Dr. Tsubokura's Radiation Lecture Vol.131

Author: Masaharu Tsubokura PhD.

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

261. Using glucose to diagnose cancer

There are 2 types of radiation exposure: external and internal exposure. Previous articles have introduced 2 types of radiation therapy for malignant cells. The first type of radiotherapy is external radiation therapy, which uses an external device placed outside of the body that aims high-energy rays at cancer cells. The second type of radiotherapy is internal radiation therapy, also called brachytherapy, which directs radiation at cancer cells with the assistance of an encapsulated radioactive material that is ingested. Similarly, there are also 2 methods to deliver radiation in radiographic examinations. For instance, X-rays and CT scans are examples of radiation diagnostic techniques using an external radiation source. One example of radiographic techniques using an internal radiation source is a PET (positron emission tomography) scan. A PET scan is an imaging test that administers glucose labeled with a radioactive isotope. Once it is ingested into the body, the radioactive glucose emits a small amount of radiation, and then its concentration and location are detected using a PET scan. Generally, ingested glucose molecules spread throughout the body via the bloodstream and are absorbed by the organs. However, cancer cells consume glucose to produce energy at a much higher level than normal cells. Therefore, PET scans can detect the malignant cells’ size and location by taking advantage of their feature of absorbing glucose at a higher rate. The patient will receive internal exposure from the ingested radioactive materials. However, since the half-life of the radioactive materials used in this examination is approximately 2 hours, their radioactivity will become undetectable in a few days.

262. Combined method of internal and external radiation for radiographic inspection As with radiotherapy used to treat cancer, internal and external sources of radiation are used in radiographic examinations routinely conducted at hospitals. For instance, X-rays and CT scans are examples of radiation diagnostic techniques using external radiation sources. On the other hand, a PET scan uses an internal radiation source to deliver radiation. A PET scan is an imaging test used to detect the location and size of cancer cells using glucose labeled with a radioactive isotope. Since the radioactive glucose used in PET scans has a short half-life of only about 2 hours, it is generally prepared within the hospital. A PET scan also takes a CT scan to assess the exact location of the tissue, which is absorbing the radioactive glucose at a higher rate. As a result, the patient will receive approximately 3.4 millisieverts of internal radiation from the injected radioactive glucose and about 10 millisieverts of external exposure from the CT scan. This accounts for several years of the average radiation exposure people receive from naturally occurring background radiation. The extent of the exposure dose determines the impact of radiation on health. It is unnecessary to search for the specific amount of radiation exposure received from X-ray, CT, or PET scans, but having a rough idea of how many digits of radiation exposure a patient will receive from a particular situation is enough to understand the safety and potential health effects of these scans.

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The Japanese version of the manuscript was originally published in Fukushima Minyu, a local newspaper in Fukushima prefecture, Japan, 19th and 26th 2020 was reproduced for MRIC Global under the author's permission.