The effects of fractionated gamma irradiation on induction of mammary carcinoma in normal and estrogen-treated rats.

The effects of dose fractionation on induction of mammary carcinoma were studied in normal and estrogen-treated female rats of the inbred WAG/Rij strain. Groups of 40 animals received total-body doses of 1 or 2 Gy of (137)Cs gamma radiation, administered in fractions of 2.5, 10 or 40 mGy with intervals of 12 h, or in fractions of 10 mGy with intervals of 2, 5 or 24 h. The irradiations were started at the age of 8 weeks. Estrogen treatment was accomplished by implantation of a pellet containing estrogen at the age of 6 weeks. All mammary tumors were resected and classified histologically as carcinoma or fibroadenoma. The age-specific incidence of mammary carcinoma was compared with that in control groups of unirradiated normal or estrogen-treated rats and was expressed as excess normalized risk, using lifetime statistical analysis with both parametric and nonparametric methods. The data were also compared to the results of single-dose experiments reported in previous papers. Fractionated irradiation increased the risk of mammary cancer in both normal and estrogen-treated rats compared to the corresponding unirradiated control group. The excess normalized risk per unit of total dose was approximately equal with or without estrogen treatment. Without estrogen treatment, the effects of the single-dose and fractionated irradiations were approximately equal. In estrogen-treated animals, however, single-dose irradiation was up to 15 times more carcinogenic than the fractionated exposures. This fractionation effect appeared to vanish for total doses below approximately 0.3 Gy. With estrogen treatment, the excess normalized risk was significantly higher for dose fractions of 40 mGy than for fractions of 10 mGy. The risk was also markedly higher for fractionation intervals of 2 or 5 h than for intervals of 12 or 24 h. The results of these experiments show that the effects of dose fractionation on the induction of mammary carcinoma may depend on hormonal status, the total dose delivered, the dose per fraction, and the fractionation interval.

Induction of mammary tumors in rats by single-dose gamma irradiation at different ages.

The effect of age at exposure on induction of mammary tumors was studied in female rats of the inbred WAG/Rij strain. Groups of 40 animals were exposed to a single total-body dose of 1 or 2 Gy of 137Cs gamma radiation at ages of 8, 12, 16, 22, 36 or 64 weeks and were observed for life. Mammary tumors, identified as nodules persisting and growing for 6 weeks, were resected and classified histologically as carcinoma or fibroadenoma. The age-specific incidence of mammary carcinoma was compared with that in a group of 120 unirradiated control rats, using lifetime statistical analysis with both parametric and nonparametric methods. The excess normalized risk of carcinoma was 0.9 for 1 Gy and 2.2 for 2 Gy in age groups 8-36 weeks, with no significant differences between the age groups. However, irradiation at 64 weeks yielded fewer carcinomas than in the controls, the excess normalized risk being -0.7 and -0.3 for 1 and 2 Gy, respectively. The occurrence of one or more fibroadenomas did not influence the incidence of carcinoma. The present data agree closely with the results reported previously for rats irradiated at age 8 or 17 weeks with a dose of 1.2 Gy. The reduced risk of radiation exposure at midlife is consistent with the available epidemiological data for exposed women. Although our findings have been obtained with a single total-body dose that is several orders of magnitude higher than the multiple doses delivered to the mammary gland during mammography, it is suggested that radiological screening for mammary cancer after the age of menopause will not increase the normal incidence of breast cancer.

The influence of estrogen treatment on induction of mammary carcinoma in rats by single-dose gamma irradiation at different ages.

The effect of age at exposure on induction of mammary carcinoma was studied in female rats of the inbred WAG/Rij strain that were treated with estrogen. Groups of 40 animals were exposed to a single total-body dose of 1 or 2 Gy of 137Cs gamma radiation at age 8, 10, 12, 15, 22, 36 or 64 weeks. Hormone levels in the animals were increased by implantation of a pellet containing Estradiol-17beta 2 weeks prior to irradiation. Animals were killed when moribund. All mammary tumors were resected and classified histologically as carcinoma or fibroadenoma. The age-specific incidence of mammary carcinoma was compared with that in control groups of unirradiated estrogen-treated rats using lifetime statistical analysis with both parametric and nonparametric methods. The excess normalized risk of carcinoma was 7.7 for both 1 and 2 Gy in the age groups 8-15 weeks, with no significant differences between the age groups. However, in the age groups 22-64 weeks, the excess normalized risk decreased with increasing age at exposure. Irradiation at 64 weeks yielded fewer carcinomas than in the controls, with an excess normalized risk of -0.6 for both 1 and 2 Gy. The excess normalized risk was 10-80 in estrogen-treated controls compared to untreated rats. The present data agree with the results reported previously for estrogen-treated rats irradiated at ages 8 or 17 weeks with doses of 0.3 or 1.2 Gy. The reduced risk of radiation exposure at midlife observed in this study in hormone-treated rats has also been reported for animals not treated with estrogens. The present findings support the earlier conclusion that radiological screening for mammary cancer after the age of menopause will not increase the normal incidence of breast cancer. Estrogen treatment at midlife may increase the risk of breast cancer in women using replacement estrogens during and after menopause.

Detection of total- and partial-body irradiation in a monkey model: a comparative study of chromosomal aberration, micronucleus and premature chromosome condensation assays.

PURPOSE: To investigate the efficacy of three cytogenetic methods (dicentrics, micronuclei (MN) and premature chromosome condensation (PCC) analysis) for assessment of the unirradiated fraction and the persistence of damage after total-body (TB) and partial-body (PB) irradiation of rhesus monkeys (Macaca mulatta). MATERIALS AND METHODS: Animals were exposed to X-rays (5 Gy), either TB or PB, with about 6% of marrow cells shielded. Blood samples were collected at different times after exposure, i.e. 1, 3 and 7 days, and cultures were set up for the different cytogenetic endpoints. In addition, blood count analysis was performed before and after irradiation. RESULTS: Blood count analysis was not suitable for discriminating between TB and PB exposure. By using Poisson or overdispersion distribution as the basis, it was not possible to distinguish TB from PB irradiation when dicentric chromosomes and MN were analysed. PCC analysis, in contrast, showed a Poisson distribution after TB exposure and overdispersion after PB exposure. Using the PCC assay, reliable dose estimates could be obtained up to 7 days after irradiation. CONCLUSIONS: For dicentrics and MN, shielding of 6% of bone marrow cells was found to be too small to estimate the unirradiated fraction accurately. The PCC technique was useful for dose assessment and the inhomogeneous exposure of 6% was detected within a short period of time after exposure.

Influence of the H-ras oncogene on radiation responses of a rat rhabdomyosarcoma cell line.

Rat R2k rhabdomyosarcoma cells were transfected with the human H-ras oncogene, which resulted in increased resistance to cell kill in vitro by a single dose of 137Cs gamma-rays. A subline carrying one oncogene showed an increase in the quasi-threshold dose (Dq) from 0.88 to 1.48 Gy. Another subline containing six oncogenes not only had an increased Dq of 1.59 Gy but also showed an increase in the dose reducing cell survival to a fraction of e-1 = 0.37 (D0) from 1.25 to 1.76 Gy. Analysis of the cell survival data according to the linear-quadratic formalism indicated that a decrease in the value of the coefficient of the linear component alpha is associated with a H-ras-mediated increase in radioresistance. In fractionated irradiation experiments it was observed that with a dose of 1 Gy/fraction a 1.8 times higher dose for an isoeffect of 10% cell survival (D10) was needed for a subline with one H-ras oncogene, while with fraction doses of 2 or 4 Gy only a 1.2 times higher D10 was found. This indicates a more efficient repair of radiation-induced damage in the transfected subline. Tumors arising in the rat gastrocnemius muscle inoculated with cultured cells were irradiated with different doses of 300-kV X-rays. A single dose of 45 Gy was found to result in a 6% cure rate for the subline containing one H-ras oncogene and a 32% for the parental line. When a priming dose of 45 Gy was followed by fractionated irradiation with 1 Gy/fraction, an extra dose of 51 Gy would be needed to obtain a 75% cure rate for the transfected subline. An extra dose of only 10 Gy would be needed for the parental line. The percentage cure per unit of dose for the parental line irradiated with 1 Gy/fraction was estimated to be 4.3%.Gy-1, whereas for the transfected tumor line it was 1.4%.Gy-1. This means that a 3.0 times higher cumulated absorbed dose would be needed for enhancing the cure rate from 32% to 75% in the subline with H-ras than for the parental line. With 2 Gy/fraction the difference in extra doses required for obtaining isolevels of cure rates was found to be small, a factor of 1.4. The results indicate that in the course of fractionated irradiation with 1 Gy/fraction, in vivo repair is much more efficient in the transfected subline.

The 5T mouse multiple myeloma model: absence of c-myc oncogene rearrangement in early transplant generations.

Consistent chromosomal translocations involving the c-myc cellular oncogene and one of the three immunoglobin loci are typical for human Burkitt's lymphoma, induced mouse plasmacytoma (MPC) and spontaneously arising rat immunocytoma (RIC). Another plasma cell malignancy, multiple myeloma (MM), arising spontaneously in the ageing C57BL/KaLwRij mice, was investigated in order to see whether the MM cells contain c-myc abnormalities of the MPC or RIC type. Rearrangement of the c-myc oncogene was found in the bone marrow cells only in 5T2 MM transplantation line in a mouse of the 24th generation and in none of the seven other MM of the 5T series which were of earlier generations. Since the mouse 5T MM resembles the human MM very closely, including the absence of consistent structural c-myc oncogene abnormalities, it can serve as a useful experimental model for studies on the aetiopathogenesis of this disease.

Hormone independent in vitro erythroid colony formation by mouse bone marrow cells.

B.M. cells of RLV-infected BALB/c mice can proliferate in methylcellulose in the absence of E.P., while normal B.M. cells cannot (12). Not only the more primitive BFU-E shows hormone-independency (18). This phenomenon is in favour of the view that the Rauscher virus induced erythroblastosis is a true neoplasia although transplantation experiments failed so far. The experiments in which transformation in vitro of B.M. cells by RLV is established (19) show that the CFU-E can serve as a target for the virus. Treatment of normal mice with CFA leads to a rapid increase in CFU-E in the bone marrow (18). Splenomegaly of RLV-infected mice is enhanced by CFA-treatment probably due to an increase in targets. Transfection with proviral DNA also can transform the CFU-E of BALB-c mice. This approach allows in vitro studies on the resistence of mouse strains to RLV in vitro. The studies are of interest for the human disease in two aspects. In vitro transformation assays are needed to study the oncogenic potential of putative human leukemia viruses. Furthermore the studies have yielded some new insight in the pathogenesis of virally induced erythroblastosis. This might serve as a model for e.g. acute myeloid leukemia in man.