Lung tumour induction in mice after uniform and non-uniform external thoracic X-irradiation

The aim of this study was to investigate the validity of the ICRP procedure of using average tissue/organ dose in estimating carcinogenic risk. It has been suggested that highly non-uniform exposure ('hot spots') is much more carcinogenic than an equivalent dose delivered uniformly. In a series of experiments, mice were irradiated with X-rays either uniformly to the thorax or non-uniformly with 72 1-mm microbeams which irradiated approximately 20 per cent of the total lung volume. Two experiments involving uniform irradiation showed a peaked tumour incidence curve with a maximum at 5 Gy. The first 'microbeam' study also produced a pronounced peak in the dose response with a maximum tumour incidence at 1 Gy average lung dose or 5 Gy to the irradiated lung tissue. This implied the use of average tissue dose might underestimate the carcinogenic hazard of non-uniform exposure. Later, more extensive, microbeam experiments failed to replicate this finding. The results were nearly similar to those for uniform irradiation, with a slight increase in tumour incidence from 2.5-5.0 Gy average lung dose. These results imply that for these irradiation conditions the ICRP dose averaging procedure remains valid.     

Cytogenetic effects of X-rays and fission neutrons in female mice

The induction by X-rays of chromosomal damage in oocytes was studied, while the genetic consequences of X- and neutron-induced damage in female mice were looked for by testing offspring for dominant lethality and semi-sterility. None out of 386 sons of hybrid females given 300 rad X-rays showed evidence of semi-sterility or translocation heterozygosity, but 9 out of 294 daughters were diagnosed as semi-sterile. At least 3 and probably 4 of these (1.4%) carried reciprocal translocations, 2 of which caused male sterility. Complete or partial loss of the X-chromosome may have been responsible for some of the other sermi-steriles. Examination of oocytes at metaphase-I during the first and third weeks after X-irradiation with 100 or 400 rad revealed both multivalents (some of the ring quadrivalent type) and fragments (mainly double). These were thought to arise mainly from chromatid intercchanges (both symmetrical and asymmetrical) and isochromatid intrachanges respectively. Since neither the proportion of asymmetrical interchanges nor the amount of hidden damage was known it was not thought possible to predict the magnitude of F₁ effects from metaphase-I findings. The aberration frequency in oocytes rose with dose and (at the 400 rad level only) with time after irradiation, reaching a maximum of 10% multivalents and 22% fragments in the third week after 400 rad. The frequency of univalents showed no consistent trend, but chiasma counts decreased in the first week after 400 rad. The increase in levels of chromosomal damage with dose and time after irradiation was reflected in dominant lethal frequencies after the same radiation-conception intervals and doses of 0–400 rad. Induced post-implantation lethality was over twice as high in the third week after 200–400 rad than in the first. Pre-implantation loss also greatly increased in the third week after 300 or 400 rad; this was associated with increased non-fertilization of ova. No evidence for the induction of translocations in oogonia or resting oocytes by fast neutron irradiation was obtained, although there was evidence for X-chromosomal loss after 200 rad to oocytes. The relative biological effectiveness (RBE) for fission neutrons vs. X-rays with respect to dominant lethal induction in oocytes was found to vary with dose, but seamed to be around 1 at lower levels.     

Relative effectiveness of neutrons and X-rays in induction of crossing over in drosophila males

Relative biological effectiveness of neutrons vs. X-rays in inducing crossing-over in males of D. melanogaster was investigated using 812 and 834 rad of neutrons and the same dose of X-rays. Crossing-over was induced in spermatocytes and spermatogonia of adults and pupae. Neutrons were 4 times more effective in spermatocytes of adults and their effectiveness in pupal spermatocytes was even more. Neutrons also induced more exchanges in spermatogonial cells including predefinitive spermatogonia. Higher effectiveness of neutrons can be attributed to their high linear energy transfer.     

Translocations induced by fast neutrons and X-rays in Delia antiqua

Summary A comparison was made using X-rays and fast neutrons for the induction of translocations in Delia antiqua. Using the same radiation dose, no difference in efficiency between the two radiation types could be observed. However, with fast neutrons many multiple translocations were induced, including a quadruple translocation involving 4 out of 5 autosomes. One male linked translocation was also induced.The reciprocal translocations were assigned into two classes: symmetrical and asymmetrical, and ten of the latter were chosen for inbreeding to produce homozygotes. Asymmetrical exchanges were chosen so that translocation homozygotes could be differentiated cytologically from the normal karyotype. In seven different translocations, homozygous larvae were observed, but often at a low frequency. In four of these lines, viable adult homozygotes were observed. Subsequent random sib-crossing failed to produce a homozygous line.     

Murine spermatogonial regeneration after exposure to either X-rays or 15 MeV neutrons*

Summary The long-term regeneration of the semeniferous epithelium after irradiation with 15 MeV neutrons was studied in the mouse on a comparative basis; 150 kV X-rays were used as reference-radiation. The mice received total-body exposures at matched doses. The spermatogonial Regeneration Index (RI) was scored from stained paraffin-sections of testes which were obtained after periodic sacrifice. A biphasic pattern of regeneration was recorded. Dose-independent and dose-dependent effects have been indicated. The radiobiological implications are discussed on a cellular basis.     

Arterial wall damage by X-rays and fast neutrons.

Radiation-induced atheromatosis has been studied for 200 kVp X-rays and 15 MeV neutrons. From the results of two earlier experiments, a r.b.e. less than 1 was expected for neutrons. (1) Irradiation of blood-vessels causes depolymerization of the mucopolysaccharides in the vessel wall, resulting in atheromatosis, and (2) in two other mucopolysaccharide systems a r.b.e. less than 1 is observed for fast neutrons. Irradiation of the carotid arteries of a total number of 120 hypercholesterolaemic rabbits, divided over several groups, with 500 and 1000 rad of X-rays or neutrons results, however, in atheromatous plaques which are more pronounced for neutrons than for X-rays at the 500 rad dose-level; for a dose of 1000 rad the effect of neutrons is less extensive than the effect of X-rays. These results lead to the assumption that the origin of the atheromatosis seems not only to be the mucopolysaccharide depolymerization, but that radiation induced damage at the cellular level must be taken into account.     

Comparative action of X-rays and fast neutrons on thymus endocrine function

Whole-body exposure of rats to X-rays (4, 6 and 8 Gy) and neutron radiation (1, 1.5 and 2 Gy) causes a dose-dependent inhibition of endocrine function of thymus and death of its lymphoid cells. Shielding of thymus somewhat reduces the degree and duration of inhibition of thymus hormone secretion.     

The interaction between X-rays and 3 MeV neutrons in the skin of the mouse foot

Mouse feet were irradiated with mixtures of 3 MeV neutrons and 140 kVp X-rays given simultaneously or within 24 hours of each other. The effects of different treatments were contrasted by comparing the doses required to give equal skin reactions. Irradiation was given as 1, 2, 4 or 8 equal fractions, in order to assess r.b.e. and the shapes of the underlying dose-response curves for mixed beams over a wide range of dose per fraction. All dose-effect curves were well fitted by a linear-quadratic (alpha, beta) model. For X-rays and neutrons given simultaneously, the linear coefficient (alpha) decreased by a factor of 4.80 while the quadratic coefficient (beta) increased by a factor of only 1.44 when the proton contamination in the beam increased from 11 to 100 per cent, with alpha/beta changing from 95.0 to 13.8. The data from simultaneous X-ray and neutron irradiation were consistent with full interaction of those effects from the two radiations which give rise to the total quadratic component of effect. When the two radiations are separated by up to 24 h, this interaction decreases but does not entirely disappear.     

Genetic background damage accompanying reciprocal translocations induced by X-rays and fission neutrons in Arabidopsis and Secale

Translocations were induced in Arabidopsis and rye by fission neutrons and X-rays using doses with equal effects. Segregations of these translocations were studied in M₃, M₄ and S₁ of backcrosses. When there was a deficit of non-translocation homozygotes it was concluded that genetic damage had occurred in chromosomes homologous with the translocation chromosomes. The frequency of families with shortage (including absence) of translocation homozygotes was much larger than families with background damage. This demonstrates that reduced viability in the former was due to true breakpoint damage and not to linked damage. The frequency of translocation breakpoint damage was the same for X-rays and fission neutrons, but the latter probably induced more serious damage. Background damage was also the same for the two types of radiation. Therefore, neutrons should not be considered a ‘cleaner’ inducer of translocation than X-rays.     

The effect of alterations in haematocrit on tumour sensitivity to X-rays

Hypoxic cells in human tumours probably contribute to the failure of radiotherapy in some sites. Changes in the oxygen carrying capacity of the blood, such as in anaemia, have been shown to influence tumour response. The effect of acute and chronic changes in haematocrit on the radiosensitivity of three mouse tumours (EMT6, KHT and RIF-1) were studied. Alterations in haematocrit were achieved by bleeding followed by retransfusion. When radiation was preceded immediately by an acute reduction in haematocrit (anaemia), radiosensitivity was markedly reduced in each tumour. An acute rise in haematocrit (polycythaemia) increased or decreased X-ray sensitivity depending on its severity. The optimum haematocrit for maximum sensitivity was always found to be at a level 5-10 per cent above normal. When the time between induction of anaemia and irradiation was increased, simulating a progressively longer duration of anaemia, marked changes in radiosensitivity of all the tumours were observed. A short duration of anaemia resulted in a resistant tumour with each cell line, but the resistance was gradually lost as the anaemia was prolonged, even though no recovery in haematocrit occurred. The rate of recovery to normal radiosensitivity varied from 24 to 72 hours in the different tumours. Therefore, only haematocrit changes which occurred within 1-3 days of a dose of radiation affect the radiosensitivity of these tumours.     

Survival of V79 cells following simultaneous irradiation with X-rays and neutrons in air or hypoxia

V79 Chinese hamster cells have been irradiated with X-rays and neutrons given simultaneously. The oxygen enhancement ratio and r.b.e. were measured as a function of the proportion of the dose due to the neutrons, which varied from 0 to 100 per cent. These were compared with the values calculated assuming the two types of radiation act independently, following an approach suggested by Curtis. The o.e.r. was less than the predicted value when the neutrons contributed less than about 40 per cent of the total dose. The r.b.e. also did not vary as predicted on the basis of independent action. The 'oxygen gain factor' reached half its maximum value when the proportion of the dose due to neutrons was only about 27 per cent. The results imply that there may be interaction between the damage caused by X-rays and neutrons and that beams having only 20 to 30 per cent of their dose due to high l.e.t. radiation, could be of therapeutic benefit.     

Immune response in mice exposed to low-dose X-rays

The present paper was conducted to evaluate the immunological effect of low dose gamma-irradiation. The splenocytes of mice (C57BL/6N), 24 hours after the irradiation from 0.087 to 0.87 Gy, were incubated with mitogens of T or B lymphocytes, allo-antigen (splenic cells of BALB/c mice) (MLC) or sheep red blood cells (SRBC) 10 days after immunization with the SRBC in vivo, and then their incorporations of 3H-thymidine were measured. On the other hand, this incorporation in the presence of T-cell growth factor (IL-2) in vitro and a drug of AET in vivo was investigated to examine their radioprotective effects. The dose-response relation, i.e. decrease of 3H-thymidine incorporation in function of increase of the irradiation dose, was demonstrated in these immunological examinations except at the dose of 0.087 Gy. More, their incorporation was remarkably promoted by the administration of the T cell growth factor and the drug, therefore, these substances represent the radioprotective effect.