High Relative Biological Effectiveness of 2 MeV Fast Neutrons for Induction of Medulloblastoma in Ptch1+/- Mice with Radiation-specific Deletion on Chromosome 13

Neutron radiation, a high-linear energy transfer radiation, has a high relative biological effectiveness (RBE) for various end points. The age at exposure is an important modifier of the effects of radiation, including carcinogenesis, with infants being generally more radiosensitive. Ptch1+/- mice offer a unique experimental system for assessing radiation carcinogenesis. Spontaneous development of medulloblastoma tumors occurs in nonirradiated animals that lose their Ptch1+ allele, most frequently by a loss of heterozygosity (LOH) of chromosome 13 via recombination or non-disjunction (referred to as S-type tumors). In contrast, tumors occur in irradiated Ptch1+/- mice as a result of chromosome 13 LOH with an interstitial deletion (R-type), making spontaneous and radiation-induced tumors discernible. To elucidate the influence of age on the effect of fast neutrons, we irradiated Ptch1+/- mice with neutrons (mean energy, ∼2 MeV) or γ rays on embryonic day (E)14 and E17 and on postnatal day (P)1, 4 or 10 and classified the resulting medulloblastomas based on chromosome 13 aberrations. Instead of LOH, some tumors harbored mutations in their Ptch1+ gene via a nonirradiation-associated mechanism such as duplication, insertion, base substitution or deletion with microhomology-mediated end joining; thus, these tumors were classified as S-type. The RBE regarding the induction of R-type tumors was 12.9 (8.6, 17.2), 9.6 (6.9, 12.3), 21.5 (17.2, 25.8), and 7.1 (4.7, 9.5) (mean and 95% confidence interval) for mice irradiated on E14, E17, P1 and P4, respectively, with the highest value seen during the most active development of the tissue and P10 being completely resistant. These results indicate that the developmental stage at exposure of the tissue influences the RBE of neutrons.

Lifetime increased cancer risk in mice following exposure to clinical proton beam-generated neutrons

PURPOSE

To evaluate the life span and risk of cancer following whole-body exposure of mice to neutrons generated by a passively scattered clinical spread-out Bragg peak (SOBP) proton beam.

METHODS AND MATERIALS

Three hundred young adult female FVB/N mice, 152 test and 148 control, were entered into the experiment. Mice were placed in an annular cassette around a cylindrical phantom, which was positioned lateral to the mid-SOBP of a 165-MeV, clinical proton beam. The average distance from the edge of the mid-SOBP to the conscious active mice was 21.5 cm. The phantom was irradiated with once-daily fractions of 25 Gy, 4 days per week, for 6 weeks. The age at death and cause of death (ie, cancer and type vs noncancer causes) were assessed over the life span of the mice.

RESULTS

Exposure of mice to a dose of 600 Gy of proton beam-generated neutrons, reduced the median life span of the mice by 4.2% (Kaplan-Meier cumulative survival, P=.053). The relative risk of death from cancer in neutron exposed versus control mice was 1.40 for cancer of all types (P=.0006) and 1.22 for solid cancers (P=.09). For a typical 60 Gy dose of clinical protons, the observed 22% increased risk of solid cancer would be expected to decrease by a factor of 10.

CONCLUSIONS

Exposure of mice to neutrons generated by a proton dose that exceeds a typical course of radiation therapy by a factor of 10, resulted in a statistically significant increase in the background incidence of leukemia and a marginally significant increase in solid cancer. The results indicate that the risk of out-of-field second solid cancers from SOBP proton-generated neutrons and typical treatment schedules, is 6 to 10 times less than is suggested by current neutron risk estimates.

Radiation-induced mammary carcinogenesis in rodent models: what's different from chemical carcinogenesis?

Ionizing radiation is one of a few well-characterized etiologic factors of human breast cancer. Laboratory rodents serve as useful experimental models for investigating dose responses and mechanisms of cancer development. Using these models, a lot of information has been accumulated about mammary gland cancer, which can be induced by both chemical carcinogens and radiation. In this review, we first list some experimental rodent models of breast cancer induction. We then focus on several topics that are important in understanding the mechanisms and risk modification of breast cancer development, and compare radiation and chemical carcinogenesis models. We will focus on the pathology and natural history of cancer development in these models, genetic changes observed in induced cancers, indirect effects of carcinogens, and finally risk modification by reproductive factors and age at exposure to the carcinogens. In addition, we summarize the knowledge available on mammary stem/progenitor cells as a potential target of carcinogens. Comparison of chemical and radiation carcinogenesis models on these topics indicates certain similarities, but it also indicates clear differences in several important aspects, such as genetic alterations of induced cancers and modification of susceptibility by age and reproductive factors. Identification of the target cell type and relevant translational research for human risk management may be among the important issues that are addressed by radiation carcinogenesis models.JRRS Incentive Award in 2009.

Radiation effectiveness factors for use in calculating probability of causation of radiogenic cancers

This paper presents so-called radiation effectiveness factors that are intended to represent the biological effectiveness of different radiation types, relative to high-energy Co gamma rays, for the purpose of estimating cancer risks and probability of causation of radiogenic cancers in identified individuals. Radiation effectiveness factors are expressed as subjective probability distributions to represent uncertainty that arises from uncertainties in estimates of relative biological effectiveness obtained from radiobiological studies of stochastic endpoints, limited data on biological effectiveness obtained from human epidemiological studies, and other judgments involved in evaluating the applicability of available information to induction of cancers in humans. Primarily on the basis of reviews and evaluations of available data by experts, probability distributions of radiation effectiveness factors are developed for the following radiation types: neutrons of energy less than 10 keV, 10-100 keV, 0.1-2 MeV (including fission neutrons), 2-20 MeV, and greater than 20 MeV; alpha particles of any energy emitted by radionuclides; photons of energy 30-250 keV and less than 30 keV; and electrons of energy less than 15 keV. Photons of energy greater than 250 keV and electrons of energy greater than 15 keV are assumed to have the same biological effectiveness as reference Co gamma rays and are assigned a radiation effectiveness factor of unity, without uncertainty. For neutrons and alpha particles, separate probability distributions of radiation effectiveness factors are developed for solid tumors and leukemias, and small corrections to represent an inverse dose-rate effect are applied to those distributions in cases of chronic exposure. A radiation effectiveness factor different from unity for 15-60 keV electrons is discussed but is not adopted due to a lack of relevant radiobiological data. Radiation effectiveness factors presented in this paper are incorporated in the Interactive RadioEpidemiological Program and were developed for use by The National Institute for Occupational Safety and Health and U.S. Department of Labor in evaluating claims for compensation for radiogenic cancers by workers at U.S. Department of Energy facilities.

Relative biological effectiveness (RBE), quality factor (Q), and radiation weighting factor (w(R)). A report of the International Commission on Radiological Protection

The effect of ionising radiation is influenced by the dose, the dose rate, and the quality of the radiation. Before 1990, dose-equivalent quantities were defined in terms of a quality factor, Q(L), that was applied to the absorbed dose at a point in order to take into account the differences in the effects of different types of radiation. In its 1990 recommendations, the ICRP introduced a modified concept. For radiological protection purposes, the absorbed dose is averaged over an organ or tissue, T, and this absorbed dose average is weighted for the radiation quality in terms of the radiation weighting factor, w(R), for the type and energy of radiation incident on the body. The resulting weighted dose is designated as the organ- or tissue-equivalent dose, H(T). The sum of the organ-equivalent doses weighted by the ICRP organ-weighting factors, w(T), is termed the effective dose, E. Measurements can be performed in terms of the operational quantities, ambient dose equivalent, and personal dose equivalent. These quantities continue to be defined in terms of the absorbed dose at the reference point weighted by Q(L). The values for w(R) and Q(L) in the 1990 recommendations were based on a review of the biological and other information available, but the underlying relative biological effectiveness (RBE) values and the choice of w(R) values were not elaborated in detail. Since 1990, there have been substantial developments in biological and dosimetric knowledge that justify a re-appraisal of w(R) values and how they may be derived. This re-appraisal is the principal objective of the present report. The report discusses in some detail the values of RBE with regard to stochastic effects, which are central to the selection of w(R) and Q(L). Those factors and the dose-equivalent quantities are restricted to the dose range of interest to radiation protection, i.e. to the general magnitude of the dose limits. In special circumstances where one deals with higher doses that can cause deterministic effects, the relevant RBE values are applied to obtain a weighted dose. The question of RBE values for deterministic effects and how they should be used is also treated in the report, but it is an issue that will demand further investigations. This report is one of a set of documents being developed by ICRP Committees in order to advise the ICRP on the formulation of its next Recommendations for Radiological Protection. Thus, while the report suggests some future modifications, the w(R) values given in the 1990 recommendations are still valid at this time. The report provides a scientific background and suggests how the ICRP might proceed with the derivation of w(R) values ahead of its forthcoming recommendations.

Long-term effects of irradiation before adulthood on reproductive function in the male rhesus monkey

Today, many patients, who are often young, undergo total body irradiation (TBI) followed by bone marrow transplantation. This procedure can have serious consequences for fertility, but the long-term intratesticular effects of this treatment in primates have not yet been studied. Testes and epididymides of rhesus monkeys that received doses of 4-8.5 Gy of TBI at 2-4 yr of age were studied 3-8 yr after irradiation. In all irradiated monkeys, at least some seminiferous tubule cross-sections lacked germ cells, indicating extensive stem cell killing that was not completely repaired by enhanced stem cell renewal, even after many years. Testes totally devoid of germ cells were only found in monkeys receiving doses of 8 Gy or higher and in both monkeys that received two fractions of 6 Gy each. By correlating the percentage of repopulated tubules (repopulation index) with testicular weight, it could be deduced that considerable numbers of proliferating immature Sertoli cells were killed by the irradiation. Because of their finite period of proliferation, Sertoli cell numbers did not recover, and potential adult testis size decreased from approximately 23 to 13 g. Most testes showed some dilated seminiferous tubules, indicating obstructed flow of the tubular fluid at some time after irradiation. Also, in 8 of the 29 irradiated monkeys, aberrant, densely packed Sertoli cells were found. The irradiation did not induce stable chromosomal translocations in spermatogonial stem cells. No apparent changes were seen in the epididymides of the irradiated monkeys, and the size of the epididymis adjusted itself to the size of the testis. In the irradiated monkeys, testosterone and estradiol levels were normal, whereas FSH levels were higher and inhibin levels lower when testicular weight and spermatogenic repopulation were low. It is concluded that irradiation before adulthood has considerable long-term effects on the testis. Potential testis size is reduced, repopulation of the seminiferous epithelium is generally not complete, and aberrant Sertoli cells and dilated tubules are formed. The latter two phenomena may have further consequences at still longer intervals after irradiation.

The carcinogenic risk of high dose total body irradiation in non-human primates

PURPOSE

High dose total body irradiation (TBI) in combination with chemotherapy, followed by rescue with bone marrow transplantation (BMT), is increasingly used for the treatment of haematological malignancies. With the increasing success of this treatment and its current introduction for treating refractory autoimmune diseases the risk of radiation carcinogenesis is of growing concern. Studies on tumour induction in non-human primates are of relevance in this context since the response of this species to radiation does not differ much from that in man.

MATERIALS AND METHODS

Since the early sixties, studies have been performed on acute effects in Rhesus monkeys and the protective action of bone marrow transplantation after irradiation with X-rays (average total body dose 6.8 Gy) and fission neutrons (average dose 3.4 Gy). Of those monkeys, which were irradiated and reconstituted with autologous bone marrow, 20 animals in the X-irradiated group and nine animals in the neutron group survived more than 3 years. A group of 21 non-irradiated Rhesus monkeys of a comparable age distribution served as controls. All animals were regularly screened for the occurrence of neoplasms. Complete necropsies were performed after natural death or euthanasia.

RESULTS

At post-irradiation intervals of 4-21 years an appreciable number of tumours was observed. In the neutron irradiated group eight out of nine animals died with one or more malignant tumours. In the X-irradiated group this fraction was 10 out of 20. The tumours in the control group, in seven out of the 21 animals, appeared at much older age compared with those in the irradiated cohorts. The histogenesis of the tumours was diverse with a preponderance of renal carcinoma, sarcomas among which osteosarcomas, and malignant glomus tumours in the irradiated groups.

CONCLUSIONS

When corrected for competing risks, the carcinogenic risk of TBI in the Rhesus monkeys is similar to that derived from the studies of the Japanese atomic bomb survivors. The increase of the risk by a factor of 8, observed in the monkeys, indicates that patients are likely to develop malignancies more frequently and much earlier in life after TBI than non-exposed individuals. This finding underlines the necessity of regular screening of long-term surviving patients subjected to TBI and BMT.

Changes in circulating atrial natriuretic peptide in relation to the cardiac status of Rhesus monkeys after total-body irradiation

PURPOSE

In order to determine the presence of cardiac damage associated with total-body irradiation (TBI), both echocardiographic parameters and circulating levels of atrial natriuretic peptide (ANP) were measured in three different age-cohorts of Rhesus monkeys (Macaca mulatta) previously treated with TBI without additional chemotherapy, at post irradiation intervals up to 30 years, at the former TNO/Radiobiological Institute at Rijswijk.

MATERIALS AND METHODS

Standard echocardiographic techniques were used to measure cardiac dimensions and left ventricular function in situ. Plasma-ANP concentration was measured by radioimmunoassay (RIA). After necropsy, tissue samples from the heart were taken for histological analysis.

RESULTS

Plasma-ANP levels of animals which received TBI were significantly (P = 0.0005) elevated when compared to age-matched controls (66.4 +/- 8.4 vs. 33.1 +/- 5.7 ng/l). Moreover, a positive correlation (P = 0.032) between plasma-ANP values and time post treatment was found in the TBI group. TBI affected cardiac dimensions; however, no significant differences in cardiac functional parameters were observed between the different treatment groups. Necropsy reports demonstrated slight but consistent cardiovascular damage in several animals treated with TBI, in terms of increased incidence of mild epicardial and coronary arterial wall fibrosis, compared to age-matched controls.

CONCLUSIONS

The concentration of plasma-ANP proved to be an important parameter for subclinical cardiac damage. In humans, serial determinations of plasma ANP in individual patients might provide relevant information about the cardiac status after TBI.

Effects of total-body irradiation on growth, thyroid and pituitary gland in rhesus monkeys

PURPOSE

To investigate the effect of total-body irradiation (TBI) on growth, thyroid and pituitary gland in primates.

METHODS AND MATERIALS

Thirty-seven rhesus monkeys (mean age 3.1+/-0.6 years) received either a low-dose (4-6 Gy) TBI (n = 26) or high-dose (7-12 Gy) TBI (n = 11) and were sacrificed together with 8 age-matched controls after a post-irradiation interval of 5.9+/-1.5 years. Anthropometric data were collected: thyroid and pituitary glands were examined; serum levels of thyroid stimulating hormone (TSH), free thyroxin (FT4), insulin-like growth factor-I (IGF-I) and its binding protein-3 (IGFBP-3) were measured.

RESULTS

Decrease in final height due to irradiation could not be demonstrated. There was a dose-dependent decrease in body weight, ponderal index, skinfold thickness and thyroid weight. The latter was not accompanied by elevation of TSH or decrease in FT4. Structural changes in the thyroid gland were found in 50% of the irradiated animals. Levels of IGF-I and IGFBP-3 did not differ between the dose groups, but the high-dose group had a lower IGF-1/IGFBP-3 ratio.

CONCLUSION

Total body irradiation had a negative effect on body fat. There was no evidence of (compensated) hypothyroidism, but dose-dependent decrease in thyroid weight and changes in follicular structure suggest some effect of TBI on the thyroid gland. The decreased IGF-I/IGFBP-3 ratio in the high-dose group can indicate that the somatotrophic axis was mildly affected by TBI. These results show that TBI can have an effect on the physical build and thyroid gland of primates even in the absence of cytostatic agents or immunosuppressive drugs.

Dosimetry for total body irradiation of rhesus monkeys with 300 kV X-rays

PURPOSE

To obtain more accurate information on the dose distribution in rhesus monkeys for total body irradiation with orthovoltage X-rays.

MATERIALS AND METHODS

Dose measurements were performed with an ionization chamber inside homogeneous cylindrical and rectangular phantoms of various dimensions and in phantoms containing lung-equivalent material. The irradiations were carried out with reference to a monitor ionization chamber placed alongside the phantom or the irradiation cage.

RESULTS

Correction factors for mass and lung dose relative to the average dose in a homogeneous reference phantom, showed linear relationships with the effective diameter of the monkey. The lung dose correction factor relative to the homogeneous phantom was about 1.12 for a 3.5 kg monkey. The stated values for the average absorbed dose in the animal of standard weight should be multiplied by a factor of 0.93 for experiments performed before 1983. All publications on total body irradiations of monkeys at TNO after 1983 contain the corrected dose values.

CONCLUSION

Dose distributions are reported for phantoms of different diameters and of cylindrical or rectangular shape. The new dosimetry has also resulted in a revised statement of the LD50 for the occurrence of bone marrow syndrome after X-irradiation; 4.9 Gy instead of 5.3 Gy.

Probable risk of tumour induction after retro-orbital irradiation for Graves' ophthalmopathy

Retrobulbar irradiation for Graves' ophthalmopathy is considered as a safe treatment and has recently been recommended as the initial treatment for patients with moderately severe eye problems. However, calculations using risk factors presently known reveal a theoretical risk of radiation-induced cancer of 1.2%. Therefore, the authors suggest that this treatment should be reserved for the elderly patient, for example above the age of 40-50 years.

Long-term consequences of high-dose total-body irradiation on hepatic and renal function in primates

Radiation effects in non-human primates were studied in order to define the long-term risk of total-body irradiation (TBI) for bone marrow transplantation patients. The long-term effects of TBI could be investigated by keeping 84 monkeys of different ages, from an experiment on acute effects, under continuous observation for a period up to 25 years. The control group consisted of non-irradiated monkeys with a comparable age distribution and identical housing conditions. Since radiation was the common toxic agent, the different age groups provided the possibility to investigate the occurrence of deterministic effects after TBI. In the present study emphasis was placed on the assessment of hepatic and renal function and the associated histopathology. The values of the liver function parameters, such as alkaline phosphatase and gamma glutamyl transferase in the irradiated group were significantly increased after TBI (p < 0.05). Also the parameters of kidney dysfunction, e.g. haematocrit and blood urea nitrogen showed a significant change in the irradiated old-aged (post-irradiated interval > 15 years) cohort (p < 0.005). The impairment of the liver and renal functions, did not lead to clinical symptoms and were only associated with mild morphologic changes in the irradiated group of monkeys. In the population of bone marrow transplant patients treated with TBI, alterations in hepatic and renal function parameters after a post-irradiated interval of > 10 years can be anticipated. This could have consequences for the tolerance and toxicity of a broad range of drugs to be administered as additional medications.