Radiation-induced cancer

Source: Wikipedia, the free encyclopedia.

Exposure to

possible carcinogen by the WHO's International Agency for Research on Cancer, but to date, no evidence of this has been observed.[2][3]

Causes

According to the prevalent model, any radiation exposure can increase the risk of cancer. Typical contributors to such risk include natural background radiation, medical procedures, occupational exposures, nuclear accidents, and many others. Some major contributors are discussed below.

Radon

Radon is responsible for the worldwide majority of the mean public exposure to ionizing radiation. It is often the single largest contributor to an individual's background radiation dose, and is the most variable from location to location. Radon gas from natural sources can accumulate in buildings, especially in confined areas such as attics, and basements. It can also be found in some spring waters and hot springs.[4]

Epidemiological evidence shows a clear link between lung cancer and high concentrations of radon, with 21,000 radon-induced U.S. lung cancer deaths per year—second only to cigarette smoking—according to the United States

Environmental Protection Agency.[5] Thus in geographic areas where radon is present in heightened concentrations, radon is considered a significant indoor air
contaminant.

Residential exposure to radon gas has similar cancer risks as passive smoking.[6] Radiation is a more potent source of cancer when it is combined with other cancer-causing agents, such as radon gas exposure plus smoking tobacco.[6]

Medical

In industrialized countries,

radiotherapy
treatments deliberately deliver lethal doses (on a cellular level) to tumors and surrounding tissues.

It has been estimated that CT scans performed in the US in 2007 alone will result in 29,000 new cancer cases in future years.[10][11] This estimate is criticized by the American College of Radiology (ACR), which maintains that the life expectancy of CT scanned patients is not that of the general population and that the model of calculating cancer is based on total-body radiation exposure and thus faulty.[11]

Occupational

In accordance with ICRP recommendations, most regulators permit nuclear energy workers to receive up to 20 times more radiation dose than is permitted for the general public.

cancer and spaceflight
.

Some occupations are exposed to radiation without being classed as nuclear energy workers. Airline crews receive occupational exposures from

US Capitol, is likely to receive a dose from natural uranium in the granite.[13]

Accidental

Chernobyl radiation map from 1996

Nuclear accidents can have dramatic consequences to their surroundings, but their global impact on cancer is less than that of natural and medical exposures.

The most severe nuclear accident is probably the Chernobyl disaster. In addition to conventional fatalities and acute radiation syndrome fatalities, nine children died of thyroid cancer, and it is estimated that there may be up to 4,000 excess cancer deaths among the approximately 600,000 most highly exposed people.[14][15] Of the 100 million curies (4 exabecquerels) of radioactive material, the short lived radioactive isotopes such as 131I Chernobyl released were initially the most dangerous. Due to their short half-lives of 5 and 8 days they have now decayed, leaving the more long-lived 137Cs (with a half-life of 30.07 years) and 90Sr (with a half-life of 28.78 years) as main dangers.

In March 2011, an earthquake and tsunami caused damage that led to

Fukushima I Nuclear Power Plant in Japan. Significant release of radioactive material took place following hydrogen explosions at three reactors, as technicians tried to pump in seawater to keep the uranium fuel rods cool, and bled radioactive gas from the reactors in order to make room for the seawater.[16] Concerns about the large-scale release of radioactivity resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. On March 24, 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures".[17]

Also in Japan was the Tokaimura nuclear accidents of 1997 and 1999. The 1997 accident was far less fatal than the 1999 accident. The 1999 nuclear accident was caused by two faulty technicians who, in their desire to speed up the process of converting uranium hexafluoride to enriched uranium dioxide, resulted in a critical mass that resulted in technicians Hisashi Ouchi being dosed with approximately 17 sieverts of radiation and Masato Shinohara to be dosed with 10 sieverts of radiation, which resulted in their deaths. The two's supervisor, Yutaka Yokokawa, who was sitting in a desk far from the tank where the uranium hexafluoride was being poured into, was dosed with 3 sieverts and survived, but was charged with negligence in October 2000.[citation needed]

In 2003, in autopsies performed on six dead children in the polluted area near Chernobyl where they also reported a higher incidence of pancreatic tumors, Bandazhevsky found a concentration of 137-Cs of 40-45 times higher than in their liver, thus demonstrating that pancreatic tissue is a strong accumulator of radioactive cesium.[18] In 2020, Zrielykh reported a high and statistically significant incidence of pancreatic cancer in Ukraine for a period of 10 year, there have been cases of morbidity also in children in 2013 compared with 2003.[19]

Other serious radiation accidents include the Kyshtym disaster (estimated 49 to 55 cancer deaths),[20] and the Windscale fire (an estimated 33 cancer deaths).[21][22]

The satellite

SNAP-3 radioisotope thermoelectric generator (RTG) with approximately 1 kilogram of Plutonium-238 on board when on April 21, 1964 it burned up and re-entered the atmosphere.[23] Dr. John Gofman claimed it increased the rate of lung cancer worldwide. He said "Although it is impossible to estimate[dubious – discuss] the number of lung cancers induced by the accident, there is no question that the dispersal of so much Plutonium-238 would add to the number of lung cancers diagnosed over many subsequent decades."[24][25] Other satellite failures include Kosmos 954 and Kosmos 1402
.

Mechanism

Cancer is a

effective radiation dose, but the severity of the cancer is independent of dose. The speed at which cancer advances, the prognosis, the degree of pain, and every other feature of the disease are not functions of the radiation dose to which the person is exposed. This contrasts with the deterministic effects of acute radiation syndrome which increase in severity with dose above a threshold. Cancer starts with a single cell whose operation is disrupted. Normal cell operation is controlled by the chemical structure of DNA molecules, also called chromosomes
.

When radiation deposits enough energy in organic tissue to cause ionization, this tends to break molecular bonds, and thus alter the molecular structure of the irradiated molecules. Less energetic radiation, such as visible light, only causes excitation, not ionization, which is usually dissipated as heat with relatively little chemical damage. Ultraviolet light is usually categorized as non-ionizing, but it is actually in an intermediate range that produces some ionization and chemical damage. Hence the carcinogenic mechanism of ultraviolet radiation is similar to that of ionizing radiation.

Unlike chemical or physical triggers for cancer, penetrating radiation hits molecules within cells randomly.

chromosome abnormalities
will turn out to be irreversible.

DNA

epigenetic markers of the DNA,[27] which regulate the gene expression. Most of the induced DSBs are repaired within 24h after exposure, however, 25% of the repaired strands are repaired incorrectly and about 20% of fibroblast cells that were exposed to 200mGy died within 4 days after exposure.[28][29][30] A portion of the population possess a flawed DNA repair mechanism, and thus suffer a greater insult due to exposure to radiation.[26]

Major damage normally results in the

cellular immortality (losing normal, life-limiting cell regulatory processes), and adaptations that favor formation of a tumor.[6]

In some cases, a small radiation dose reduces the impact of a subsequent, larger radiation dose. This has been termed an 'adaptive response' and is related to hypothetical mechanisms of hormesis.[34]

A latent period of decades may elapse between radiation exposure and the detection of cancer. Those cancers that may develop as a result of radiation exposure are indistinguishable from those that occur naturally or as a result of exposure to other carcinogens. Furthermore, National Cancer Institute literature indicates that chemical and physical hazards and lifestyle factors, such as smoking, alcohol consumption, and diet, significantly contribute to many of these same diseases. Evidence from uranium miners suggests that smoking may have a multiplicative, rather than additive, interaction with radiation.[6] Evaluations of radiation's contribution to cancer incidence can only be done through large epidemiological studies with thorough data about all other confounding risk factors.

Skin cancer

Prolonged exposure to

UVB, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world.[35]

Skin cancer may occur following ionizing radiation exposure following a latent period averaging 20 to 40 years.

radiographers are among the earliest occupational groups exposed to radiation. It was the observation of the earliest radiologists that led to the recognition of radiation-induced skin cancer—the first solid cancer linked to radiation—in 1902.[44] While the incidence of skin cancer secondary to medical ionizing radiation was higher in the past, there is also some evidence that risks of certain cancers, notably skin cancer, may be increased among more recent medical radiation workers, and this may be related to specific or changing radiologic practices.[44] Available evidence indicates that the excess risk of skin cancer lasts for 45 years or more following irradiation.[45]

Epidemiology

Cancer is a stochastic effect of radiation, meaning that it only has a probability of occurrence, as opposed to deterministic effects which always happen over a certain dose threshold. The consensus of the nuclear industry, nuclear regulators, and governments, is that the incidence of cancers due to ionizing radiation can be modeled as increasing linearly with

Chernobyl accident, both of which were internal exposure events. Chris Busby of the self styled "European Committee on Radiation Risk", calls the ICRP model "fatally flawed" when it comes to internal exposure.[48]

Radiation can cause cancer in most parts of the body, in all animals, and at any age, although radiation-induced solid tumors usually take 10–15 years, and can take up to 40 years, to become clinically manifest, and radiation-induced

nevoid basal cell carcinoma syndrome or retinoblastoma, are more susceptible than average to developing cancer from radiation exposure.[6] Children and adolescents are twice as likely to develop radiation-induced leukemia as adults; radiation exposure before birth has ten times the effect.[6]

Radiation exposure can cause cancer in any living tissue, but high-dose whole-body external exposure is most closely associated with leukemia,[50] reflecting the high radiosensitivity of bone marrow. Internal exposures tend to cause cancer in the organs where the radioactive material concentrates, so that radon predominantly causes lung cancer, iodine-131 for thyroid cancer is most likely to cause leukemia.

Data sources

Increased Risk of Solid Cancer with Dose for atomic blast survivors

The associations between ionizing radiation exposure and the development of cancer are based primarily on the "

atomic bomb survivors, the largest human population ever exposed to high levels of ionizing radiation. However this cohort was also exposed to high heat, both from the initial nuclear flash of infrared light and following the blast due their exposure to the firestorm and general fires that developed in both cities respectively, so the survivors also underwent Hyperthermia therapy to various degrees. Hyperthermia, or heat exposure following irradiation is well known in the field of radiation therapy to markedly increase the severity of free-radical insults to cells following irradiation. Presently however no attempts have been made to cater for this confounding
factor, it is not included or corrected for in the dose-response curves for this group.

Additional data has been collected from recipients of selected medical procedures and the 1986 Chernobyl disaster. There is a clear link (see the UNSCEAR 2000 Report, Volume 2: Effects) between the Chernobyl accident and the unusually large number, approximately 1,800, of thyroid cancers reported in contaminated areas, mostly in children.

For low levels of radiation, the biological effects are so small they may not be detected in epidemiological studies. Although radiation may cause cancer at high doses and high dose rates,

radiation doses
, researchers rely on models of the process by which radiation causes cancer; several models that predict differing levels of risk have emerged.

Studies of occupational workers exposed to chronic low levels of radiation, above normal background, have provided mixed evidence regarding cancer and transgenerational effects. Cancer results, although uncertain, are consistent with estimates of risk based on atomic bomb survivors and suggest that these workers do face a small increase in the probability of developing leukemia and other cancers. One of the most recent and extensive studies of workers was published by Cardis, et al. in 2005 .[51] There is evidence that low level, brief radiation exposures are not harmful.[52]

Modelling

Alternative assumptions for the extrapolation of the cancer risk vs. radiation dose to low-dose levels, given a known risk at a high dose: supra-linearity (A), linear (B), linear-quadratic (C) and hormesis (D).

The linear dose-response model suggests that any increase in dose, no matter how small, results in an incremental increase in risk. The linear no-threshold model (LNT) hypothesis is accepted by the International Commission on Radiological Protection (ICRP) and regulators around the world.[53] According to this model, about 1% of the global population develop cancer as a result of natural background radiation at some point in their lifetime. For comparison, 13% of deaths in 2008 are attributed to cancer, so background radiation could plausibly be a small contributor.[54]

Many parties have criticized the ICRP's adoption of the linear no-threshold model for exaggerating the effects of low radiation doses. The most frequently cited alternatives are the "linear quadratic" model and the "hormesis" model. The linear quadratic model is widely viewed in

radiotherapy as the best model of cellular survival,[55] and it is the best fit to leukemia data from the LSS cohort.[6]

Linear no-threshold F(D)=α⋅D
Linear quadratic F(D)=α⋅D+β⋅D2
Hormesis F(D)=α⋅[D−β]

In all three cases, the values of alpha and beta must be determined by regression from human exposure data. Laboratory experiments on animals and tissue samples is of limited value. Most of the high quality human data available is from high dose individuals, above 0.1 Sv, so any use of the models at low doses is an extrapolation that might be under-conservative or over-conservative. There is not enough human data available to settle decisively which of these model might be most accurate at low doses. The consensus has been to assume linear no-threshold because it the simplest and most conservative of the three.

Radiation hormesis is the conjecture that a low level of ionizing radiation (i.e., near the level of Earth's natural background radiation) helps "immunize" cells against DNA damage from other causes (such as free radicals or larger doses of ionizing radiation), and decreases the risk of cancer. The theory proposes that such low levels activate the body's DNA repair mechanisms, causing higher levels of cellular DNA-repair proteins to be present in the body, improving the body's ability to repair DNA damage. This assertion is very difficult to prove in humans (using, for example, statistical cancer studies) because the effects of very low ionizing radiation levels are too small to be statistically measured amid the "noise" of normal cancer rates.

The idea of radiation hormesis is considered unproven by regulatory bodies. If the hormesis model turns out to be accurate, it is conceivable that current regulations based on the LNT model will prevent or limit the hormetic effect, and thus have a negative impact on health.[56]

Other non-linear effects have been observed, particularly for

Graves disease have failed to find any increase in thyroid cancer, even though there is linear increase in thyroid cancer risk with I-131 absorption at moderate doses.[57]

Public safety

Low-dose exposures, such as living near a

coal-fired power plant, which has higher emissions than nuclear plants, are generally believed to have no or very little effect on cancer development, barring accidents.[6]
Greater concerns include radon in buildings and overuse of medical imaging.

The International Commission on Radiological Protection (ICRP) recommends limiting artificial irradiation of the public to an average of 1 mSv (0.001 Sv) of effective dose per year, not including medical and occupational exposures.[1] For comparison, radiation levels inside the US capitol building are 0.85 mSv/yr, close to the regulatory limit, because of the uranium content of the granite structure.[13] According to the ICRP model, someone who spent 20 years inside the capitol building would have an extra one in a thousand chance of getting cancer, over and above any other existing risk. (20 yr × 0.85 mSv/yr × 0.001 Sv/mSv × 5.5%/Sv ≈ 0.1%) That "existing risk" is much higher; an average American would have a one in ten chance of getting cancer during this same 20-year period, even without any exposure to artificial radiation.

Internal contamination due to ingestion, inhalation, injection, or absorption is a particular concern because the radioactive material may stay in the body for an extended period of time, "committing" the subject to accumulating dose long after the initial exposure has ceased, albeit at low dose rates. Over a hundred people, including

radium girls, have received committed doses in excess of 10 Gy and went on to die of cancer or natural causes, whereas the same amount of acute external dose would invariably cause an earlier death by acute radiation syndrome.[58]

Internal exposure of the public is controlled by regulatory limits on the radioactive content of food and water. These limits are typically expressed in becquerel/kilogram, with different limits set for each contaminant.

History

Although radiation was discovered in late 19th century, the dangers of radioactivity and of radiation were not immediately recognized. Acute effects of radiation were first observed in the use of X-rays when Wilhelm Röntgen intentionally subjected his fingers to X-rays in 1895. He published his observations concerning the burns that developed, though he attributed them to ozone rather than to X-rays. His injuries healed later.

The genetic effects of radiation, including the effects on cancer risk, were recognized much later. In 1927

atomic bomb survivors
.

Before the biological effects of radiation were known, many physicians and corporations had begun marketing radioactive substances as patent medicine and radioactive quackery. Examples were radium enema treatments, and radium-containing waters to be drunk as tonics. Marie Curie spoke out against this sort of treatment, warning that the effects of radiation on the human body were not well understood. Curie later died of aplastic anemia, not cancer. Eben Byers, a famous American socialite, died of multiple cancers in 1932 after consuming large quantities of radium over several years; his death drew public attention to dangers of radiation. By the 1930s, after a number of cases of bone necrosis and death in enthusiasts, radium-containing medical products had nearly vanished from the market.

In the United States, the experience of the so-called Radium Girls, where thousands of radium-dial painters contracted oral cancers, popularized the warnings of occupational health associated with radiation hazards. Robley D. Evans, at MIT, developed the first standard for permissible body burden of radium, a key step in the establishment of nuclear medicine as a field of study. With the development of nuclear reactors and nuclear weapons in the 1940s, heightened scientific attention was given to the study of all manner of radiation effects.

Notes

  1. Transuranic
    elements are believed to have a chemical affinity for DNA, and any radioactive element could be part of a chemical compound that targets certain molecules.

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