This was going to be a reply to a comment on one of my posts. However, I think it is an important issue, so I am writing this post.

The comment referenced a study¹ on DNA repair centres in human cells. I have no problem with the study itself, however, I do with one of the conclusions:

“However, our discovery of DSB clustering over such large distances casts considerable doubts on the general assumption that risk to ionizing radiation is proportional to dose, and instead provides a mechanism that could more accurately address risk dose dependency of ionizing radiation.”

This idea of DNA repair is nothing new – I remember reading some similar research on DNA repair centres in bacteria many years ago. While this research is looking at repairing the DNA before any cancers begin, the human body also has defences against cancers once they form – an idea first postulated by Paul Ehrlich over 100 years ago and supported by more recent research. It is unclear what the relative affect these different mechanisms have on the bodies self-defence against cancer.

If such mechanisms did not exist, then the rates of cancer deaths would be much higher. Such mechanisms also explain why there is not a direct casual link between radiation exposure or smoking and cancer. Radiation increases your risk of getting cancer. A person may be exposed to a high radiation level or smoke 20 a day and not get cancer – someone else who is not exposed or does not smoke will.

There are many other risk factors as well including stress, amount of exercise, exposure to carcinogenic chemicals, number of friends and close family etc. It is an accumulation of these factors which overwhelms our defences and results in cancer.

Although DNA damage is undoubtedly an important factor in cancer development, it is probably not the only one.

Therefore, the effect of ‘DSB clustering’ does not bring into question the effect of radiation at low doses, which many studies have shown^{2,3,4,5,6,7,8,9,10,11,12,13}. This is one of many mechanisms that need to be understood to address risk dose dependency of ionizing radiation.

Not only are there difference cancer defences and mechanisms for the formation of cancer that need to be considered but also the different types of radiation. X-rays and ?-rays which are energetic light waves and have zero mass may have very different effects from ?-particles which are heavy and doubly charged. Different elements will bind to different parts of the body, so Uranium will have a different effect from Iodine or Strontium. A radioactive dust consisting of hundreds of short-lived ? emitting atoms will deliver an acute dose to a small area, whereas a long lived ? emitter will give a chronic dose to the whole body. When we have a decent theoretical understanding of these and their associated mechanisms, it may at some point (way into the future) be able to have some better theoretical understanding of the dose dependency of ionizing radiation. Until then, we must rely on the epidemiological studies referenced above.

¹ Evidence for formation of DNA repair centers and dose-response nonlinearity in human cells, Neumaier et al, Proceedings of the National Academy of Scinces of the United States (http://www.pnas.org/content/109/2/443.full.pdf)

The following references are thanks to Ian Fairlie (www.ianfairlie.com)

² Cardis E et al (2005) Risk of cancer after low doses of ionising radiation: retrospective cohort study in 15 countries. BMJ 2005;331:77.

³ Dale L. Preston, Yukiko Shimizu, Donald A. Pierce, Akihiko Suyama, and Kiyohiko Mabuchi (2003) Studies of Mortality of Atomic Bomb Survivors. Report 13: Solid Cancer and Noncancer Disease Mortality: 1950–1997. Radiation Research: October 2003, Vol. 160, No. 4, pp. 381-407.

⁴ Darby et al. Radon in homes and risk of lung cancer: collaborative analysis of individual data from 13 European case-control studies. BMJ 2005;330:223.

⁵ Doody MM et al Land CE for the US Scoliosis Cohort Study Collaborators. Breast cancer mortality following diagnostic x rays: Findings from the US Scoliosis Cohort Study. Spine 25 (2000): 2052-2063.

⁶ Jacob P et al. Childhood exposure due to the Chernobyl accident and thyroid cancer risk in contaminated areas of Belarus and Russia. British Journal of Cancer 80.9 (1999): 1461.

⁷ Leuraud, Klervi et al (2015) Ionising radiation and risk of death from leukaemia and lymphoma in radiation-monitored workers (INWORKS): an international cohort study. The Lancet Haematology Published Online: 21 June 2015. (http://www.thelancet.com/journals/lanhae/article/PIIS2352-3026%2815%2900094-0/fulltext)

⁸ Noschenko et al. 2001. Patterns of Acute Leukemia Occurrence Among Children in the Chernnobyl Region. Intl J of Epidemiology 30, 125-129.

⁹ Ron E and Schneider AB. Thyroid cancer. Cancer epidemiology and prevention 3 (1996): 975-994.

¹⁰ Sont WN, Zielinski JM, Ashmore JP, Jiang H, Krewski D, Fair ME, et al. 2001. First analysis of cancer incidence and occupational radiation exposure based on the National Dose Registry of Canada. Am J Epidemiol 153:309-318.

¹¹ Stevens et al Leukemia in Utah and Radioactive Fallout From the Nevada Test Site: A Case-Control Study. JAMA. 1990;264(5):585-591. doi:10.1001/jama.1990.03450050043025.

¹² Stewart A, Webb J, Giles D, and Hewitt D (1956) Malignant disease in childhood and diagnostic irradiation in utero. Lancet 271, 447.

¹³ Zablotska et al (2012) Radiation and the Risk of Chronic Lymphocytic and Other Leukemias among Chornobyl Cleanup Workers. Environmental Health Perspectives. http://dx.doi.org/10.1289/ehp.1204996 Online 8 November 2012.