Is Ionizing Radiation Causing Neurodevelopment Impairment, Even at Low Doses?

Marie Curie used to say “nothing in life has to be feared, only to be understood”. With her discoveries on radioactivity, she opened a big box full of things that need to be understood. Indeed, her discoveries helped us understand how ionizing radiation can be exploited in medicine and, today, thanks to it, a large number of people recover from cancer every year, doctors can substitute heart valves without surgery, and disease diagnosis is more precise and faster.

We need to better understand the risk from exposure to low levels of radiation doses, meaning below the threshold of 100 mSv

However, in the big box of things that need to be understood, there are also questions on the safety of ionizing radiation. Almost in parallel with the first application of ionizing radiation technologies in medicine and other industrial sectors, scientists started to characterize the health risk of ionizing radiation exposure. Today, we have in place a radiation protection system for workers, patients and for the general population based on the most advanced scientific evidence.

In the field of radiation protection, there are still questions that need to be addressed, so that we understand more and fear less. In particular, we need to better understand the risk from exposure to low levels of radiation doses, meaning below the threshold of 100 mSv which is the lowest dose clearly linked to an increase in cancer risk.

In fact, the general population is exposed to low levels of ionizing radiation coming mostly from natural sources (around 3.5 and 4 mSv per year) and eventually from the medical setting (for example around 0.02 mSv during a chest X-ray and 7 mSv during a chest CT-scan). Thus, the radiation protection research community is trying to study the health effects of such low doses, to which we are all somewhat exposed. The risk, if it exists, is likely to be very low, and therefore its detection will require large efforts. But it is, very important for our society.

Research community has started to explore effects other than cancer, including neurodevelopment

Traditionally cancer has been the most studied radiation-related health effect, but in recent years, the radiation protection research community has started to explore effects other than cancer, including neurodevelopment.

Neurodevelopment allows us to think, reason, learn, remember, coordinate our movement and behave in society. We know that cancer patients receiving cranial radiotherapy (more than 20.000 mGy to the cancer cells) may experience some neurodevelopmental deficit, which can be addressed by appropriate neuropsychological rehabilitation. However, we do not know if smaller amounts of ionizing radiation can also cause a certain degree of neurodevelopmental deficit and we have addressed this question by evaluating the strength of the current scientific evidence on this topic.

Computed Tomography Scan Unit at Sant Joan de Déu Hospital, Barcelona.[Foto: Sant Joan de Déu Hospital]

Evaluating the strength of scientific evidence means that scientists collect and critically read all studies that have been published to date around a specific topic. You can imagine that studies may have different results and methodologies, so the challenge is to interpret the different results together, in order to conclude if there is (or not) sufficient scientific data in support of a causal effect. This work is also important to identify gaps in the knowledge since, as you know, science advances with good questions!

The scientific evidence is currently limited to inadequate, meaning that even if some studies have shown an effect, a causal effect cannot be definitely established

We collected a total of 26 studies based on data from the atomic bomb survivor’s study, from patients exposed to radiotherapy or diagnostic radiation, from people exposed to the Chernobyl accident and to other environmental fallouts.

We found that the scientific evidence is currently limited to inadequate, meaning that even if some studies have shown an effect, a causal effect cannot be definitely established. Our work contributed to raising the debate around this issue, and the European Radiation protection community has identified this topic as a high priority for research (SRA MELODI).

The whole radiation protection system builds on the idea that doses should be kept as low as reasonably possible

Researchers are already involved in setting up studies to better understand this effect and ISGlobal is currently coordinating a study on neurodevelopment among childhood cancer survivors. In the meanwhile, even if risk at low dose is not fully characterized, the whole radiation protection system builds on the idea that doses should be kept as low as reasonably possible in order to protect the people from any possible increase in health risk.

Reference:

Elisa Pasqual, Magda Bosch de Basea, Mónica López-Vicente, Isabelle Thierry-Chef, Elisabeth Cardis. Neurodevelopmental effects of low dose ionizing radiation exposure: A systematic review of the epidemiological evidence; Environment International, Volume 136, 2020, 105371, ISSN 0160-4120, https://doi.org/10.1016/j.envint.2019.105371.