When is a positron emission tomography (PET) scan referral indicated?

PET Scan Referral Indications

PET scanning is primarily indicated in oncology for staging, detecting recurrence, and differentiating viable tumor from post-treatment changes in specific malignancies, with the strongest evidence supporting its use in lung cancer staging, colorectal cancer recurrence detection, and assessment of residual masses in lymphoma. 1, 2

Core Oncologic Indications

Lung Cancer (Strongest Evidence)

• Staging locoregional disease: FDG-PET is recommended when there is no evidence of distant metastatic disease on CT scan to complement anatomic imaging 1
• Solitary pulmonary nodules: Standard indication for diagnosis of malignancy in lesions >1 cm suspicious on initial imaging (Level A evidence) 1
• Mediastinal lymph node assessment: Biopsy required for nodes >1 cm on CT or positive on PET; negative PET does not eliminate need for biopsy of enlarged nodes 1
• Metastatic screening: Assessment of locoregional involvement and distant metastases, particularly adrenal glands (Level A evidence) 1
• Adrenal/liver masses: Any isolated adrenal or liver mass on imaging requires biopsy to rule out metastatic disease if patient is otherwise resectable 1

Colorectal Cancer

• Recurrence detection: Diagnosis of recurrence in patients with confirmed elevation of serum carcinoembryonic antigen (CEA) without localizing findings on conventional imaging (Level B2 evidence), with sensitivity 87-100% and specificity 66-100% 1, 3
• Preoperative staging: Assessment of operability of recurrent disease and metastases 1
• Lung metastases: Preoperative staging when lung metastases are deemed resectable on CT, as PET changes management in 21% of cases by detecting extrapulmonary disease 4

Lymphoma

• Post-treatment assessment: Differentiation between residual active tumor and fibrosis in patients with residual masses on CT after chemotherapy ± radiation 5
• Hodgkin lymphoma: Interim restaging with PET/CT after 2-4 cycles of chemotherapy, though optimal timing remains under investigation 1
• Critical caveat: Surveillance PET should NOT be performed routinely due to high false-positive rates; management decisions should not be based on PET alone 1

Melanoma

• High-risk disease: Initial metastatic screening in patients with Stage III AJCC melanoma at high risk of metastases (Level B2 evidence) 1
• NOT indicated: Screening for nodal micrometastases (Level B1 evidence against) 1

Breast Cancer

• Metastatic screening: Locoregional and distant metastatic screening for patients with invasive tumors (Level B2 evidence) 1
• Recurrence evaluation: Suspicion of local or metastatic recurrence (Level B2 evidence) 1
• NOT indicated: Detection of nodal micrometastases is a standard contraindication 1

Gastrointestinal Malignancies

Esophageal cancer: Pretreatment evaluation of nodal and metastatic status to complement CT and endoscopic ultrasound (Level B2 evidence), with particular benefit in detecting stage IV disease 1, 3

Pancreatic cancer: Differential diagnosis and staging in patients with normal serum glucose (Level B2 evidence), with sensitivity 68-96% and specificity 78-100% for detecting malignancy in suspicious pancreatic masses 1, 3

Hepatocellular carcinoma: Differential diagnosis of hepatic metastases, cholangiocarcinomas, and benign tumors in patients with solitary hepatic lesions 1

Head and Neck Cancer

• Initial staging: Metastatic screening for patients with untreated head and neck cancers, particularly pharyngeal cancers (Level B2 evidence) 1
• Differential diagnosis: When biopsy has not been conclusive for distinguishing benign from malignant tumors (Level B2 evidence) 1

Gynecologic Malignancies

Cervical cancer: Assessment of nodal involvement (Level B2 evidence) 1

Ovarian cancer: Suspicion of local or metastatic recurrence (Level C evidence) 1

NOT indicated: Endometrial or vaginal cancer outside clinical trials (standard contraindication) 1

Testicular Cancer (Seminoma Only)

• Post-chemotherapy assessment: Detection of malignancy in residual masses >3 cm with normal tumor markers, performed ≥6 weeks after chemotherapy completion (Level B2 evidence) 1
• High predictive value: Both positive and negative predictive values are high for determining residual disease 1
• NOT indicated: Routine use in nonseminoma is not recommended 1

Thyroid Cancer

• Well-differentiated thyroid cancer: Suspicion of residual disease or recurrence when standard imaging including radioactive iodine scans are not conclusive (Level B2 evidence) 1
• NOT indicated: Diagnosis of thyroid nodules (standard contraindication, Level B2 evidence) 1

Non-Oncologic Indications

Cardiac Imaging

• Myocardial viability: Identification of left ventricular myocardium with residual glucose metabolism and reversible loss of systolic function in patients with coronary artery disease and left ventricular dysfunction, when used with myocardial perfusion imaging 2

Neurologic Imaging

• Epilepsy: Identification of regions of abnormal glucose metabolism associated with foci of epileptic seizures 2

Contraindications and Situations Where PET is NOT Indicated

• Mediastinal tumors: No indication outside clinical trials (standard) 1
• Gastric carcinoma: No indication outside clinical trials (expert agreement) 1
• Bladder cancer: No indication outside clinical trials (standard) 1
• Neuroendocrine tumors: Only indicated when octreotide scan is normal; NOT first-line imaging (Level B2 evidence) 1
• Prostate carcinoma: Limited role; only considered for initial staging or suspicion of recurrence (Level C evidence) 1

Critical Technical Considerations

Timing: PET should be performed ≥6 weeks after chemotherapy completion to reduce false-positives 1

False-positives: Granulomatous diseases (e.g., sarcoid) are common sources of false-positive results 1

Combined imaging: Side-by-side viewing of PET and CT is essential in 42% of cases for accurate diagnosis; integrated PET/CT provides additional benefit in only 6.7% of patients 6

Confirmation required: Positive PET findings in potentially resectable disease require histologic confirmation or corroboration by additional radiologic testing before altering surgical management 1