collapsed Cone algorithm yielded values around 3%. A more complex behavior was found for 6 MV where the Collapsed Cone performed less well, overestimating the dose inside the heterogeneity in 3%-5%. The rebuildup in the interface bone-water and the penumbra shrinking in high-density media were not predicted by any of the calculation algorithms except the Collapsed Cone, and only the MC simulations matched the experimental values within the estimated uncertainties. The TLD and MOSFET detectors were suitable for dose measurement inside bone-equivalent materials, while parallel ionization chambers, applying the same calibration and correction factors as in water, systematically underestimated dose by 3%-5%.

Hip osteoplasty by an anterior minimally invasive approach for active patients with femoroacetabular impingement

Femoroacetabular regularization in hip impingement is currently performed by means of trochanter osteotomy and hip dislocation or more recently by means of hip arthroscopy. We present a novel alternative through a unique mini-invasive anterior approach. Our first series consisted of 35 hips (32 patients) with a mean follow-up of 29.2 months. Range of motion (ROM) and clinical scores were evaluated preoperatively at six weeks, three months, six months and one-year follow-up (FU). Impingement test was negative in 33 out of 35 cases six weeks after surgery. Mean hospitalisation time was 2.6 days (2-5 days). Mean improvement in internal rotation was 23 degrees (p=0.006) and 21 degrees in flexion (p=0.011). There was a significant improvement in hip score according to the Merle d'Aubign evaluation (13.8 points preoperative vs. 16.9 at one-year visit) (p=0.017). No Trendelenburg, heterotopic calcifications or osteonecrosis were observed. Complications related to the femorocutaneous nerve appeared in six cases (17.1%) although all but one were neuroapraxia and disappeared before one year. Mean rehabilitation time was 4.4 weeks. We conclude that the anterior surgical approach for the treatment of femoroacetabular impingement enables early resumption of sport while accuracy in bone resection is maintained.

Dose evaluation in lung-equivalent media in high-energy photon external radiotherapy

In high-energy photon external radiotherapy treatment planning systems (TPSs) are used to calculate the dose to the target volume and the dose distribution around it. Commonly used TPSs include algorithms based on measurements in water and often fail in the estimate of dose in the presence of heterogeneities. In this study TL detectors were used to study the reliability of the Cadplan (Varian) TPS in the presence of low-density heterogeneities such as the lung for 6 and 18 MV photon beams at different field sizes. TL measurements were compared with TPS calculations and Monte Carlo simulations performed with the PENELOPE MC code. In a phantom with lung heterogeneity, TL measurements and MC simulations agreed, with an average deviation inside the lung of 2%. In contrast, TPS results overestimated the dose inside the lung, with a maximum deviation of 39% for the 18 MV photon beam and a field size of 2 x 2 cm(2).

Pantak Therapax SXT 150: performance assessment and dose determination using IAEA TRS-398 protocol

The performance assessment and beam characteristics of the Therapax SXT 150 unit, which encompass both low and medium-energy beams, were evaluated. Dose determination was carried out by implementing the International Atomic Energy Agency (IAEA) TRS-398 protocol and measuring all the dosimetric parameters in order to have a solid, consistent and reliable data set for the unit. Mechanical movements, interlocks and applicator characteristics agreed with specifications. The timer exhibited good accuracy and linearity. The output was very stable, with good repeatability, long-term reproducibility and no dependence on tube head orientation. The measured dosimetric parameters included beam first and second half-value layers (HVLs), absorbed dose rate to water under reference conditions, central axis depth dose distributions, output factors and beam profiles. Measured first HVLs agreed with comparable published data, but the homogeneity coefficients were low in comparison with typical values found in the literature. The timer error was significant for all filters and should be taken into consideration for the absorbed dose rate determination under reference conditions as well as for the calculation of treatment times. Percentage depth-dose (PDD) measurements are strongly recommended for each filter-applicator combination. The output factor definition of the IAEA TRS-398 protocol for medium-energy X-ray qualities involves the use of data that is difficult to measure. Beam profiles had small penumbras and good symmetry and flatness except for the lowest energy beam, for which a heel effect was observed.

Comparison of dose calculation algorithms in phantoms with lung equivalent heterogeneities under conditions of lateral electronic disequilibrium

An extensive set of benchmark measurement of PDDs and beam profiles was performed in a heterogeneous layer phantom, including a lung equivalent heterogeneity, by means of several detectors and compared against the predicted dose values by different calculation algorithms in two treatment planning systems. PDDs were measured with TLDs, plane parallel and cylindrical ionization chambers and beam profiles with films. Additionally, Monte Carlo simulations by means of the PENELOPE code were performed. Four different field sizes (10 x 10, 5 x 5, 2 x 2, and 1 x 1 cm2) and two lung equivalent materials (CIRS, p(w)e=0.195 and St. Bartholomew Hospital, London, p(w)e=0.244-0.322) were studied. The performance of four correction-based algorithms and one based on convolution-superposition was analyzed. The correction-based algorithms were the Batho, the Modified Batho, and the Equivalent TAR implemented in the Cadplan (Varian) treatment planning system and the TMS Pencil Beam from the Helax-TMS (Nucletron) treatment planning system. The convolution-superposition algorithm was the Collapsed Cone implemented in the Helax-TMS. The only studied calculation methods that correlated successfully with the measured values with a 2% average inside all media were the Collapsed Cone and the Monte Carlo simulation. The biggest difference between the predicted and the delivered dose in the beam axis was found for the EqTAR algorithm inside the CIRS lung equivalent material in a 2 x 2 cm2 18 MV x-ray beam. In these conditions, average and maximum difference against the TLD measurements were 32% and 39%, respectively. In the water equivalent part of the phantom every algorithm correctly predicted the dose (within 2%) everywhere except very close to the interfaces where differences up to 24% were found for 2 x 2 cm2 18 MV photon beams. Consistent values were found between the reference detector (ionization chamber in water and TLD in lung) and Monte Carlo simulations, yielding minimal differences (0.4%+/-1.2%). The penumbra broadening effect in low density media was not predicted by any of the correction-based algorithms, and the only one that matched the experimental values and the Monte Carlo simulations within the estimated uncertainties was the Collapsed Cone Algorithm.

Comparison study of MOSFET detectors and diodes for entrancein vivodosimetry in 18 MV x-ray beams

The feasibility of dual bias dual metal oxide semiconductor field effect transistors (MOSFETs) for entrance in vivo dose measurements in high energy x-rays beams (18 MV) was investigated. A comparison with commercially available diodes for in vivo dosimetry for the same energy range was performed. As MOSFETs are sold without an integrated build-up cap, different caps were tested: 3 cm bolus, 2 cm bolus, 2 cm hemispherical cap of a water equivalent material (Plastic Water) and a metallic hemispherical cap. This metallic build-up cap is the same as the one that is mounted on the in vivo diode used in this study. Intrinsic precision and response linearity with dose were determined for MOSFETs and diodes. They were then calibrated for entrance in vivo dosimetry in an 18 MV x-ray beam. Calibration included determination of the calibration factor in standard reference conditions and of the correction factors (CF) when irradiation conditions differed from those of reference. Correction factors for field size, source surface distance, wedge, and temperature were determined. Sensitivity variation with accumulated dose and the lifetime of both types of detectors were also studied. Finally, the uncertainties of entrance in vivo measurements using MOSFET and diodes were discussed. Intrinsic precision for MOSFETs for the high sensitivity mode was 0.7% (1 s.d.) as compared to the 0.05% (1 s.d.) for the studied diodes. The linearity of the response with dose was excellent (R2 = 1.000) for both in vivo dosimetry systems. The absolute values of the studied correction factors for the MOSFETs when covered by the different build-up caps were of the same order of those determined for the diodes. However, the uncertainties of the correction factors for MOSFETs were significantly higher than for diodes. Although the intrinsic precision and the uncertainty on the CF was higher for MOSFET detectors than for the studied diodes, the total uncertainty in entrance dose determination, once they were calibrated, was of 2.9% (1 s.d.) while for diodes it was 2.0% (1 s.d.). MOSFETs showed no sensitivity variation with accumulated dose or temperature. When used in the high sensitivity mode, after approximately 50 Gy of accumulated dose MOSFETs could no longer be used as radiation dosimeters. In conclusion, MOSFETs can be used for entrance in vivo dosimetry in high energy x-rays beams if covered by an appropriate build-up cap. Metallic build-up caps, such as those used for in vivo diodes, have the advantage of greater patient comfort and less perturbation of the treatment field than the other build-up caps tested, while keeping the correction factors of the same order.

Assessment of cumulated genetic alterations in colorectal cancer

Widespread genetic alterations are a common feature of most colorectal cancers. While specific recurrent alterations may reveal the involvement of a gene or set of genes in the biology of the disease, the cumulated genomic damage is likely to reflect the biological history of the neoplastic cells. Furthermore, the functional implications behind many of these genetic changes may show the evolutionary potential of the neoplastic cells. Different approaches, ranging from the gross determination of total nuclear DNA content to cytogenetic and molecular approaches, reveal different types of chromosomal and subchromosomal alterations and have been applied to measure generalized genomic damage in colorectal carcinomas. High levels of genomic damage usually appear associated with increased aggressiveness in colorectal cancer, and the use of different assessments of genomic damage as independent prognostic factors has been proposed. Therefore, appropriate definition of the extent of cumulated alterations and their functional consequences may be of interest in the understanding and management of cancer. The different methodologies and clues to the interpretation and integration of the results obtained with each technique are discussed in this review.

Radiation-induced chromosome breaks in ataxia-telangiectasia cells remain open

PURPOSE

To determine if broken chromosome-end healing mechanisms through the addition of new telomeric sequences exist in cells having difficulties in rejoining the ends of broken chromosomes.

MATERIALS AND METHODS

A full-colour painting protocol of all human chromosomes by FISH was combined with a telomeric and centromeric labelling using PNA probes to characterize the rejoining pattern and telomere status of radiation-induced chromosome breaks in ataxia-telangiectasia (A-T) and normal lymphoblastoid cell lines.

RESULTS

It was first established that the cell lines used for chromosome healing analysis were chromosomally stable. FISH analysis provided evidence that the frequency of deleted chromosomes, apparently unrejoined, was much higher in A-T than in normal cells, as expected by the role of ATM in cell-cycle control, as well as in DNA repair. In spite of their high frequency, broken chromosome ends in A-T cells do not seem to act as substrates for telomerase since additional terminal telomere sequences (more than the 92 expected pairs) indicative of chromosome healing were never observed. Broken chromosome ends in A-T cells remained open.

CONCLUSION

The disability of cells to rejoin broken chromosome ends does not lead to the healing of DSBs by the acquisition of new telomeric sequences.

Suitability of FISH painting techniques for the detection of partial-body irradiations for biological dosimetry

Peripheral blood was irradiated with 2, 3, 4 or 5 Gy of X rays and was mixed with nonirradiated blood at five different dilutions to simulate partial-body irradiations. Analysis by FISH was performed using whole-chromosome painting probes for chromosomes 1, 4 and 11 in combination with a pancentromeric probe. Chromosome aberrations affecting the painted fraction were classified according to the PAINT nomenclature; other unstable aberrations affecting the unpainted material were also recorded. To evaluate the suitability of painting for dose assessment in partial-body irradiations, the ability of the u test and a proposed s test to detect the expected overdispersion and the similarity between the real doses and the doses estimated using Dolphin's approach were considered. For short-term biodosimetry, compared with solid-stained dicentric analyses, the suitability of FISH painting techniques for the detection of partial-body exposures is reduced, because of the decrease in the frequency of aberrations detected by FISH and in the number of cells with two or more aberrations. For reconstruction of past doses, when only complete apparently simple translocations in cells free of unstable aberrations were considered, the detection of the overdispersion and the accuracy of dose estimations were dramatically reduced. In a partial-body exposure, as the original dose increased, the whole-body dose estimated a long time after irradiation would tend to be lower, and the difference from the original dose would tend to be greater.

Polymorphisms in human apolipoprotein(a) kringle IV-10 and coronary artery disease: relationship to allele size, plasma lipoprotein(a) concentration, and lysine binding site activity

Elevated plasma levels of lipoprotein(a) [Lp(a)] represent a major independent risk factor for the development of atherosclerosis. The kringle IV type 10 of apolipoprotein(a) [apo(a)] is the primary lysine binding site (LBS) of Lp(a) and is associated with lesion formation in transgenic mice. The purpose of this study was to search for mutations in the apo(a) kringle IV type 10 which could alter the LBS activity of Lp(a) from patients with coronary artery disease. We found the DNA region of kringle IV type 10 of apo(a) to be mutable but relatively well preserved in the Spanish population. We identified a novel mutation which probably leads to a truncated form of apo(a) in a patient heterozygous for the mutation and with low lysine binding activity and low plasma Lp(a) concentration. Two other mutations have been previously identified in humans, the substitutions W81R and M75T. The W81R was not found in our sample, but the M75T mutation was present in 43% of patients with coronary artery disease and 23% of age-matched controls. The genotype TT conferred a significant risk for myocardial infarction (odds ratio 2.53). This association was not due to linkage disequilibrium with kringle IV repeats. The M75T polymorphism was not associated with the LBS function of apo(a), but it influenced plasma Lp(a) concentration.

Impact of apolipoprotein(a) isoform size heterogeneity on the lysine binding function of lipoprotein(a) in early onset coronary artery disease

Elevated plasma Lp(a) is an independent risk factor for cardiovascular disease. Unique to Lp(a) is the apoprotein, apo(a) which can vary from 250 to 800 kDa in molecular weight. Small isoforms are also associated with the risk of cardiovascular disease. The purpose of this study was to examine the association of Lp(a) concentration, apo(a) size, and Lp(a) lysine-binding site(s) (LBS) function in patients with early onset heart disease, and age-matched controls. Mean values of Lp(a) were significantly higher in the patients than for the age-matched group. The smallest molecular weight isoform for each subject had significantly fewer kringles for the patients than the age-matched controls. There was a significant correlation between LBS activity and kringle number in the single-banded phenotypes of the patients, but not the controls. LBS activity was significantly higher in patients with small isoforms (< or =18 kringles) compared to controls. The odds ratio for coronary artery disease for high LBS activity and high Lp(a) concentration was 4.4 (p = 0.002) and for high LBS activity and small isoforms was 10.1 (p = 0.002). In the patients, Lp(a) concentration was higher, apo(a) size was smaller, and LBS activity higher in the small isoforms compared to the controls. This study suggests an association of high LBS activity in small isoforms of Lp(a) with disease in humans.

Cytogenetic analyses by fluorescence in situ hybridization (FISH) in hospital workers occupationally exposed to low levels of ionizing radiation

An occupationally exposed population has been studied to evaluate the suitability of FISH painting techniques to detect chronic exposures to very low doses of ionizing radiation by the analysis of translocations. Whole-chromosome painting probes for chromosomes 1, 4 and 11 in combination with a pancentromeric probe have been employed. For comparison, a matched control population has also been studied. The mean genomic frequencies per 100 cells of total translocations in the control and exposed populations were 0.90 +/- 0.12 and 1.04 +/- 0.11, respectively. In the occupationally exposed population, no correlation between the frequencies of translocations and the doses received was found. When the two populations were compared, no significant differences were observed for the frequencies of the different chromosomal abnormalities examined. The absence of differences between control and exposed populations could be attributed to the very low-dose exposures recorded in the occupationally exposed population and to the wide range of individual frequencies of translocations observed.