In last week’s article, we discussed the role of scans in the management of cancer and focused on two types of scans—CT scans and MRI scans. This week, we will address the third major type of scan utilized in oncology—the PET scan.
On some level, it is useful to think of a PET scan (or positron emission tomography) as a form of nuclear medicine since it utilizes radioactive substances (radiotracers) to visualize organs and metabolic activity. These radiotracers are injected prior to the procedure, circulate in the bloodstream, and then emit photons or other forms of radiation which are detected by the scanner to form images. The exact radiotracer that is utilized for a PET scan depends on the purpose of the scan, as the tracers may be metabolized differently by different organs, such as bone or lung.
While some of the theoretical concepts surrounding these devices go back as far as the 1950s, the first practical PET scanners were developed in the 1970s, but really went into widespread use in the 1990s. The PET scan was named by Time magazine as the medical invention of the year in 2000.
For the purposes of oncology and cancer detection, the radiotracer that is utilized in PET scanning is 18F-FDG. FDG is an analogue of glucose that is preferentially taken up by malignant cells that are rapidly growing, and thus this tracer is concentrated in these malignant cells and therefore in these tumors. The 18F is a radioactive isotope of fluorine which generates the radiation that is used for the imaging of the PET scanner. About 90% of PET scans conducted nowadays utilize this radiotracer. The exposure to radiation that is caused by the radiotracer utilized in the scan is considered trivial and very low risk to the patient from the point of view of future carcinogenesis.
Most cancers will have a high glucose uptake due to what is known as the Warburg effect. (We have discussed this phenomenon previously.) This also applies to metastases. Thus, a PET scan will show intense radiolabeling of these tumors as compared to benign tissues or tumors and can distinguish benign from malignant.
Most PET scans are now conducted in combination with CT scans. This provides a better sense of the anatomic structure (from the CT scan) in conjunction with the metabolic information generated by the PET scan, so the two types of data are viewed together. Some newer methodologies are doing PET scans in conjunction with MRI scans for similar purposes.
PET scans are mainly utilized as follow-up for evaluation of lesions observed on prior CT scans in order to clarify benign from malignant disease. They may also be chosen for use when a CT scan cannot be done, for example when the patient is allergic to the contrast dye used for CT scans or has renal failure, which also precludes the use of the contrast dye.
PET scans do tend to be less accurate for slow-growing or less-active tumors since they work as a function of glucose uptake. They also are less accurate for small tumors, e.g., less than 8 or 10 mm.
Why are PET scans not utilized upfront as the initial radiological scans for diagnosis, staging and surveillance? On a number of occasions I have had patients requesting (or demanding) that I order PET scans in lieu of CT scans, thinking that they were superior. The main side effect of a PET scan is its cost. While CT scans typically run a few hundred dollars and don’t normally get much pushback from insurance companies, PET scans nowadays run above $5,000 or even more and usually get denied by insurance companies unless very well justified.
The truth is that for most purposes, the CT scan or MRI scan are perfectly adequate and the PET scan is unnecessary. There are some specialized situations or settings for which a PET scan is very useful and there may be some settings in which CT scans cannot be used. Under those circumstances, the high cost of the PET scan can be justified.
The bottom line is that all three types of scans (CT, MRI, PET) have made major contributions to cancer diagnosis, staging and follow-up and have greatly improved the care of most cancer patients. Other modalities also contribute—ultrasound, nuclear scans, etc.—but these three tests have revolutionized the day-to-day management of cancer patients and almost surely improved survival as well.
Alfred I. Neugut, MD, PhD, is a medical oncologist and cancer epidemiologist at Columbia University Irving Medical Center/New York Presbyterian and Mailman School of Public Health in New York.
This article is for educational purposes only and is not intended to be a substitute for professional medical advice, diagnosis, or treatment, and does not constitute medical or other professional advice. Always seek the advice of your qualified health provider with any questions you may have regarding a medical condition or treatment.
