A 45-year-old woman with a known history of breast cancer presents for staging evaluation. A PET-CT scan is performed using 18F-FDG. The scan shows increased uptake in the right axillary lymph nodes and a focal area of uptake in the left femur. Which of the following statements regarding the interpretation of this PET scan is MOST accurate?

Explanation

## Interpretation of 18F-FDG PET-CT in Oncology ### Understanding 18F-FDG Uptake **Key Point:** 18F-FDG (fluorodeoxyglucose) is a glucose analog that accumulates in tissues with high metabolic activity. While malignant cells typically show increased uptake, this is NOT pathognomonic for cancer. ### Specificity Limitations of PET Imaging 18F-FDG uptake occurs in: - **Malignant lesions** (high sensitivity, moderate specificity) - **Inflammatory conditions** (tuberculosis, sarcoidosis, chronic osteomyelitis) - **Infections** (active bacterial, fungal, or viral infections) - **Benign processes** (granulation tissue, post-radiation changes, healing fractures) Therefore, **increased uptake alone cannot exclude infection or inflammation** in the axillary nodes, making option 1 incorrect. ### Standardized Uptake Value (SUV) **High-Yield:** SUV is calculated as: $$SUV = \frac{\text{Activity concentration in tissue (kBq/mL)}}{\text{Injected dose (kBq) / Body weight (kg)}}$$ **Warning:** SUV is **NOT independent of body weight** — it is normalized to body weight by design. SUV varies with: - Time of imaging (delayed imaging increases SUV) - Glucose levels (hyperglycemia reduces uptake) - Reconstruction algorithms - Partial volume effects Option 2 is factually incorrect. ### Bone Metastasis Detection **Clinical Pearl:** While increased 18F-FDG uptake in bone (femur in this case) is highly suggestive of metastatic disease, **false positives occur** with: - Benign bone lesions (hemangioma, osteoid osteoma) - Fractures (healing callus) - Degenerative joint disease - Bone infarction **Correct approach:** Correlation with CT morphology (lytic vs. sclerotic lesion) or MRI is essential to confirm malignancy. Option 3 correctly acknowledges this limitation. ### PET Limitations in Bone Imaging **Key Point:** PET-CT has **reduced sensitivity for sclerotic (osteoblastic) metastases** because: - Sclerotic lesions have lower glucose metabolism than lytic lesions - Osteoblastic response may not generate sufficient 18F-FDG uptake - Conventional bone scintigraphy or CT may be more sensitive for detection Option 4 is incorrect — PET is not superior for all bone lesion types. ### Summary Table: PET vs. Conventional Imaging | Feature | PET-CT | CT/MRI | |---------|--------|--------| | Metabolic activity detection | Excellent | Poor | | Morphologic detail | Limited | Excellent | | Sclerotic metastases | Reduced sensitivity | Good sensitivity | | Specificity for malignancy | Moderate (60–80%) | High | | Benign false positives | Common | Rare | **Mnemonic:** **SUV-ER** — SUV is **U**seful but **V**ariable, **E**nhanced by correlation, **R**equires morphologic confirmation.