Possible positive effects of ionising radiation - hormesis

"Bio-positive" effects, observed sporadically in the low dose range, include among other things

• the acceleration of growth or development processes,
• an increased cellular survival rate through the stimulation of repair processes as well as
• the reduced sensitivity of cells to high radiation doses after pre-irradiation with low doses ("conditioning", also referred to as "adaptive response").

Hormesis is also used to justify the radiotherapy of inflammatory conditions, stimulative irradiation as well as the treatment of rheumatism in radon (health) mines containing radon gas. The terms "hormesis" and "adaptive response" are often used synonymously.

Contradictory data from studies and trials on hormesis

Knowledge on the hormetic effects of ionising radiation in the low dose range in humans can only be gained through powerful epidemiological studies. Population groups exposed to different exposure levels due to natural background radiation are of particular interest in this context. However, the data gathered worldwide on this subject are contradictory; the same applies to data from animal studies.

Cell biology investigations show detailed research results on possible mechanisms of action. Most of these investigations were performed in an artificial environment outside the human body ("in vitro") using human lymphocytes (cellular components of blood).

Endpoints of these investigations are, for example, alterations in the number or structure of chromosomes after irradiation. In some trial series, such "aberrations" occurred significantly less often in pre-irradiated lymphocytes than in cells not pre-irradiated.

Adaptive response

Further studies have demonstrated that "adaptive response" may be induced by both acute and chronic radiation exposure. "Adaptive Response" effects were also found for other endpoints than aberrations, such as for the survival, for mutations as well as for the transformation of the cells. These effects could also be observed for other cell systems, for example, for the precursor cells of human connective tissue cells (fibroblasts) or also for breast cancer cell lines.

Significance for radiation protection

From the point of view of the most important international panels such as the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), the International Commission on Radiological Protection (ICRP) or the Advisory Committee on the Biological Effects of Ionizing Radiation (BEIR), the results of these studies are insufficient to deviate from the assumption of the potentially detrimental effects of ionising radiation in the form of a linear no-threshold dose-response relationship in radiation protection. This assumption is based on the observations of population groups exposed to radiation and refers to the risk of developing or dying from radiation-induced cancer. It is striking that the results of such investigations on large exposed groups in the low-dose range are commonly best described by a linear no-threshold dose-response relationship. Effects in cellular systems on a laboratory scale, however, often show dose-response relationships diverging from linearity.

Possible positive effects of ionising radiation refer to individual cases and must not be transferred to the population. Applications in this sense, such as radon balneotherapy for rheumatic diseases, should only be prescribed by a physician upon strict indication after balancing the individual risks and benefits based on his assessment.