PET/CT for Oncologic Interventions



PET/CT for Oncologic Interventions


Don C. Yoo



Positron emission tomography (PET) uses 18F-fluorodeoxyglucose (18F-FDG), a molecular imaging agent, which couples a fluorine radioisotope (18F) with a glucose analog (FDG), to show physiologically active tumor. When administered intravenously, the radiopharmaceutical is transported into all cells of the body, which actively use glucose (1). As most cancer cells have higher metabolic activity and accumulate more 18F-FDG compared to normal cells, they are highlighted by comparison to less active cells. By fusing radionuclide PET images with those from x-ray computed tomography (CT), PET/CT distinguishes purely anatomic findings on CT from active tumor.


PET/CT Imaging with 18F-FDG



Indications

1. To plan an initial treatment strategy (formerly referred as diagnosis and initial staging). For all solid tumors except in the following circumstances:

a. Prostate cancer

b. Cervical cancer—for assisting in diagnosis


c. Breast cancer—for assisting in diagnosis and detection of regional nodal metastases

d. Melanoma—for detection of regional nodal metastases in newly diagnosed melanoma

Note: PET/CT can be performed for initial treatment strategy of suspected distant metastatic disease for cervical, breast, and melanoma in high-risk patients.

2. Subsequent treatment strategy (formerly referred as restaging, detection of suspected recurrence, and treatment monitoring)

a. Lifetime insurance limit—three FDG-PET scans are covered for oncologic indications.

Additional scans will be permitted at the discretion of Medicare Administrative Contractor (based on Centers for Medicare & Medicaid Services Coverage Decision effective June 11, 2013) (2).



Contraindications

1. Elevated glucose levels above 200 mg per dL

The American College of Radiology (ACR) and the Society of Nuclear Medicine and Molecular Imaging (SNMMI) both recommend checking glucose levels on all patients prior to administration of 18F-FDG (3,4). SNMMI guidelines recommend that patients with glucose of greater than 150 to 200 mg per dL be rescheduled, if possible, as hyperglycemia will dilute the FDG uptake by tumors through competitive inhibition. Therefore, tumor uptake of 18F-FDG will be reduced in hyperglycemic states (4).

2. Subcutaneous insulin

Subcutaneous insulin should not be administered to a patient with diabetes with high glucose within 4 hours of a PET/CT scan as insulin will stimulate FDG uptake by skeletal muscle resulting in an altered biodistribution which can severely limit image interpretation (5).


Patient Preparation

1. Fasting: minimum fasting of 4 hours. Recent meal can result in a hyperinsulinemic response resulting in altered biodistribution (3,5).

2. Limiting exercise: Strenuous activity for 24 hours prior to injection should be avoided to decrease muscle uptake, which could limit interpretation (3,5).

3. Hydration with water is encouraged to promote urinary excretion and decrease whole-body radiation dose (3,4,5).

4. Low-carbohydrate diet for 24 hours before the study should be considered to decrease the amount of blood glucose at the time of injection (5).




Procedure

1. Current PET-CT systems essentially consist of a CT scanner and a PET scanner that are combined in tandem allowing for sequential imaging. The scout image from the CT determines the extent of the imaged area on the CT and PET portions of the study. Major vendors offer integrated PET-CT systems that combine different models of dedicated PET scanners and CT scanners.

2. In terms of performing PET/CT scans, there is considerable variability in PET/CT imaging protocols.

a. For many PET/CT scans, the CT portion of the study is performed without contrast and at a low kilovolt peak (kVp) and milliampere (mA) for attenuation correction and localization.

b. Some PET/CT scans will have the CT portion performed with both intravenous (IV) and oral contrast with a high kVp and mA. Utilizing an automated milliampere protocol can help limit the radiation dose while optimizing image quality.


c. The dose of the FDG is weight based and can vary depending on the recommendations of the PET/CT vendor. For an adult patient, the dose is typically 10 to 15 mCi (370 to 555 MBq) of 18F-FDG.

d. Imaging is typically performed at 60 to 75 minutes after injection to allow 18F-FDG to be taken up by tumors and cleared from background soft tissue and blood pool (1).


PET/CT Tumor Treatment Assessment

PET/CT has a growing role in the evaluation of tumor response after transarterial chemoembolization (TACE) and radiofrequency ablation (RFA).

It is important to have pretreatment PET/CT imaging to determine the baseline metabolic activity of the neoplasm. In the liver, this is especially true for hepatocellular carcinoma, which can have variable amounts of FDG avidity. In the lung, low-grade or well-differentiated cancers can have mild metabolic activity (6,7).

False-positive findings on PET/CT after image-guided interventions may be seen and are attributable to an inflammatory response and tissue regeneration that occurs at the periphery of the treatment site, which could be mistaken for active malignancy (6).

Tags:
Jun 17, 2016 | Posted by in INTERVENTIONAL RADIOLOGY | Comments Off on PET/CT for Oncologic Interventions

Full access? Get Clinical Tree
Get Clinical Tree app for offline access