Human coronary artery restenosis after percutaneous revascularization is a response to mechanical injury. Smooth muscle cell proliferation is a major component of restenosis, resulting in obstructive neointimal hyperplasia. Because ionizing radiation inhibits cellular proliferation, this study tested in a porcine coronary injury model the hypothesis that the hyperplastic response to coronary artery injury would be attenuated by X-irradiation. Deep arterial injury was produced in 37 porcine left anterior descending coronary artery segments with overexpanded, percutaneously delivered tantalum wire coils. Three groups of pigs were irradiated with 300-kV X-rays after coil injury: Group I (n = 10), 400 cGy at 1 day; Group II (n = 10), 400 cGy at 1 day and 400 cGy at 4 days and Group III (n = 9), 800 cGy at 1 day. Eight pigs in the control group underwent identical injury but received no radiation. Treatment efficacy was histologically assessed by measuring neointimal thickness and percent area stenosis. Mean neointimal thickness in all irradiated groups was significantly higher than in the control groups and thickness was proportional to X-ray dose. X-irradiation delivered at these doses and times did not inhibit proliferative neointima. Rather, it accentuated the neointimal response to acute arterial injury and may have potentiated that injury.
Regulations for protecting humans against stochastic biological effects from ionizing radiation are based on the linear no-threshold (LNT) risk assessment model, which states that any amount of radiation exposure may lead to cancer in a population. Based on the LNT model, risk from low-dose radiation increases linearly with increasing doses of radiation. Imaging procedures in medicine and dentistry are an important source of low-dose ionizing radiation. The increased use of computerized tomography (CT) and cone beam computerized tomography (CBCT) has raised health concerns regarding exposure to low-dose ionizing radiation. In oral and maxillofacial surgery and implant dentistry, CBCT is now at the forefront of this controversy. Although caution has been expressed, there have been no direct studies linking radiation exposure from CT and CBCT used in dental imaging with cancer induction. This article describes the concerns about radiation exposure in dental imaging regarding the use of CT.
Solar ultraviolet radiation (UVR) is recognized as a major cause of non-melanoma skin cancer in man. Skin cancer occurs most frequently in the most heavily exposed areas and correlates with degree of outdoor exposure. The incidence of skin cancer is also increased by contact with photosensitizing drugs and chemicals such as psoralens, coal tars and petroleum stocks. Other substances which do not act as photosensitizers, such as immunosuppressants taken by organ transplant recipients, also increase the risk of skin cancer. The U.S. Food and Drug Administration requests, on a case-by-case basis, that risk of enhanced photocarcinogenesis is assessed for many classes of drugs. Health Canada's Therapeutic Products Programme has issued a Notice of Intent to regulate pharmaceutical products which may enhance carcinogenicity of the skin induced by ultraviolet radiation. Other national regulatory agencies review such data when they exist, but their own requirements emphasize batteries of short-term in vitro and in vivo tests. While they may support drug development strategies, short-term tests have yet to be validated as predictors of the ability of drugs or chemicals to enhance photocarcinogenesis. Published protocols now describe study designs and procedures capable of determining whether test agents enhance the rate of formation of UVR-induced skin tumors.
Twelve cell lines representing 10 genera of three orders (Diptera, Lepidoptera, and Orthoptera) of the class Insecta and one cell line (Acarina) from the class Arachnida were examined to discern their sensitivity to the lethal effects of x-irradiation. Radiosensitivity was measured by a combination of colony formation and population growth curve techniques. Each of these arthropod cell lines is significantly more radioresistant than mammalian cells, though the degree of resistance varies greatly with order. Dipteran cells are 3 to 9 times and lepidopteran cells 52 to 104 times more radioresistant than mammalian cells. Orthopteran and acarine cells are intermediate in radiosensitivity between dipteran and lepidopteran cells. These cells, especially the lepidopteran, should be valuable in determining the molecular nature of repair mechanisms that result in resistance to ionizing radiation.The pronounced radioresistance of adult insects has been well documented (1, 2). This resistance has been attributed primarily to the lack of cell division in adult insects, in that the sensitivity of cells to irradiation is directly proportional to their reproductive activity (1-3), and because insects have minimal or no cell division in the adult state, they are extremely resistant to radiation (1, 2).. Recent evidence has suggested that cells of insects have an intrinsic radioresistance (4). This cellular basis of resistance derives largely from studies with the TN-368 lepidopteran cell line. These mitotically active cells are approximately two orders of magnitude more radioresistant than mammalian cells (4-6). The unique nature and the ubiquity of radioresistance in insect cells have important implications concerning the innate ability of cells to tolerate large doses of radiation. Therefore, the present experiments examined 13 cell lines representing three orders of the class Insecta, along with 1 arachnid cell line ( Population Doubling Times. Doubling times were derived from the logarithmic portion of growth curves obtained by seeding 106 cells in replicate 25-cm2 flasks and counting at 24-hr intervals until the populations were well into stationary phase (each point on these growth curves represents at least two separate experiments having two duplicate flasks per experiment and four hemocytometer cell counts per flask).X-Irradiations. A Picker Vanguard x-ray machine operated at 260 kV and 15 mA was used for all irradiations. X-rays passed through approximately 5 mm of glass but were otherwise unfiltered. Dosimetry, utilizing the same geometry as for experiments, was performed according to the method of Fricke and Hart (7).Growth Curves. Exponentially growing cells were diluted in fresh growth medium to a concentration of 2 x 0L cells per ml. They were irradiated at room temperature in 2.0-ml aliquots as an aerated, stirred suspension at a dose rate of just under 0.83 Gy (83 cGy)/sec. Cells irradiated with a given dose were pooled and 1 x 106 cells were seeded into Costar 25-cm2 flasks. Pooling was done to insure...
TN-368 lepidopteran insect cells display a multiphasic survival response in both air and nitrogen. In each case the survival curve is characterized by an initial small- shouldered component having a steep slope, a plateau or broad- shouldered region near the 0.1 survival level, and finally a shallow slope component. The D0, Dq, and n values for the initial steep slope component in air and nitrogen are, respectively, 65.7 Gy, 9.0 Gy, and 1.2, and 104.4 Gy, 28.8 Gy, and 1.3. The oxygen enhancement ratio (OER) for this portion of the curve is 1.6. The D0, Dq, and n values for the shallow slope component in air and nitrogen are, respectively, 130.2 Gy,--36.1 Gy, and 0.8, and 226.8 Gy, 121.0 Gy, and 1.7. The OER for this portion of the curve is 1.7. The D0 values for each slope and the width of the plateau region all increase proportionally for the nitrogen curve over that of air, the OER being approximately the same for both curve components. A similar multiphasic response was observed at dose rates of 202, 49.6, and 9.1 Gy/min. In addition, the survival of cells which had previously been irradiated with a dose well into the logarithmic region of the more resistant shallow slope portion of the curve retained a multiphasic response. Although cell cycle variations in radiosensitivity may contribute slightly to the response, an inducible or activated repair process would be consistent with the results.
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