Positron Emission Tomography
A patient with lung cancer recently underwent a PET scan. The result confirmed that he has a solitary tumor that has not metastasized or spread to other parts of the body. The cancer is then ruled operable and thus, the patient was scheduled for the procedure.
In another instance, another patient with lymphoma recently underwent serial PET scans. The oncologist needed to know if the patient was responding to treatment. Since the multiple lymph nodes seen in the initial PET scan were no longer present in the succeeding scans, the doctor and the patient welcomed the news that the cancer had responded well to the treatment.
These are just examples of how PET imaging has been helping cancer or oncologic patients.
PET or Positron Emission Tomography is a nuclear medicine imaging modality used to show the molecular nature of diseases. The PET scan was initially used as a research tool in neurology and cardiology. It remains helpful in patients suspected of having Alzheimer’s disease and in patients with seizure disorders to localize an epileptic focus.
PET is also indicated in patients suspected of coronary artery disease and is considered the gold standard in the detection of viable myocardium prior to coronary artery bypass or cardiac transplant. Its greatest clinical impact, however, is in oncology.
To produce PET scan images, biological compounds, such as glucose or amino acids, labeled with positron-emitting radionuclides like fluorine-18 (F18) are needed. Once injected intravenously, these radiolabelled compounds or radiotracers enter biological processes in cells such as glucose metabolism.
The most commonly used radiotracer is F18 Fluorodeoxyglucose (F18-FDG), which enters cells through the glucose facilitated carrier system. F18-FDG is phosphorylated but does not undergo glycolysis and is trapped instead in the cell. Its F-18 label emits energy signals that are recorded by the PET camera. These signals, once reconstructed, localize areas of greatest glucose metabolism in the body. Since most types of cancers have cells with higher glucose metabolism, they accumulate and trap more F18-FDG than normal cells. Thus, PET images are helpful in giving doctors a record of where these hypermetabolic cancer cells are found.
Localization of cancer improved even more with the introduction of the PET-CT camera. A CT scanner was incorporated with the PET camera allowing patients to be imaged with both cameras in the same day and in the same position. Images from PET and CT are now fused for better disease evaluation.
The molecular images from PET benefit from the anatomic landmarks provided by the CT scan images, leading to more accurate localization of tumors. CT anatomic images also benefit from the molecular information from PET, specially since anatomic changes may lag behind molecular or metabolic changes. Now, PET generally refers to PET-CT scans as almost all cameras produced today are combined PET-CTs.
As cited in the two cases described above, PET helps in the staging of cancer as well as determining response of the patient to treatment. It helps detect primary tumors and metastatic tumors.
Benign tumors are differentiated from malignant ones based on the avidity of the lesion for F18-FDG. This may lead to changes in the patient’s cancer stage and subsequent changes in management. It is also helpful in treatment planning prior to radiotherapy.
PET is useful in monitoring treatment of cancers. It can help differentiate responsive from nonresponsive patients, in some as early as after the first course of chemotherapy.
Early identification of patients not responding to one form of treatment may avoid unnecessary toxicity. A patient’s response to treatment also gives clinicians information regarding the prognosis.
Detection of recurrent malignant disease is another indication for PET, particularly for patients whose anatomy may have been altered by therapeutic procedures done. PET is very helpful in distinguishing scarred tissue from recurrent disease.
PET scans are safe as it is a noninvasive imaging procedure. Patient preparation is mainly done by fasting several hours prior to the radiotracer injection. Additional preparation may be necessary for diabetic patients or if contrast media will be used for the CT part of the imaging.
Like most nuclear medicine procedures, a PET scan entails radiation introduced to the body. Fortunately, only a small amount of F18-FDG radiotracer needs to be injected to patients and it decays within a few hours.
PET scans can be done at the PET Center of St. Luke’s Medical Center, Quezon City, the first PET facility in the country. It has its own cyclotron operated by radiochemists and physicists to produce its own F18-FDG.
Its medical staff is composed of board certified nuclear medicine physicians and radiologists who have had extensive experience in performing PET imaging on a wide variety of cancers. They are ready to help and answer queries from doctors and patients interested in having PET scans done.
For more information, please call St. Luke’s Medical Center - Quezon City PET Center at tel. no. 7230101 ext. 4157/4158 or St. Luke’s Medical Center-Global City PET Center at tel. no. 7897700 ext. 1004/1152