The story of fluorine (F) in connection with IYPT 2019

The year 2019 marks the 150th anniversary of the discovery of the periodic law of the elements by a Russian chemist named Dmitri Mendeleev in 1869. The UNESCO and UN therefore established the International Year of the Periodic Table of Chemical Elements (IYPT 2019), so various types of events linked to this discovery are being held throughout the world at this time.

Even in Japan, through the campaign (website) called Our Elements,we can see essay contests held for junior high school, high school, and college students who will be the scientists of the future and also messages from universities and corporations.

This article takes up the topic of fluorine (symbol F) as an element from the periodic table of chemical elements in connection with the International Year of the Periodic Table of Chemical Elements and also gives a description of how it’s used in medical diagnosis.

Cancer is the number one cause of death among all diseases for both men and women in Japan. However, medical treatment is making progress and cancer is a disease that can be cured. The most important thing about cancer treatment is finding it while it is still small. An effective diagnostic technique for finding a small cancer is called PET (positron emission tomography) that makes use of the characteristics of fluorine and light.

The so-called “PET cancer screening” takes advantage of the properties of cancer cells to find a small cancer that was very difficult to detect up to now. The cells of the human body utilize the glucose produced from food and elsewhere as an energy source for daily activities. Cancer cells, however, are characterized by taking in large amounts of glucose compared to normal cells. This means if we can find points with high concentrations of glucose then we can detect cancer.

But, how do we find glucose in a living human body and even glucose concentrated in the tiny cells in the body?

PET diagnosis does this by way of a test drug called FDG. This FDG has nearly the same properties as glucose except that it contains atoms that emit light called gamma rays. So this FDG is a drug with a “marker of light” that is put on the favorite energy source of the cancer cells.

When this FDG is injected into a human body, the cancer cells ingest a large amount of FDG in the same way as they take in glucose, and emit gamma rays. By detecting and measuring these gamma rays with high-sensitivity light sensors placed outside the body, it is possible to search out concentrations of FDG, in other words, the locations where cancer is probably present.

So FDG in this way lets us know where cancer cells are present by emitting gamma rays that serve as a marker of light. But why does FDG emit gamma rays? To find the answer, we have to know what FDG is made from.

The correct name for FDG is ¹⁸F-FDG. The FDG is an abbreviation for fluorodeoxyglucose which is its full name in English. The F stands for fluoro (has fluorine), D stands for deoxy (takes in oxygen), and G stands for glucose. The name of the chemical in this way indicates its structure and the name also gives a hint that FDG resembles glucose.

So what is ¹⁸F anyway? This ¹⁸F apparently holds the secret behind how it works as a “marker of light.” Actually, this ¹⁸F is what is called an “isotope” of the fluorine and this isotope is heavily involved with the phenomenon of gamma ray emission. Now let’s dive a little deeper into this topic, though it will be a little more specialized.

Some isotopes immediately change into another element and are called unstable isotopes. The ¹⁸F is also one of these unstable isotopes called radioactive isotopes. The isotope ¹⁸F emits positive electrons (called positrons which are particles having a positive charge and the same properties as an electron) and changes into ¹⁸O which is a stable isotope of oxygen. The positrons emitted at this time then bind to the surrounding electrons and emit light called gamma rays. In PET diagnosis these gamma rays are measured to search out cancer cells.

The “PET” in PET diagnosis is capital letters expressing “positron” for P, “emission” for E, and “tomography” for T.

The drawing below shows the reaction where ¹⁸F changes into ¹⁸O. A proton inside the ¹⁸F emits a positron and changes into a neutron. The ¹⁸F in this way loses 1 proton and the atomic number (or number of protons) decreases by one. In other words, the fluorine changes into oxygen that is one position to the left on the periodic table. The protons decrease but the neutrons increase, so there is no change in the mass number (total number of protons and neutrons).

A unique feature of cancer cells is that they increase more rapidly than normal cells. This means they need large amounts of nourishment (glucose) and so take in more glucose and ¹⁸F-FDG that is an analogue of glucose.
Images captured by a PET camera show a lot of gamma rays (light) being emitted from cancer cells that have taken in large amounts of ¹⁸F-FDG. Cancer cells are detected by comparing these images with normal images and normal cells.

These are images of breast cancer (stage 1) detected early by PET screening. The small points shown by the orange arrows are FDG accumulations. By combining PET images with the images of the body structure such as CT images, even more information including the form, position and functioning (how the cells are functioning) of the cancer can be obtained to help detect cancer in its early stage.

Everything in our world is made up of a mere 118 elements. By discovering the wonderful principles of the periodic law, we have come to understand that the elements of the earth are finite and also how natural processes work.
As mentioned at the start, this year is International Year of the Periodic Table of Chemical Elements. Please take this opportunity to have fun and learn about the interesting elements, element symbols and the periodic table, and think about how we make use of these elements in our daily lives!