Cancer

Ionising radiation can cause cancer. Cancers are malignant neoplasms. Malignant tumours affect an organ and leukaemia is a disease of the blood-forming system that has an effect on the whole organism. Much the same applies to lymphomas, malignant neoplasms of the lymphatic system which also affect the whole organism.

Ionising radiation may cause cancer in various body tissues or organs

In principle, it can be assumed that ionising radiation may have a cancer-inducing effect on all body tissues or organs. According to UNSCEAR (2000 and 2006) there is sufficient epidemiological documentation for an increased cancer risk due to radiation for the following organs: oesophagus, stomach, intestine, liver, lung, bones, skin, female breast, bladder, brain, central nervous system, thyroid and throat. The same applies to leukaemias and various malignant neoplasms originating from the lymphoid tissue.

Until now there has been no epidemiological evidence for the radiation-dependent induction of Hodgkin's disease, a certain type of lymphoma. It was long assumed that chronic lymphocytic leukaemia (CLL) could not be induced by radiation. More recent epidemiological evaluations suggest that this is not the case. However, the relationship between radiation and CLL appears to be considerably different from the relationship between radiation and other types of leukaemia. The risk of CLL after radiation exposure is significantly lower than that of other leukaemia types.

Radiation-induced cancers can only be determined by statistical methods

Radiation-induced cancers do not occur until years or decades after the radiation exposure. The time between the exposure and the occurrence of a radiation-induced cancer is called "latency period". The clinical manifestations of radiation-induced and spontaneous diseases cannot be distinguished. Thus, they can only be determined by statistical methods if the incidence in a sufficiently large group of individuals is notably higher than in a comparable non-exposed group.

Latency periods vary for different types of cancer

Latency periods vary for different types of cancer. The shortest latency periods have been observed for radiation-induced leukaemias and thyroid cancer. With a radiation exposure during childhood, the shortest latency periods stated for leukaemias and thyroid cancer are two to three years. For other types of cancer latency periods are rather longer than ten years.

No threshold dose

No threshold dose is known for the radiation-induced risk of cancer. It is generally assumed that low doses can also increase the probability of cancer incidence in individuals exposed to radiation. The risk of disease increases with increasing dose.

Studies and investigations on the health effects of ionising radiation

Owing to epidemiological investigations on groups of individuals who, for various reasons, had been subjected to radiation exposure, extensive knowledge exists about the detrimental effects of ionising radiation.

The most important study is the one on the survivors of the nuclear bombings of Hiroshima and Nagasaki. To this day, it remains the foundation of radiation risk estimates in radiation protection.

Further knowledge was gained from investigations on groups of individuals who had been exposed to increased radiation for other reasons:

• through their occupation (radiologists, radium dial painters, employees in nuclear installations etc.),
• as patients (radiotherapy, X-ray diagnostics),
• individuals affected by nuclear weapons tests (for example in the Marshall Islands or in Kazakhstan), by radioactive contaminations from the nuclear facility Mayak in South Urals or the nuclear accident at Chornobyl.

Findings on radiation risks are collected and assessed regularly by the "United Nations Scientific Committee on the Effects of Atomic Radiation" (UNSCEAR) and by the International Commission on Radiological Protection (ICRP). In addition, extensive documentation on radiation risks is available from the International Agency for Research on Cancer (IARC) of the World Health Organization (WHO) and the National Research Council of the USA ("Committee on the Biological Effects of Ionizing Radiation", "BEIR" report).

The level of risk

The epidemiological study on the survivors of the nuclear bombings of Hiroshima and Nagasaki shows a linear dose-response relationship up to a dose of 3 sieverts (Sv) for all types of malignant tumours combined. By contrast, the dose-response relationship for leukaemia, is best described by a linear-quadratic function, according to UNSCEAR.

Based on the epidemiological data available, UNSCEAR estimates the average excess lifetime risk of mortality following an acute dose of 100 millisieverts (mSv) at 0.4 to 0.8 % for malignant tumours and at 0.03 to 0.05 % for leukaemia in its report from the year 2010. An excess lifetime risk of mortality of 1 % equates to one additional death per 100 people.

The probability of developing cancer is about twice as high as that of dying of cancer. To determine the risk of developing the disease, not of cancer mortality, the values have to be doubled accordingly.

Whereas the dose-response relationship for malignant tumours can be described as linear, for leukaemia a dose ten times lower (e.g. from 1,000 mSv to 100 mSv) leads to a 20 times lower risk of leukaemia.

What are the factors that determine the radiation risk?

The radiation risk level is mainly determined by the following factors:

• The dose: the higher the dose, the higher the risk.
• The radiation type: densely ionising radiation is more effective with the same energy dose than sparsely ionising radiation.
• The affected type of tissue or organ: the blood-forming system, breast tissue, stomach and colon are rather radiosensitive, bones, muscle and nerve tissue are rather less radiosensitive.
• Age at radiation exposure: children and adolescents are in general more sensitive than adults.

The varying radiosensitivity of various tissues can also be observed in the studies on the Japanese survivors of the nuclear bombings: the proportion of cancer cases that can be attributed to radiation of the total cancer incidence varies significantly in cancer localisation. The following table shows that, for the leukaemias (relating to all types except for chronic lymphocytic leukaemia and adult T-cell leukaemia) occurring in survivors of nuclear bombings, about 30 % are attributed to radiation exposure compared to about 7 % for stomach cancer.

Information according to Grant 2017, Hsu 2013, Preston 2007, Cahoon 2017 and Sakata 2019 (Radiation Research)