Nuclear medicine in oncology

Dr Lizette Louw educates us about nuclear medicine, a molecular imaging and therapy technique, and its use in oncology.

What is nuclear medicine?

Nuclear medicine is a specialised area of medical imaging that uses small amounts of radioactivity to examine organ function. The radioactive atom is linked to a specific molecule, depending on which organ system will be imaged. This radioactive molecule, called a radiopharmaceutical or radiotracer, then images a physiological process in the body, without interfering with the physiological process.

Understanding radiation

An unstable atom is called radioactive. The atom gives off energy to stabilise itself. Some atoms give off energy from the outer electron shell, called X-rays, and are used in X-ray and computerised tomography (CT). 

Other atoms give off energy from the nucleus, called gamma rays, and these are the ones used in nuclear medicine. This is where the name nuclear medicine comes from. 

Both gamma rays and X-rays are part of the electromagnetic spectrum. These are called ionizing radiation as they have sufficient energy to cause ionization within the body, which in high enough doses can cause harm to the body. 

The amount of radiation used in medical imaging is insufficient to cause harm. However, cumulative exposure to radiation may cause harm. Therefore, medical radiation imaging is kept to a minimum as far as possible. 

Other types of radiation on the electromagnetic spectrum (e.g. microwave rays, TV, radio or even cellular phone rays) don’t generate high enough energy to cause ionization and as such can’t cause cellular damage or cause cancer, despite what mass media tries to scare us with. 

In radiation oncology therapy, the same ionizing radiation is used, but in much higher amounts than used in imaging, as the very purpose of radiation therapy is to use the radiation damage as a therapy form.

How is nuclear medicine different from an X-ray or CT scan?

Nuclear medicine is also called functional imaging, as the function of an organ or organ system is imaged. Radiology is called structural imaging, as it images the size, shape and structure of various body parts. 

When an organ or organ system is affected by disease, the function is affected first, before any structural changes take place. As such, nuclear medicine can detect certain abnormalities before X-ray or CT scans can. For example, cancer that has spread.

In nuclear medicine, the radiation is inside the patient and the camera merely captures the image but doesn’t give off any additional radiation. 

In X-ray or CT, the camera itself emits the radiation, which then moves through the patient, and the image captures how that radiation moved through the body, slowed down differently as it moved through bone, organs, soft tissues, etc.

How are nuclear medicine images captured?

In nuclear medicine, the radiotracers are either injected into your bloodstream, inhaled or swallowed. The radioactive atom gives off radiation, that is then detected by specialised cameras and converted into an image. 

The amount of radiation used in general nuclear medicine scans is too low to cause any significant harm. In fact, most general nuclear medicine scans use less or similar radiation amounts than most X-ray examinations. 

The radioactivity decreases in strength over time, called decay. 

The most common radioactivity used is Technetium, which has a half-life of six hours. That means after six hours, only half of the original activity will be left over. After four to six half-lives, there is negligible amount of radiation left over. This is also why patient injections are freshly prepared every day and not available “off the shelf” at any time. 

Nuclear therapy as a cancer treatment

Nuclear medicine can be used for therapy as well, specifically for thyroid disease, e.g. overactive thyroid; thyroid cancer; neuroendocrine cancers; and recently prostate cancer. 

What types of nuclear medicine scans are done in oncology?

Bone scan

This is the most common scan done in oncology. A phosphate derivative is used and is linked to a radioactive atom, called Technetium. 

When cancer spreads to bone, the bone tries to heal itself and this causes increased bone formation at that site. Phosphate is used in bone formation and thus the radiolabelled phosphate will go to sites of increased bone formation and show up as bright spots on the bone scan. 

We also look at the pattern of bright spots to determine if they are due to joint inflammation or truly cancer that has spread to the bone. 

There is no specific preparation before the scan, although an appointment needs be made in advance, as the injection needs to be specially ordered and prepared for each patient. 

Images of the whole body are taken three to four hours after injecting the radiolabelled phosphate, called MDP. 

PET/CT

The word PET stands for positron emission tomography. Positron is a specific type of gamma radiation. For this scan a different type of camera is used than for other nuclear medicine scans. 

Three different radiotracers

The PET scan is combined with a CT scan, hence the name PET/CT. Three different radiotracers are available for PET/CT scans, namely a form of radioactive glucose (FDG), DOTATATE (only used to image and treat neuroendocrine tumours), and PSMA (only for prostate cancer). 

PSMA PET/CT can detect very small lesions, before they are big enough to appear abnormal on CT. It can even detect small lesions in bone before those lesions show up on a bone scan. Currently, PSMA PET/CT is only done in patients with high-risk prostate cancer, or in patients already on hormone treatment but with rising levels of PSA.FDG PET/CT is the most common PET/CT radiotracer. Most cancers use glucose as an energy fuel, and they use much more glucose than normal body tissues. Using radioactive glucose enables us to pick up all the spots of abnormal high glucose use. Often small lesions are identified that are not yet big enough to appear abnormal on a CT scan and would therefor easily be missed. 

With a PET/CT scan the whole body is imaged, whereas often only the chest or abdomen is imaged with a CT only scan. 

The half-life of FDG is only two hours, so it’s important to be on time for your PET/CT appointment. It also means the radiation is cleared rapidly from the body and by the evening there will be negligible amounts of radioactivity left in your body. 

To avoid normal glucose use in the body competing with the cancer spots for the FDG, the patient must avoid sugary drinks and foods the day before and must fast from the night before the scan. 

Patients also mustn’t exercise the day before their scan. This is to avoid the muscles using more glucose.

Final thought

Nuclear medicine is a highly-specialised and rapidly evolving field of medical imaging and therapy. Because it can detect early functional changes before structural changes take place, it forms an essential part of cancer imaging for initial staging, treatment planning, as well as early identification of cancer recurrence or spread.

Radiation in medical imaging is not something to fear, but it should be used responsibly.