PET/CT in Oncology 18F-FDG IMAGING

PET/CT in Oncology 18F-FDG IMAGING

Objectives

  • Upon completion of this section, participants will be able to:

    • Select the correct PET radiopharmaceuticals per FDG oncology protocol and indication, enhancing diagnostic and treatment accuracy.

    • Choose the correct adjunct medication and dosage per procedure protocol, ensuring patient safety and optimal imaging outcomes.

    • Calculate appropriate dosage for FDG oncologic procedures, including adjunctive medications, using patient-specific factors and clinical considerations to tailor the approach.

    • Identify appropriate indications and contraindications for the use of contrast in FDG oncologic imaging, minimizing risks and ensuring effective imaging results.

    • Describe PET imaging protocols for FDG oncologic imaging, including patient preparation, imaging techniques, and post-imaging responsibilities.

    • Identify normal biodistribution and normal variants on FDG oncologic PET images, facilitating accurate interpretation and differentiation from pathological findings.

    • Identify pathology and image artifacts on FDG oncologic procedures to enhance diagnostic confidence and reduce misinterpretation of results.

Cancer Metabolism and Imaging Tool

  • Many types of cancers and malignant cells exhibit significantly increased metabolic functions, resulting in heightened glucose uptake, which is critical for the production of ATP and biosynthesis of macromolecules.

  • Intracellular glucose activity varies distinctly among different types of cancer, indicating the need for tailored imaging approaches for accurate diagnosis and staging.

  • PET imaging using F-18-FDG has become the most accurate imaging tool for diagnosis and staging of various cancers due to its ability to visualize metabolic activity in cells, enabling oncologists to detect tumors earlier and monitor the treatment response effectively.

F-18 Fluorodeoxyglucose (FDG)

  • Definition and Characteristics:

    • F-18 Fluorodeoxyglucose (FDG) is produced in a cyclotron and decays by positron emission, emitting 511 KeV photons with a maximum energy of 635 KeV, which facilitates advanced imaging techniques.

    • The half-life (T_p) is 110 minutes, allowing for the administration of FDG and subsequent imaging within a clinically relevant timeframe.

    • FDG is a glucose analog that concentrates in cells that rely heavily on glucose for energy, such as cancer cells, and in cells with pathophysiological conditions that demand more glucose, serving as a critical biomarker for metabolic imaging.

  • Chemical Composition:

    • Denoted as 2-[18F]-fluoro-2-deoxy-D-glucose, it closely resembles glucose in structure and function, thereby serving as a marker of glucose metabolism.

    • Classifies cells as hypermetabolic or hypometabolic based on their glucose uptake characteristics, providing invaluable information for cancer diagnosis and treatment planning.

Cellular Metabolism

  • Metabolic Pathway:

    • Involved enzymes: Hexokinase (HK) facilitate the phosphorylation of glucose to Glucose-6-Phosphate (G-6-P), which is critical for cellular metabolism.

    • FDG is metabolized to 18 F-FDG-6P within the cell, where it gets trapped, allowing for the visualization of metabolic activity in tumors.

Kit Contents for FDG

  • Required isotonic, sterile, pyrogen-free, clear, colorless citrate buffered solution containing:

    • 10-100 mCi in each ml for suitable dosages based on protocol.

    • 4.5 mg of NaCl for osmotic balance.

    • 7.2 mg of citrate ions to maintain optimal pH conditions.

    • pH range of 5.0 to 7.5 ensures compatibility with human physiology.

    • No preservatives included to prevent potential interference with imaging results.

Indications for FDG Imaging

  • Assessment of abnormal glucose metabolism assists in the evaluation of malignancy:

    • Confirmed abnormalities through adjunct testing, emphasizing the necessity of comprehensive diagnostic evaluation.

    • In patients with known cancer (CA), guiding treatment decisions based on metabolic activity of lesions.

    • Identify left ventricular myocardium with residual glucose metabolism in patients with known coronary artery disease (CAD) and ventricular dysfunction, aiding in cardiac viability assessments.

    • Evaluation of cardiac viability, critical post-myocardial infarction or before interventions.

    • Identify abnormal glucose metabolism in foci associated with epileptic seizures, underscoring the utility of FDG PET in neurology as well.

  • FDG PET scanning is commonly recommended:

    • At first diagnosis as part of treatment planning and baseline imaging, establishing a reference point for future evaluations.

    • During treatment to gauge efficacy and restage the disease, allowing for timely adjustments to therapeutic strategies.

    • After therapy to assess for residual disease activity and inform follow-up management.

Contraindications for FDG Imaging

  • Patients with unstable blood glucose levels that may compromise imaging integrity.

  • Patients with a glucose level above 200 mg/dL, as this can lead to erroneous interpretations

  • Patients who have not followed preparation protocols, emphasizing the importance of stringent adherence to pre-imaging guidelines.

  • Pregnant patients should consult the ordering physician and radiologist before proceeding due to potential risks to fetal health.

Indications for PET/CT Oncological Imaging

  • Indications include a broad spectrum of malignancies:

    • Solitary Pulmonary Nodules (SPN).

    • Non-small cell lung cancer (CA).

    • Head and Neck cancer.

    • Esophageal cancer.

    • Recurrent Breast cancer.

    • Thyroid Carcinoma.

    • Melanoma.

    • Small Cell Lung cancer.

    • Mesothelioma.

    • Hodgkin's Lymphoma.

    • Non-Hodgkin’s Lymphoma.

    • Multiple Myeloma.

    • Colorectal Cancer.

    • Brain Cancer.

    • Cervical Cancer.

    • Ovarian Cancer.

    • Testicular Cancer.

    • Pancreatic Cancer.

Dosage and Administration of FDG

  • Typical dosage varies based on malignancy, cardiac, or epilepsy conditions:

    • Recommended dosage: 5-10 mCi, delivered intravenously, adjusted for patient-specific factors

    • Patients should fast for 4-6 hours prior to injection to minimize circulating glucose levels.

    • Blood glucose levels must be stabilized between 150-200 mg/dL before administration of FDG to ensure accuracy of the imaging results.

    • For cardiac imaging: fasting limits FDG accumulation in ischemic myocardium, enhancing diagnostic clarity.

    • Administration of approx. 50-75 grams of glucose 1-2 hours before FDG will enhance visualization of ischemic areas during imaging.

Patient Preparation for F-18FDG Imaging

  • Preparation Steps:

    • Avoid strenuous exercise for 24 hours before the scan to prevent false readings caused by increased muscle metabolism.

    • If directed, follow a high protein/restricted carbohydrate diet 24 hours prior to optimize metabolic states.

    • Ensure patient is well-hydrated to facilitate vascular access and imaging quality.

    • Must fast for 4-6 hours before the injection (NPO) to avoid interference with imaging results.

    • Record height, weight (BMI), and fasting blood sugar for accurate dosage calculations and assessments.

    • Patient should rest quietly before the injection to stabilize conditions and obtain optimal imaging results, and lactating mothers must pump and waste breast milk for a day after the procedure to prevent contamination.

    • Most medications can be taken; diuretics, bowel prep, and urinary catheters are optional per protocol.

    • Patients should void before the imaging procedure to ensure comfort and reduce activity during imaging.

Patient Preparation - Summary

  • For Non-Diabetics:

    • No strenuous activity for a minimum of 24 hours.

    • Fasting period of 4-6 hours is critical to ensure accurate imaging results.

    • Ensure patient has voided before imaging and recover for 60-90 minutes post injection for monitoring any immediate effects of the procedure.

Patient Preparation for Diabetic Patients

  • Consult patients to understand their baseline and management of diabetes, ensuring a comprehensive approach to preparation:

    • Ask questions regarding fasting blood sugar practices and journal any fluctuations or irregularities relevant to their condition.

    • Type I insulin-dependent: Schedule early in the day, fast and refrain from insulin unless absolutely necessary, to mitigate risks of hypoglycemia.

    • Type II non-insulin-dependent: Allow a light breakfast, then mandate a minimum 4-hour fast before the procedure to stabilize blood levels prior to imaging.

    • Require patients to bring medications and record medical history for any metabolic/hormonal conditions that could affect imaging.

    • Monitor for hypoglycemia, adhering to institutional policies for managing low/high glucose levels effectively before and during the procedure.

    • Perform fasting glucose test, treating and measuring until levels are within the desired range prior to FDG injection for optimal imaging accuracy.

Blood Glucose Monitoring

  • Ideal fasting blood glucose: less than 120 mg/dL.

  • Some facilities may accept up to 200 mg/dL, though recent trends favor tighter controls, maintaining the maximum at 150 mg/dL to enhance imaging quality.

  • Patients with diabetes exhibit inhomogeneous tissue activity, often requiring supplemental glucose or insulin to stabilize imaging conditions.

Use of Contrast in Imaging

  • Oral Contrast:

    • Low-density oral contrast can be utilized to assist in the evaluation of gastrointestinal FDG uptake. It distends the bowel and minimizes FDG uptake for clearer imaging results.

    • Example Dose: 20 mL (0.6 oz) of Iohexol (Omnipaque) diluted in 26 oz of water or flavored zero-sugar water, improving patient compliance and uptake monitoring.

    • Patients consume the contrast mix during the uptake period, providing valuable anatomical detail during imaging.

    • Avoid high-density contrast due to artifact creation; use diluted low-density contrast to prevent imaging distortion.

  • IV Contrast:

    • Administer only with the diagnostic quality CT portion. Some regulations require specific certifications for nuclear medicine technologists (NMTs) to ensure patient safety.

    • Ensure proper CT protocol compliance for the anatomy being imaged, contributing to accurate diagnoses.

    • No statistically significant interference with standardized uptake values (SUVs) from IV contrast, maintaining imaging integrity.

Alprazolam Use in Imaging

  • Definition:

    • Alprazolam, commonly known as Xanax, is used specifically in adult patients with head and neck cancer to mitigate anxiety and improve cooperation during imaging procedures.

  • Dosage:

    • Oral alprazolam 0.5 mg administered immediately post-FDG injection can minimize skeletal muscle uptake and enhance lesion detectability, ultimately aiding in clearer scan interpretation.

Sample PET/CT Procedure for Oncology

  1. Verify patient identity to prevent mix-ups and ensure safety.

  2. Measure patient's height, weight, and glucose level, and obtain medical history, including medication and existing conditions.

  3. Follow department protocol for blood glucose levels; do not inject over 200 mg/dL, ensuring readiness for imaging.

  4. Inject 10-15 mCi F-18 FDG and flush with at least 20 mL saline to ensure proper distribution.

  5. Provide patient a warm blanket for comfort and reduce anxiety, ensuring a resting period of 60-90 minutes for FDG distribution and metabolism.

  6. Confirm patient has voided before imaging to prevent discomfort during the procedure.

  7. Position patient, typically supine, ensuring comfort and use immobilization devices if necessary for stability.

  8. Perform SCOUT CT, usually anterior/posterior, utilizing low mA settings to minimize radiation exposure while maintaining image quality.

Advanced PET/CT Procedure Steps

  1. Identify patient and address/remove all metal objects which could interfere with imaging results.

  2. Measure height and weight, review patient history, and perform medication reconciliation to ensure safety throughout the procedure.

  3. Measure blood glucose levels and commence IV access, ensuring readiness for FDG administration.

  4. Inject 5-15 mCi of F-18 FDG, followed by flushing the IV to facilitate FDG utilization for imaging.

  5. Provide a warm blanket to promote comfort, followed by a rest period of at least 60-90 minutes for optimal imaging conditions.

  6. Ensure patient voids before imaging and properly position for scan, enhancing comfort and quality of results.

  7. Provide any breathing instructions if necessary to facilitate clear imaging.

  8. Select the appropriate protocol on the scanner, proceed to scout, perform CT, and follow with PET scan, which typically lasts approximately 2-3 mins per bed position to gather comprehensive diagnostic information.

Total Body Scan Protocol

  • Definition: Total body scanning spans the orbitomeatal line to the mid-thigh, also commonly referred to as skull to thigh.

  • Applications:

    • Identifies tumors likely to metastasize to areas such as the head, skull, brain, and lower extremities, aiding in comprehensive staging.

    • Notable cancers include Melanoma, multiple myeloma, various sarcomas (e.g., osteosarcoma)

Normal Biodistribution of FDG

  1. FDG is transported into cells via facilitative glucose transporter proteins, phosphorylated within cells, resulting in trapping until dephosphorylated, which is critical for imaging.

  2. Tumor cells typically possess increased numbers of glucose transporters compared to normal tissues, significantly enhancing FDG uptake.

  3. At 45 minutes, FDG accumulation in tumors plateaus, with heightened activity levels observed in:

    • Brain

    • Heart (if not fasting), which can affect diagnostic interpretations.

    • Urinary system, which is responsible for excretion of FDG.

  4. Primary excretion occurs via urination, emphasizing the need to monitor urinary output pre-and post-imaging.

Normal Biodistribution Locations

  • Typically observed in:

    • Brain

    • Liver

    • Kidneys

    • Bladder

    • Salivary glands

    • Thyroid

    • Heart and vascular structures

    • Thymus (especially in children)

    • Spleen

    • Esophageal ampulla

    • Stomach

    • Bowel (in colon)

    • Endometrium (during menstruation)

    • Bone marrow

    • Muscles

    • Testicles

Normal Variations of FDG Localization

  • Observed in areas of:

    • Infection, contributing to potential false positives in imaging results.

    • Recent biopsy sites, which can lead to inflammation and altered glucose metabolism.

    • Exercised muscles (may alter imaging), necessitating careful pre-imaging instructions to patients.

    • Small and large bowel may necessitate bowel prep prior to imaging to enhance imaging clarity.

Situations Leading to Benign FDG Uptake

  • Physiological Uptake:

    • Areas such as salivary glands, lymphoid tissues, thyroid, muscle activity due to physical exertion, brown adipose tissue, and thymus (particularly in children), all of which require careful interpretation to avoid false positives in cancer detection.

  • Infectious and Inflammatory Processes:

    • Post-procedural inflammation, local infections, post-surgery effects, post-radiation effects, and conditions such as sarcoidosis can mimic malignancy.

  • Benign Tumor or Tumor-like Conditions:

    • Conditions such as pituitary adenoma, adenomatous polyps, salivary gland tumors, etc., necessitate comprehensive analytical review to distinguish from malignant pathology.

  • Artifacts:

    • Misalignment between PET and CT leading to attenuation-correcting artifacts, the influence of implanted metal devices, and motion or edge artifacts that must be accounted for during interpretation.

Future Directions in PET Imaging

  • The future of PET imaging involves continuous improvement and innovation in:

    • Development of new instrumentation and software to enhance imaging quality and speed.

    • Sophisticated reconstruction and processing methods to improve clarity and accuracy.

    • Targeted gene therapy approaches that personalize treatment based on specific tumor biology.

    • Advanced radiochemistry to create more refined imaging agents, increasing diagnostic capabilities.

    • Expansion of radiopharmaceutical options for clinical applications, thus improving the specificity and sensitivity of cancer diagnosis and management.