What are the guidelines for a PET (Positron Emission Tomography) scan referral order?

PET Scan Referral Order Guidelines

PET scanning with FDG should be ordered based on specific cancer type and clinical scenario, with established indications primarily for staging, recurrence detection, and treatment response assessment in oncology—not for routine surveillance or screening. 1

General Principles for PET Referral

Patient Preparation Requirements

• Blood glucose must be stabilized before FDG administration, achieved by 4-6 hours of fasting in non-diabetic patients 2
• Diabetic patients require glucose stabilization on both the day preceding and day of the scan 2
• The recommended adult dose is 185-370 MBq (5-10 mCi) administered intravenously 2
• Imaging should optimally begin within 40 minutes of FDG injection, with static scans acquired 30-100 minutes post-injection 2

Key Limitations to Consider

• FDG accumulation is not cancer-specific—it increases in benign tumors, inflammatory diseases, sarcoidosis, and granulomatosis 1
• This creates false-positive results that must be interpreted in clinical context 1

Cancer-Specific Indications

Lung Cancer

Standard indications (order PET for these):

• Assessment of locoregional involvement and metastatic screening, particularly for adrenal gland evaluation 1
• Pretreatment evaluation of nodal and metastatic status to complement CT and endoscopic ultrasound 1

Options (consider PET for these):

• Differential diagnosis of recurrence versus post-treatment fibrosis 1
• Optimization of radiotherapy fields when combined with CT 1

Timing caveat: Do not order PET earlier than 3 months post-treatment, as timing depends on treatment type used 3

Colorectal Cancer

Standard indication:

• Assessment of operability of recurrent disease and metastases 1

Options:

• Diagnosis of recurrence when serum carcinoembryonic antigen (CEA) is elevated 1
• Preoperative staging of local and metastatic recurrence 1

Breast Cancer

Standard indication:

• Locoregional and distant metastatic screening for patients with invasive tumors 1

Options:

• Suspicion of local or metastatic recurrence 1
• Evaluation of response to neoadjuvant chemotherapy 1

Do NOT order for:

• Routine surveillance in patients who completed curative-intent therapy—increased cost and radiation exposure outweigh benefits 4
• Deciding whether to continue or stop hormonal therapy 4
• Detection of nodal micrometastases (not standard practice) 1

Melanoma

Standard indication:

• Initial metastatic screening in high-risk patients (Stage III AJCC) 1

Options:

• Screening for recurrence during follow-up 1

Do NOT order for:

• Non-cutaneous melanomas outside clinical trials 1

Head and Neck Cancer

Standard indication:

• Initial metastatic screening for pharyngeal cancer 1

Options:

• Metastatic screening for untreated head and neck cancers 1
• Differential diagnosis between benign and malignant tumors when biopsy is inconclusive 1

Esophageal Cancer

Standard indication:

• Pretreatment evaluation of nodal and metastatic status to complement other imaging and endoscopic ultrasound 1

Pancreatic Cancer

Standard indication:

• Differential diagnosis and staging in patients with normal serum glucose 1

Cervical Cancer

Standard indication:

• Assessment of nodal involvement 1

Thyroid Cancer

Standard indication for well-differentiated thyroid cancer:

• Suspicion of residual disease or recurrence when standard imaging (including radioactive iodine scans) is not conclusive 1

Standard indication for medullary thyroid cancer:

• Preoperative staging if further surgery is indicated for persistent or recurrent disease 1

Do NOT order for:

• Diagnosis of thyroid nodules 1

Testicular Cancer

Standard indication:

• Detection of malignancy in post-treatment residual mass 1

Options:

• Search for recurrent disease when tumor markers are elevated at follow-up 1

Lymphoma

Appropriate indications:

• Staging of Hodgkin's disease and diffuse large B-cell non-Hodgkin's lymphoma 5
• Evaluation during and after chemotherapy to predict relapse 5
• End-of-therapy assessment for prognostic information 5

Hepatocellular Carcinoma

Standard indication:

• Differential diagnosis of hepatic metastases, cholangiocarcinomas, and benign tumors in patients with a solitary hepatic lesion 1

Options:

• Metastatic screening 1

Carcinoma of Unknown Primary

Option:

• Search for primary tumor in patients with metastatic cervical lymphadenopathy 1

Do NOT Order PET For

Absolute contraindications (no indication outside clinical trials):

• Gastric carcinoma 1
• Bladder cancer 1
• Endometrial or vaginal cancer 1
• Mediastinal tumors 1
• Neuro-endocrine tumors as first-line imaging 1

Common Pitfalls to Avoid

1. False-positive lymph nodes: PET-positive lymph nodes measuring less than 1 cm have high false-positive rates; consider clinical background and obtain histological confirmation when necessary 6

2. False-negative small nodes: Lymph nodes less than 7 mm with low metastatic burden may be missed by PET 6

3. Timing errors: Ordering PET too early post-treatment (before 3 months) reduces accuracy 3

4. Inappropriate surveillance: PET is for characterizing suspicious findings, not routine follow-up in disease-free patients 4

5. Glucose interference: Elevated blood glucose reduces FDG uptake accuracy; ensure proper patient preparation 2