RAD 114 RADIATION PROTECTION

Transcription

1 RAD 114 RADIATION PROTECTION RADIATION BIOLOGY, PART II CELL RADIOSENSITIVITY In addition to the type of radiation as having an impact on cell radiosensitivity, the presence of oxygen in tissues also plays an important role. In general, oxygenated cells are more sensitive to radiation than hypoxic cells. CELL RADIOSENSITIVITY One way to relate the degree to which oxygen affects cell radiosensitivity is the OXYGEN ENHANCEMENT RATIO (OER). It is the ratio of the dose required to affect a response under hypoxic conditions to the dose required to affect the same response under oxygenated conditions.

2 OER = Dose Under Hypoxic Conditions Dose Under Oxygenated Conditions LAW OF BERGONIE & TRIBONDEAU At the turn of the century, two French scientists determined that cell radiosensitivity depended on three things... CELL RADIOSENSITIVITY INCREASES AS... DIFFERENTIATION DECREASES REPRODUCTIVE ACTIVITY INCREASES THE MITOTIC HISTORY INCREASES (NUMBER OF DIVISIONS OVER THE COURSE OF THE CELL S LIFE)

3 CELL RADIOSENSITIVITY If cells are highly specialized such as a neuron, radiosensitivity is less than if cells are non-specialized such as neuroblasts. CELL RADIOSENSITIVITY Cells which proliferate rapidly are more sensitive to radiation than those cells which do not. Since lymphocytes live for a very short time (~24 hours) they must be replaced rapidly to maintain immune system health. These are the most sensitive blood cells in the body. CELL RADIOSENSITIVITY Cells with relatively long mitotic histories (over the course of a cell s lifetime) are more sensitive to radiation damage than those whose histories are relatively short.

4 RELATIVE CELL RADIOSENSITIVITY LYMPHOCITES EPITHELIAL TISSUES MUSCLE TISSUE NERVE TISSUE RELATIVE CELL RADIOSENSITIVITY Reproductive cells vary in their degree of radiosensitivity with the immature cells being more sensitive than the mature cells. Female reproductive cell radiosensitivity declines from birth until about 20 or 30 years. ORGANIC EFFECTS Organic effects of radiation are those that can be observed at the macroscopic level. These have been observed in insects, animals and human beings (atomic bomb and nuclear reactor victims).

5 TWO MAIN TYPES: EARLY SOMATIC EFFECTS LATE SOMATIC EFFECTS ORGANIC EFFECTS Early somatic effects are seen early on following exposure to radiation and are the result of acute high-dose radiation exposure. TYPES OF EARLY EFFECTS NAUSEA FATIGUE SKIN REDDENING EPILATION DESQUAMATION

6 EARLY EFFECTS One or more of the early effects outlined may appear following radiation therapy treatments. However, the area of the body exposed is small (compared to whole-body exposures). Consequently, the effects are localized. EARLY EFFECTS When a large dose is delivered to the whole body, the early effects are more severe and may occur in succession. The manifestation of these effects is called ACUTE RADIATION SYNDROME. ACUTE RADIATION SYNDROME PRODROME LATENT PERIOD MANIFEST ILLNESS RECOVERY OR DEATH

7 ACUTE RADIATION SYNDROME HEMATOPOIETIC EFFECTS GASTROINTESTINAL EFFECTS CEREBROVASCULAR EFFECTS HEMATOPOIETIC EFFECTS Due to the relative radiosensitivity of cells in the body, the blood- forming organs are primarily affected. Above 100 rad, chances of survival diminish with increasing dose. Recovery may occur in weeks or months. GASTROINTESTINAL EFFECTS At doses from 600 rad to 1000 rad, in addition to blood-forming organ effects, the digestive system is also effected. Death occurs in days as the epithelial cells are destroyed, causing bleeding, infection and dehydration.

8 CEREBROVASCULAR EFFECTS At doses of 5000 rad or more, death occurs within hours or days due to cardiovascular collapse and neurological impairment. Given the extent of radiation dose, the victim does not have time to exhibit hematopoietic or gastrointestinal effects (thankfully!). LATE SOMATIC EFFECTS Are the result of either wholebody or partial body doses of radiation given acutely or over a long period. They manifest themselves months or years later. MAJOR TYPES OF LATE EFFECTS STOCHASTIC EFFECTS: CARCINOGENESIS BIRTH DEFECTS (TERATOGENESIS) NONSTOCHASTIC EFFECTS: CATARACTOGENESIS FIBROSIS STERILITY NON-SPECIFIC LIFE SPAN SHORTENING

9 CANCER RISK The result of exposure to high doses of radiation over long periods is well documented. Early radiology pioneers such as Clarence Dally and uranium dial painters were painful reminders of the dangers of ionizing radiation. CANCER RISK Low-dose effects cannot be directly known. Cancer risk is extrapolated from high dose data obtained from atomic bomb survivors. Consequently, any dose is assumed to be potentially carcinogenic. CANCER RISK A report from the National Academy of Sciences called BEIR V, concluded that except for leukemia, the risk of cancer followed a linear, non-threshold dose-response relationship.

10 CANCER RISK Still, it is believed that the risk of cancer is outweighed by the benefit that radiological studies provide. This does not excuse carelessness in the use of ionizing radiation by technologists. BIRTH DEFECTS It is a well-known fact that unborn are highly susceptible to ionizing radiation during the first trimester of pregnancy. Therefore, unless a lifeor-death situation is involved, women should not have elective radiological procedures during the first three months of pregnancy. CATARACTOGENESIS It can be shown that a single dose of 200 rad given to the eyes will induce cataracts. The relationship between radiation dose and cataractogenesis follows a threshold, non-linear pattern.

11 LIFE SPAN SHORTENING Long-term exposure can, if unchecked, lead to non-specific life span shortening. Recent studies of radiologists i have shown that, t compared to other medical specialists, the average life span appears to be the same. This was not true 50 years ago. GENETIC EFFECTS These are effects in reproductive cells BEFORE fertilization. Normally, a genetic defect is recessive unless the defect is present in both parents. GENETIC DEFECTS Radiation-induced genetic defects have not been clearly demonstrated in humans. However, this does not mean that they do not occur. Therefore, it is important that not only the occupationally exposed, but the population as a whole limit exposure to ionizing radiation.

12 DOUBLING DOSE CONCEPT The dose that, when received by an entire population, would result in a doubling of the natural incidence of mutations. It is believed to be around 156 rem. GENETICALLY SIGNIFICANT DOSE CONCEPT The average gonadal dose that takes into account the number of people in a population and the expected contribution of children. GENETICALLY SIGNIFICANT DOSE CONCEPT It assumes that if every member of the population received the same dose of radiation (above normal background levels) it would have the same genetic effect as a few individuals receiving a proportionally larger dose.

13 GENETICALLY SIGNIFICANT DOSE CONCEPT For example, the genetic effect of 100,000 people each receiving 12 mrad would be identical to 10,000 people each receiving 120 mrad. GENETICALLY SIGNIFICANT DOSE CONCEPT This concept also assumes that t radiation-induced i d d genetic mutations are cumulative and that any dose is potentially mutagenic. RISK ESTIMATES Scientists can estimate the amount of radiation-induced cancer risk ikon the basis of three measures, each of which depends on specific kinds of data.

14 RISK ESTIMATES RELATIVE RISK EXCESS RISK ABSOLUTE RISK RELATIVE RISK In the absence of precise dose data, relative risk is calculated as the ratio of observed cases of cancer to the number of expected cases (in the absence of radiation). RELATIVE RISK For example, if 12 cancer cases are observed in a population of 20,250, 250 yet the expected cases of cancer in a population of 100,000 is 45, what is the relative risk?

15 RELATIVE RISK Expected Cases = ,250 = Observed Cases = 12 Relative Risk = = = 1.32 EXCESS RISK A measure of radiation-induced cancer risk based on the number of observed cases that exceed expected cases. Using the previous example, the excess risk is 12-9=3 cases. ABSOLUTE RISK Using specific dose data, absolute risk is calculated as: Number of Expected Cases per million population, multiplied by Dose in RAD

16 ABSOLUTE RISK The incidence of cancer deaths in the population is 10 per million per rad per year. What is the expected rate of cancer deaths among the 300,000 American radiographers if they receive an annual effective dose equivalent of 8 mrad? ABSOLUTE RISK Cancer Deaths = ,000 = 3 Absolute Risk = = = cases/yr