Dr. Tsubokura's Radiation Lecture Vol.69
Author: Masaharu Tsubokura
Editors: Akihiko Ozaki M.D., Yuki Senoo
137. Radiation therapy takes advantage of the difference in the deoxyribonucleic acid (DNA) damage repair speed between cancer cells and normal cells.
Today, surgery, chemotherapy, and radiation therapy constitute the standard for cancer therapy. Radiation therapy is a procedure that exposes cancer cells to radiation, and it has been proved to be no less effective than surgery or chemotherapy is.
Similarly to surgery, radiation therapy is categorized as a local therapy, in which the procedure is directed only at cancer lesions, partly affecting their surrounding tissues. Although an appropriate radiation dose differs among cancer types based on their locations and susceptibility to radiation, it is normally fragmented from several times to several tens of times for a few minutes of radiation therapy. In a radiation therapy, such a short period is scheduled five days per week for several weeks in a row.
Why is radiation therapy designed in such a way? This is because normal cells are better able to repair the damage caused by radiation than malignant cells are. In other words, cancer cells are not able to fully recover from the damage caused by radiation therapy during the interval in which normal cells do so. By repeatedly irradiating the cancer cells before they fully repair the damage, we can predominantly destroy cancer cells while minimizing the damage to the surrounding normal cells.
In this respect, radiation therapy is a treatment that takes advantage of the difference in how quickly normal cells and malignant cells repair from the damage caused by radiation.
138. The important role of oxygen in radiotherapy
In cancer treatment, radiation therapy has been demonstrated to be no less effective than surgery or chemotherapy is. Radiation therapy is fragmented from several times to several tens of times for a few minutes. There are reasonable explanations for why radiation therapy is designed in such a way.
In fact, the susceptibility to radiation differs even among cancer cells in a single cancer lesion. For this reason, even when a high level of radiation is delivered to these lesions, the destruction of all cancer cells is not achievable with a single therapy, as some parts of the lesions are not susceptible to the radiation therapy.
It is known that the susceptibility to radiation differs depending on the oxygen level of the cancer lesion. When cancer cells are well oxygenated, the effect of radiation is amplified, and its damage will be two to three times larger compared with cells without oxygen. This is because a chemical reaction between oxygen and radiation creates a harmful reactive oxygen, which ionizes and damages the DNA of the cancer cells.
Cancer cells are well oxygenated when blood vessels are located nearby, whereas when they are apart from blood vessels, they do not receive enough oxygen. Nonetheless, when cancer cells close to the blood vessels are destroyed, these blood vessels will extend to reach another part of the cancer lesion. By gradually destroying the cancer cells from the part located close to the blood vessels, one may eventually destroy the central part of the cancer.
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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, on 27th August and 3rd September 2017 was reproduced for MRIC Global under the author's permission.