John F. Dicello

In vivo mammary tumourigenesis in the Sprague?Dawley rat and microdosimetric correlates

Standard methods for risk assessments resulting from human exposures to mixed radiation fields in Space consisting of different particle types and energies rely upon quality factors. These are generally defined as a function of linear energy transfer (LET) and are assumed to be proportional to the risk. In this approach, it is further assumed that the risks for single exposures from each of the radiation types add linearly. Although risks of cancer from acute exposures to photon radiations have been measured in humans, quality factors for protons and ions of heavier atomic mass are generally inferred from animal and/or cellular data. Because only a small amount of data exists for such particles, this group has been examining tumourigenesis initiated by energetic protons and iron ions. In this study, 741 female Sprague-Dawley rats were irradiated or sham irradiated at approximately 60 days of age with 250 MeV protons, 1 GeV/nucleon iron ions or both protons and iron ions. The results suggest that the risk of mammary tumours in the rats sequentially irradiated with 1 GeV/nucleon 56Fe ions and 250 MeV protons is less than additive. These data in conjunction with earlier results further suggest that risk assessments in terms of only mean LETs of the primary cosmic rays may be insufficient to accurately evaluate the relative risks of each type of particle in a radiation field of mixed radiation qualities.

Analysis of MIR-18 results for physical and biological dosimetry: radiation shielding effectiveness in LEO

We compare models of radiation transport and biological response to physical and biological dosimetry results from astronauts on the Mir space station. Transport models are shown to be in good agreement with physical measurements and indicate that the ratio of equivalent dose from the Galactic Cosmic Rays (GCR) to protons is about 3/2:1 and that this ratio will increase for exposures to internal organs. Two biological response models are used to compare to the Mir biodosimetry for chromosome aberration in lymphocyte cells; a track-structure model and the linear-quadratic model with linear energy transfer (LET) dependent weighting coefficients. These models are fit to in vitro data for aberration formation in human lymphocytes by photons and charged particles. Both models are found to be in reasonable agreement with data for aberrations in lymphocytes of Mir crew members: however there are differences between the use of LET dependent weighting factors and track structure models for assigning radiation quality factors. The major difference in the models is the increased effectiveness predicted by the track model for low charge and energy ions with LET near 10 keV/micrometers. The results of our calculations indicate that aluminum shielding, although providing important mitigation of the effects of trapped radiation, provides no protective effect from the galactic cosmic rays (GCR) in low-earth orbit (LEO) using either equivalent dose or the number of chromosome aberrations as a measure until about 100 g/cm 2 of material is used.

The genotype of the human cancer cell: Implications for risk analysis

An extremely large database describes genotypes associated with the human cancer phenotype and genotypes of human populations with genetic predisposition to cancer. Aspects of this database are examined from the perspective of risk analysis, and the following conclusions and hypotheses are proposed: (1) The genotypes of human cancer cells are characterized by multiple mutated genes. Each type of cancer is characterized by a set of mutated genes, a subset from a total of more than 80 genes, that varies between tissue types and between different tumors from the same tissue. No single cancer-associated gene nor carcinogenic pathway appears suitable as an overall indicator whose induction serves as a quantitative marker for risk analysis. (2) Genetic defects that predispose human populations to cancer are numerous and diverse, and provide a model for associating cancer rates with induced genetic changes. As these syndromes contribute significantly to the overall cancer rate, risk analysis should include an estimation of the effect of putative carcinogens on individuals with genetic predisposition. (3) Gene activation and inactivation events are observed in the cancer genotype at different frequencies, and the potency of carcinogens to induce these events varies significantly. There is a paradox between the observed frequency for induction of single mutational events in test systems and the frequency of multiple events in a single cancer cell, suggesting events are not independent. Quantitative prediction of cancer risk will depend on identifying rate-limiting events in carcinogenesis. Hyperproliferation and hypermutation may be such events. (4) Four sets of data suggest that hypermutation may be an important carcinogenic process. Current mechanisms of risk analysis do not properly evaluate the potency of putative carcinogens to induce the hypermutable state or to increase mutation in hypermutable cells. (5) High-dose exposure to carcinogens in model systems changes patterns of gene expression and may induce protective effects through delay in cell progression and other processes that affect mutagenesis and toxicity. Paradigms in risk analysis that require extrapolation over wide ranges of exposure levels may be flawed mechanistically and may underestimate carcinogenic effects of test agents at environmental levels. Characteristics of the human cancer genotype suggest that approaches to risk analysis must be broadened to consider the multiplicity of carcinogenic pathways and the relative roles of hyperproliferation and hypermutation. Further, estimation of risk to general human populations must consider effects on hypersusceptible individuals. The extrapolation of effects over wide exposure levels is an imprecise process.

Tumor response to radiotherapy is dependent on genotype-associated mechanisms in vitro and in vivo

BackgroundWe have previously shown that in vitro radiosensitivity of human tumor cells segregate non-randomly into a limited number of groups. Each group associates with a specific genotype. However we have also shown that abrogation of a single gene (p21) in a human tumor cell unexpectedly sensitized xenograft tumors comprised of these cells to radiotherapy while not affecting in vitro cellular radiosensitivity. Therefore in vitro assays alone cannot predict tumor response to radiotherapy.In the current work, we measure in vitro radiosensitivity and in vivo response of their xenograft tumors in a series of human tumor lines that represent the range of radiosensitivity observed in human tumor cells. We also measure response of their xenograft tumors to different radiotherapy protocols. We reduce these data into a simple analytical structure that defines the relationship between tumor response and total dose based on two coefficients that are specific to tumor cell genotype, fraction size and total dose.MethodsWe assayed in vitro survival patterns in eight tumor cell lines that vary in cellular radiosensitivity and genotype. We also measured response of their xenograft tumors to four radiotherapy protocols: 8 × 2 Gy; 2 × 5Gy, 1 × 7.5 Gy and 1 × 15 Gy. We analyze these data to derive coefficients that describe both in vitro and in vivo responses.ResultsResponse of xenografts comprised of human tumor cells to different radiotherapy protocols can be reduced to only two coefficients that represent 1) total cells killed as measured in vitro 2) additional response in vivo not predicted by cell killing. These coefficients segregate with specific genotypes including those most frequently observed in human tumors in the clinic. Coefficients that describe in vitro and in vivo mechanisms can predict tumor response to any radiation protocol based on tumor cell genotype, fraction-size and total dose.ConclusionsWe establish an analytical structure that predicts tumor response to radiotherapy based on coefficients that represent in vitro and in vivo responses. Both coefficients are dependent on tumor cell genotype and fraction-size. We identify a novel previously unreported mechanism that sensitizes tumors in vivo; this sensitization varies with tumor cell genotype and fraction size.

Cytomorphologic differentiation of benign and malignant mammary tumors in fine needle aspirate specimens from irradiated female Sprague-Dawley rats

BACKGROUND Fine needle aspiration (FNA) offers a rapid and minimally invasive means to distinguish malignant from benign neoplasms. However, few studies have been published regarding the cytopathology of mammary tumors in rats despite widespread use of the rat model for breast cancer formation and inhibition. OBJECTIVE The purpose of this study was to determine the diagnostic accuracy of FNA cytology and to develop distinguishing cytologic criteria for the diagnosis of radiation-induced benign and malignant mammary tumors in rats. METHODS In a study of radiation-induced mammary carcinogenesis, 100 Sprague-Dawley rats with cutaneous masses were randomly chosen for FNA. The aspirates were smeared, fixed, and stained with a modified Papanicolaou procedure for diagnostic evaluation. Cytologic and histologic diagnoses (benign vs malignant) were compared, and diagnostic accuracy was calculated using the histologic diagnosis as the criterion standard. FNA smears were scored semiquantitatively on a scale of 1-4 for cellularity, atypia, nuclear size, chromatin pattern, nuclear membrane thickness, nucleoli, and mitoses. The background was evaluated for necrosis, hemorrhage, inflammation, and mucosecretory material. Cytomorphologic features were compared statistically between benign and malignant tumors, based on the histologic diagnosis. RESULTS The sensitivity of FNA was 92.3% and specificity was 89.4% for the detection of malignancy. However, 14% of specimens, all fibroadenomas by histology, had insufficient cells for cytologic evaluation, for an overall accuracy rate of 78.0%. Malignant tumors had significantly higher scores for all cytomorphologic features, and were significantly more likely to contain cell clusters and necrotic debris. CONCLUSIONS FNA is an accurate method for differentiating benign and malignant rat mammary tumors.

Microdosimetric comparison of scanned and conventional proton beams used in radiation therapy

Multiple groups have hypothesised that the use of scanning beams in proton therapy will reduce the neutron component of secondary radiation in comparison with conventional methods with a corresponding reduction in risks of radiation-induced cancers. Loma Linda University Medical Center (LLUMC) has had FDA marketing clearance for scanning beams since 1988 and an experimental scanning beam has been available at the LLUMC proton facility since 2001. The facility has a dedicated research room with a scanning beam and fast switching that allows its use during patient treatments. Dosimetric measurements and microdosimetric distributions for a scanned beam are presented and compared with beams produced with the conventional methods presently used in proton therapy.

Sequentially-induced responses define tumour cell radiosensitivity

Purpose: Our aim was to define dose-dependent and genotype-dependent components of radiosensitivity by resolving patterns of radiation-induced clonal inactivation into specific responses. Methods: In a set of 10 tumour cells with varying expression of radiosensitivity and genotype, we identified doses at which all tumour cells change in their rate of clonogenic inactivation. We tested intervening dose-segments as to whether inactivation was constant, expressing inactivation as a log-linear function of dose. We compared these segments to components proposed in the Hit-target (HT) model and the Linear-quadratic (LQ) model. Temporal changes in redistribution in cell-cycle prevalence and apoptosis were examined as essential components of cellular radiosensitivity. Results: We identified four distinct responses induced sequentially in all cells independent of genotype. Rates of inactivation within each response varied with expression of genotype and identified: (i) A hypersensitive component H (0.0–0.10 Gy); (ii) a resistant component R (0.1–0.2 Gy); (iii) an induced repair response alpha* (0.2 Gy and higher); and (iv) a more sensitive component omega* (3.0 Gy and higher). The H, alpha* and omega* components were fitted well by log-linear patterns, the R response did not. Conclusions: Four distinct, sequentially-induced responses comprise cellular radiosensitivity. H and R responses are associated with low dose hyper-radiosensitivity and early apoptosis, while the alpha* and omega* responses share characteristics of the HT and LQ models and are associated with post-repair apoptosis. Radiation induces these four responses at the same doses in all cells, but the rate of inactivation over each response depends on genotype.

A NEW NANO-ENHANCED TECHNOLOGY PROPOSED TO QUANTIFY INTRACELLULAR DETECTION OF RADIATION-INDUCED METABOLIC PROCESSES

A new approach to intracellular detection and imaging of metabolic processes and pathways is presented that uses surface plasmon resonance to enhance interactions between photon-absorbing metabolites and metal nanoparticles in contact with cells in vitro or in vivo. Photon absorption in the nanoparticles creates plasmon fields, enhancing intrinsic metabolite fluorescence, thereby increasing absorption and emission rates, creating new spectral emission bands, shortening fluorescence lifetimes, becoming more photo-stable and increasing fluorescent resonance energy transfer efficiency. Because the cells remain viable, it is proposed that the method may be used to interrogate cells prior to and after irradiation, with the potential for automated analyses of intracellular interactive pathways associated with radiation exposures at lower doses than existing technologies. The design and concepts of the instrument are presented along with data for unexposed cells.

Analysis of a hormesis effect in the leukemia-caused mortality among atomic bomb survivors

Yakovlev and Polig (1996) developed a mechanistically motivated stochastic model of radiation carcinogenesis allowing for cell death. The key feature of the model is that it allows for radiation-induced cell killing to compete with the process of tumor promotion. This model describes and explains a wide range of experimental findings documented in the radiobiological literature, including the inverse dose-rate effect and radiation hormesis. The model has successfully been applied to various sets of experimental and epidemiological data to gain quantitative insight into the processes of tumorigenesis induced by radiation and chemical carcinogens. In this paper, we discuss the most recent application of the Yakovlev-Polig model to the analysis of epidemiological data on the mortality caused by radiation-induced leukemia (all types) among the atomic bomb survivors (Hiroshima and Nagasaki). Nonparametric estimates of the hazard function for leukemia latency time were obtained for three different dose groups identified in the Hiroshima cohort. The behavior of these estimates suggests the presence of the hormesis-type effect in relation to leukemia-caused mortality. A parsimonious version of the mechanistic model yields parametric estimates that are in good agreement with their nonparametric counterparts. Using the parametric model, we corroborated the presence of a moderate hormesis effect in the Hiroshima data. However, we have been unable to uncover the same effect with the Nagasaki cohort of the atomic bomb survivors.