The study of biological and medical effects of ionizing radiation continues to be enigmatic. After over ten decades of intensive and extensive studies, specialists concluded that high doses of radiation may cause cancer in the exposed individual.
At low doses there is some uncertainty; however in the field of radiation protection, specialists assume that radiation doses at all levels are carcinogenic.
Evidence on links
Evidence on links between radiation exposure and non cancer diseases such as heart disease has emerged more recently.
For many years, scientists suspected these links. On October 7, 2010, the UK Health Protection Agency's (HPA) Advisory Group on Ionizing Radiation (AGIR) published a report titled “Circulatory Disease Risk”, reviewing the recently published epidemiological studies and experimental work on the risks and potential causes of circulatory diseases following exposures to ionizing radiation.
The report urged the clinicians who use medical radiation procedures in diagnosis and therapy to examine their working practices.
AGIR recommended that where possible they should keep the radiation doses to the brain and heart of the patients as low as possible while maintaining essential medical benefits (HPA Release, October 7, 2010).
The expert group also highlighted the need for further research to better understand the link between radiation exposure and circulatory disease.
The AGIR concluded that radiation exposure to the heart and circulatory system can occur in several contexts. For instance, the circulatory system of the entire population is exposed to a part of the natural back ground radiation. These are low levels.
“Radiation workers may receive higher doses and those receiving medical diagnostics, some medical interventional radiological procedures and, particularly, radiotherapy may receive doses to the circulatory system, or parts of it, up to the level of several gray (absorbed dose)”, the specialist group added
(gray-Gy- is a unit of absorbed dose; a tissue is said to receive one gray of dose, when the energy due to ionizing radiation absorbed by it is one joule per kilograme. AGIR defined doses thus: Very high – doses above 15 Gy; High – doses of 5–15 Gy; Medium –doses of 0.5–5 Gy; Low – doses below 0.5 Gy)
“Even small relative risks due to radiation could have a major impact…….. as circulatory diseases are already common in the population”, the specialists cautioned. For instance, circulatory diseases are common in Western populations and are the main cause of death in the UK, accounting for some 193,000, or 34 per cent, of deaths each year.
AGIR noted that evidence from radiotherapy follow-up studies and from experimental animal models indicates that irradiation at high and very high doses increases circulatory disease risk. But the use of cardio-toxic drugs in chemotherapy complicates the precise estimation of risk.
Clinicians have detected a statistically significant increase in the risk of certain circulatory diseases (notably, stroke, heart disease and specifically ischemic heart disease) at low and moderate dose (below 5 Gy) epidemiological studies, notably the atomic bomb survivor studies and nuclear worker studies.
While heterogeneity between the studies is considerable, statistically significant excess risk can be detected at around 0.5 Gy; contributory risk factors such as cigarette smoking, diet and alcohol consumption may confound these studies.
Convincingly strong association with, circulatory disease below doses of 0.5 Gy is considered to be very unlikely to emerge from human population studies in the near future. Insights from mechanistic experimental studies may eventually show whether cardiac diseases may be caused by low radiation doses.
According to AGIR, there is currently little evidence to justify the inclusion of circulatory disease while calculating radiation risk at doses of 0.5 Gy and below. This is a pointer towards more research. Radiation protection specialists can breathe easy.
AGIR concluded that it would be appropriate to incorporate circulatory disease risks when we estimate health risks to individuals exposed to doses above 0.5 Gy. This is a clear message to radiation oncologists. Dedicated use of Intensity Modulated Radiotherapy (IMRT), if available, may be useful.
The Association of Radiation Oncologists of India may review the HPA report and examine how best they may modify their practices wherever appropriate, to give maximum benefits to their patients.
K. S. PARTHASARATHY
Raja Ramanna Fellow, Department of Atomic Energy
( email@example.com )