Long-Term Somatic Effects of Radiation

In This Lecture Somatic Cell Mutations and Cancer Radiation Induced Cancer in Humans Extrapolating to Low Doses Radiation Induced Cataracts

TYPE OF RADIATION DAMAGE The effects of radiation on the human population can be classified as either somatic or genetic: Somatic effects are harm that exposed individuals suffer during their lifetime, such as radiation induced cancers (carcinogenesis), sterility, opacification of the eye lens and life shortening. Genetic or hereditary effects are radiation induced mutations to an individual’s genes and DNA that can contribute to the birth of defective descendants.

Long-Term Somatic Effects Somatic cells  are those cells in your body other than reproductive cells . They can be damaged in a variety of ways, such as by chemicals, biological and physical agents or by ionizing radiation . The effects of the damage from ionizing radiation can be long-term  or short-term   depending on the type and severity of the damaging agent. The long-term effects of radiation exposure are cancer  and cataract  formation.

Somatic and Genetic Effects Carcinogenesis expresses itself as a late somatic effect. Sources of human data on carcinogenesis: Low level occupational exposure. Atomic bomb survivors in Hiroshima and Nagasaki. Medical radiation exposure of patient: treatment of ankylosing spondylitis with orthovoltage x rays treatment of thyroid abnormalities with radiation radiotherapy in cancer treatment Exposure of staff during medical procedures Early radiologists Early radiation oncologists using brachytherapy

Somatic Cell Mutations and Cancer A long-term somatic effect  is the damage to cells that are continually reproducing. These cells are the most sensitive to radiation because any changes made in the parent cell's chromosome  structure will be transmitted to its daughters. Also, radiation can affect the delicate chemistry of the cell causing changes in the rate of cell division or even the destruction of that cell. An event which causes a somatic cell  to behave in this way is called a mutation . Mutations in the reproductive cells  cause damage that affects future generations. However, a mutation in a somatic cell has consequences only for the individual.

Somatic Cell Mutations and Cancer If the mutation in the somatic cell  increases the rate of its reproduction in an uncontrolled manner, then the number of daughter cells may increase rapidly in that area. When this occurs, it often happens that the daughter cells divide before reaching their mature state. The result then is an ever increasing number of cells that have no beneficial function to the body, yet are absorbing body nutrition at an increasing rate. The tissue could now be called a tumor.

Somatic Cell Mutations and Cancer Benign tumor :cells remain in their place of origin and do not directly invade surrounding tissues. Malignant tumor : tumor invades neighboring tissue and causes distant secondary growths (called metastasis ). A malignant tumor is what we call “cancer”, whether it is fatal or not depends on: The tissue in which it is located How rapidly it grows How soon it is detected and treated

Radiation Induced Cancer in Humans Leukemia :is a disease characterized by a great excess of immature white cells in the blood and can be likened to a blood cancer. Marie Curie , for example, who first isolated radium from uranium ore died of leukemia as did her daughter- assistant, (her husband on the other hand died in a traffic accident). In the 1920's, watch dials were painted with a radium based luminous paint. The employees, all women, who did this work often licked their paint brushes to give them a sharp point and ingested a small quantity of the paint each time they did this. The radium in the paint collected in the bones of these employees and resulted in bone tumors 8 to 40 years later.

Radiation Induced Cancer in Humans The distribution of cancers among the 1346 radium dial painters  that were followed up. It demonstrates quite clearly that: The risk of cancer increases with the radiation dose.

Radiation Induced Cancer in Humans In UK 6500 patients with arthritis of the spine were treated with large doses of x-rays . The average dose was 3Gy. The disease called ankylosing spondylitis , causes a painful stiffening of the joints in the backbone. Of the 6500 patients, 30 developed leukemia  compared with an expected incidence of 7 cases. The survivors of the Hiroshima  and Nagasaki  atom bomb attacks. Nearly 80,000 of these people have been carefully studied in the years since the war. Of this number of survivors, 126 died of leukemia. This provided clear evidence of the dose dependent relationship of leukemia to radiation, i.e.: The higher the dose, the greater the risk. The study of these survivors also indicated an increase in frequency of stomach , lung  and breast cancers . These have taken much longer to develop and some are still appearing now. The A-bomb data from Japan shows that that there is a delay (called the latency ) between the radiation exposure and the death from cancer  it induces. The mean latency from leukemia is in the region of ten years, and for the other cancers it is more than twenty years. Even now, more than 40 years after the explosions, excess cancer deaths are still occurring.

Extrapolating to Low Doses There is no doubt that various forms of cancer are produced in people who have received large doses (of up to several Gray) in short periods of time. It is known that for low LET radiation  (x-rays , - rays  and beta rays ) exposures received over a long period of time (such as a year) have less effect than if received over a period of a few hours. For high LET radiation the opposite appears to be true. Similarly the biological effect is not strictly proportional to the received dose and smaller doses become less and less efficient in producing an effect.

Extrapolating to Low Doses The body does not contain any organs whose specific function is to undo radiation damage although it is known that individual cells do have the ability to repair some types of damage. The question of whether a threshold radiation dose for cancer exists is a subject of some debate in scientific circles. Because of the lack of definitive data it is at present assumed that: There is no radiation dose which can be said to be perfectly safe. There may be a threshold dose for cancer but its existence has not been demonstrated, and therefore it is prudent to assume that even the smallest dose has the possibility of producing a harmful effect (albeit with a very small probability). It is for this reason that: All unnecessary exposure should be avoided. Effects such as cancer induction and genetic damage are referred to as stochastic effects , because the initiating event occurs randomly and there is no definite threshold. The severity of the effect  also does not depend on the dose, but rather the probability  that the effect occurs increases with increasing dose.

Extrapolating to Low Doses Nominal probability coefficients for stochastic effects  Determent ( % per Sv) Exposed population Fatal cancer Non-fatal cancer Severe hereditary effects Total Adult workers 4.0 0.8 5.6 Whole Population 5.0 1.0 1.3 7.3 This means that if your whole body is exposed to 1 Sv of ionizing radiation , you have an extra 4% chance of contracting a cancer  that will be fatal many years after the exposure. The word ‘extra’ is used because you normally have a 20 to 25% chance of dying from cancer.

Radiation Induced Cataracts Cataract : used to describe a loss of transparency of the lens of the eye . Cataracts are commonly associated with aging and also with metabolic conditions such as diabetes. The lens of the eye is peculiar in that there is no cell replacement system and therefore damaged cells that have become opaque are not replaced naturally. Unlike the induction of cancer, radiation damage to the lens of the eye shows a definite threshold effect. As long as the life- time equivalent dose is less than 7.5 Sv , no lens opacities due to ionizing radiation will be produced that will interfere with vision. Radiation effects that show a definite threshold are called deterministic effects  . Deterministic effects do not occur below the threshold, but above the threshold: The severity of the effect increases with increasing dose.

Radiation Induced Cataracts The Degree of Opacity: 1+ 3+ 2+ 4+