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Dr. Tsubokura's Radiation Lecture Vol.109

Author: Masaharu Tsubokura PhD.

Editors: Akihiko Ozaki M.D.,PhD., Yuki Senoo

117. Bone marrow is a radiosensitive organ

Soft and spongy tissue called bone marrow is present within human bones. It functions as a factory to produce blood-forming cells. The condition in which the factory can no longer produce normal blood-forming cells is called leukemia. Therefore, leukemia is categorized as a type of blood cancer. 

Along with certain viral infections and the health effects of carcinogens and drugs, exposure to high-dose radiation is known to increase the risk of leukemia.

 Unfortunately, a marked increase in the number of deaths due to radiation-associated stomach, lung, colorectal, and breast cancers, as well as leukemia, was observed among atomic bomb survivors of Hiroshima and Nagasaki. On the other hand, there was no increase in the incidence of pancreatic, uterine, prostate, and malignant lymphoma cancers.

One of the reasons the association between radiation exposure and the risk of leukemia is currently well studied is because of the radiosensitivity of bone marrow. When the same dose of radiation is applied to organs and bone marrow, the effect on bone marrow is greater than on other organs. However, the number of patients diagnosed with leukemia is much lower than the number of patients diagnosed with other types of cancer. Therefore, the total number of radiation-associated leukemia cases after the bombing of Hiroshima and Nagasaki was also lower than that of other types of cancer.

Of course, the level of radiation exposure that would increase the risk of developing cancer is an order of magnitude higher than that of external radiation exposure caused by the Fukushima Daiichi Nuclear Power Plant accident in 2011. Consequently, an increase in the incidence of leukemia is not expected from the radiation exposure caused by this nuclear accident.

118. The exact cause of cancer is undetectable

Bone marrow is like a factory where blood-forming cells are produced.

A malfunction in the factory production system is called leukemia. The previous study reported an increased number of deaths due to both leukemia and other types of cancer including stomach, lung, colorectal, and breast cancer among atomic bomb survivors of Hiroshima and Nagasaki.

However, the radiation emitted by the atomic bombs was not the cause of all cancer cases among atomic bomb survivors who were exposed to high-dose external radiation. This is because a certain proportion of the population will always develop cancer.

 However, with current technology and knowledge, we cannot yet detect the exact cause of cancer, and there is no method to clearly distinguish between cancer caused by radiation and cancer caused by other risk factors. This means that when a patient is diagnosed with cancer, we cannot determine whether he or she developed cancer due to radiation exposure. On the other hand, we are able to compare the cancer incidence among people who were exposed to radiation and those who were not.

For example, during a 40-year study of 40,000 people who received a certain level of radiation exposure, 8,000 people were diagnosed with cancer other than leukemia in Hiroshima and Nagasaki. It was estimated that this number was larger by approximately 800 people compared with the number of those who developed cancer among the population that was not exposed to radiation.


The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 10th and 17th March 2019 was reproduced for MRIC Global under the author's permission.

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