Ionizing Radiation and Cancer

That is the whole definition. Ionizing radiation carries enough energy to strip electrons off atoms. What is left behind is a free electron with a negative charge. It leaves an ionized atom with a positive charge too. Radiation that cannot do this is non-ionizing. Visible light, radio waves, and the energy from a cell phone all fall in that group. NCI notes non-ionizing radiation has not been found to cause cancer in people.

Ionizing radiation splits into two families. Photons are packets of energy with no charge and no mass. X-rays and gamma rays are photons. Particles are matter, and there are three that matter here: alpha particles, beta particles, and neutrons.

X-rays and gamma rays behave almost alike, but differ in where they are born. X-rays come from processes outside the atomic nucleus. Fast electrons striking heavy atoms is the usual one. Gamma rays come from inside the nucleus, during radioactive decay. Medical X-rays typically carry 5 to 100 keV of energy. A few millimeters of lead will stop them. Gamma rays are stronger and go deeper.

The National Toxicology Program does not treat ionizing radiation as one entry. It lists five types or sources as known to be human carcinogens, across four separate listings:

• X-radiation and gamma radiation. These share one listing, added in the Eleventh Report on Carcinogens in 2004.
• Neutrons. Also first listed in 2004.
• Radon, including radon-220 and radon-222. These emit mainly alpha particles. Radon was first listed in the Seventh Annual Report in 1994.
• Thorium dioxide, which decays by alpha emission. It was first listed in the Second Annual Report in 1981.

IARC reached the same place from a different direction. Its Volume 100D covers radiation. Volume 100 exists to re-review every agent already in Group 1, carcinogenic to humans. Volume 100D takes in solar and ultraviolet radiation, X- and gamma radiation, and neutron radiation. It also covers radionuclides taken into the body that emit alpha or beta particles.

Four cancers carry the strongest link to X-radiation and gamma radiation. They are leukemia, thyroid cancer, breast cancer, and lung cancer. Links have been reported at absorbed doses below 0.2 gray.

Age at exposure changes which cancer shows up. Childhood exposure is mainly responsible for the raised leukemia and thyroid cancer risk. Exposure during reproductive years drives the breast cancer risk. Some evidence suggests lung cancer risk relates most strongly to exposure later in life.

A second tier of cancers has been reported, usually at higher doses near 1 gray. That tier is salivary gland, stomach, colon, urinary bladder, ovary, central nervous system, and skin. Two studies pointed to liver cancer above 100 millisieverts. One covered workers at a Russian nuclear weapons and fuel reprocessing plant. The other covered Japanese atomic-bomb survivors, whose liver cancer risk rose in a straight line with dose. A study of children given radiation for non-cancer reasons found some evidence of raised lymphoma and melanoma.

Animal data line up. X-rays and gamma rays caused cancer in every species tested. They produced tumors at 17 or more tissue sites, including the four seen in people.

Radiation dose has three layers. Mixing them up makes numbers meaningless.

Absorbed dose is energy deposited per unit mass of tissue. The unit is the gray, equal to 1 joule per kilogram. The older unit was the rad; 1 Gy equals 100 rads.

Absorbed dose alone is not enough. The same energy does more damage when it arrives in dense tracks. Alpha particles and neutrons are high-LET radiation. They lay down energy along a short, concentrated path. X-rays and gamma rays are low-LET and scatter their energy more widely. So equivalent dose multiplies the absorbed dose by a radiation weighting factor. The International Commission on Radiological Protection sets that factor. It runs from 1 for low-LET radiation up to 20 for high-LET radiation. The unit is the sievert, and 1 Sv equals 100 rem. The factor is 1 for X-rays and gamma rays, so their absorbed and equivalent doses match.

Effective dose goes one step further. It weights each organ by how much harm a given dose does there, then adds those up. It is also reported in sieverts.

For all types of ionizing radiation combined, natural sources contribute over 80% of human exposure. Radon and its decay products account for roughly two-thirds of that natural share. The remaining third comes from cosmic radiation, terrestrial radiation, and radionuclides already deposited inside the body.

Everything humans added comes to less than 20%. Medical procedures make up about 15%. Consumer products add about 3%. Everything else is under 1% combined, including work exposure, nuclear fallout, and the nuclear fuel cycle.

The worldwide average annual effective dose from natural background was put at 2.4 mSv. The range ran from 1 to 20 mSv, depending on where a person lives. Average medical diagnostic exposure was 0.4 mSv, ranging from 0.04 to 1 mSv. In countries with the most physicians per person, the average dose from medical X-rays reached about 1.2 mSv. That is roughly half the natural background figure.

Not all imaging is equal. Radiation therapy, computed tomography, and positron emission tomography deliver higher doses than a standard diagnostic X-ray. So do procedures guided by fluoroscopy. Radiation therapy beams are typically high-energy X-rays of 4 to 50 MeV, or cobalt-60 gamma rays.

Among workers whose exposure is not individually monitored, about 5 million people worldwide take in more than natural background. About 75% of them are coal miners, at an estimated 1 to 2 mSv a year. Another 13% are other underground miners, at 1 to 10 mSv. About 5% are airline crews, at up to 3 mSv. Miners get theirs mostly by breathing radon, so mostly as alpha particles. Airline crews get gamma radiation and neutrons.

Nuclear Regulatory Commission rules set hard numbers. Under 10 CFR 20.1201, the annual dose for an adult worker is capped at whichever of two figures is more limiting. One is a total effective dose equivalent of 5 rem, which is 0.05 Sv. The other is 50 rem to any single organ or tissue other than the lens of the eye. The lens has its own annual limit of 15 rem. Skin of the whole body, or of any extremity, is capped at 50 rem.

A separate rule, 10 CFR 20.1208, covers pregnancy. Once a worker declares a pregnancy, the dose to the embryo or fetus is limited across the whole pregnancy. The cap is 0.5 rem, which is 5 mSv. The employer also has to avoid bunching that exposure into a short stretch.

X-rays and gamma rays cause a wide spectrum of genetic damage. The list runs from gene mutations and micronucleus formation to chromosomal aberrations, changes in chromosome number, DNA strand breaks, and lasting chromosomal instability. This has been seen in people exposed by accident, at work, and in the environment. It has also been seen in laboratory animals and in cultured human cells.

DNA can be hit two ways. Radiation can strike the molecule directly. Or it can split water inside the cell into free electrons, hydrogen radicals, and hydroxyl radicals. Those then attack DNA. Most of the damage seen traces back to errors made while repairing the harm, rather than to the first break.

Three stranger mechanisms are proposed for damage that shows up late, or in the wrong place. One is lasting genomic instability. One is mutation triggered by irradiating only the cytoplasm rather than the nucleus. The third is the "bystander effect," where neighboring cells that were never hit sustain genetic damage through cell signaling.

For the largest single source of everyday exposure, see radon and cancer. For the scans and their doses, see medical imaging radiation and cancer, and for one specific treatment exposure, radioactive iodine and cancer.

• NCI — Radiation
• NTP Report on Carcinogens, 15th edition — Ionizing Radiation profile
• IARC Monographs Volume 100D — Radiation
• govinfo — 10 CFR 20.1201, occupational dose limits for adults
• govinfo — 10 CFR 20.1208, dose equivalent to an embryo/fetus