When is a referral for a Positron Emission Tomography (PET) scan recommended?

When to Refer for PET Scan in Oncology

PET scan referral is recommended for specific cancer types and clinical scenarios where it will change staging, guide treatment decisions, or detect occult disease—primarily in lung cancer staging, lymphoma response assessment, melanoma metastatic workup, suspected recurrence in colorectal/gynecological cancers, and when conventional imaging is equivocal. 1

Cancer-Specific Indications for PET Referral

Non-Small Cell Lung Cancer (NSCLC)

• Refer for PET/CT when there is no evidence of distant metastatic disease on chest CT scan to complete staging workup 1
• PET is essential for evaluating indeterminate solitary pulmonary nodules, where it significantly outperforms CT in distinguishing benign from malignant lesions 2
• For mediastinal lymph node evaluation: Refer when CT shows nodes >1.0 cm in shortest axis, though negative PET does not eliminate need for biopsy of radiographically enlarged nodes 1
• PET detects unsuspected distant metastases and changes management in approximately 72% of lung cancer patients 3
• Do not refer for routine staging in clinical stage I-II disease if conventional imaging is clearly negative 1

Malignant Lymphoma

• Refer for PET/CT for initial staging of Hodgkin's lymphoma and diffuse large B-cell lymphoma (essential, not optional) 1
• PET is recommended for response evaluation during treatment and end-of-therapy assessment, as it predicts progression-free survival 1
• Refer before high-dose chemotherapy with stem cell support in relapsed patients, as PET positivity predicts shorter progression-free survival 1
• Obtain PET at least 6 weeks after completion of chemotherapy when evaluating residual masses >3 cm to minimize false-positives from inflammation 1

Malignant Melanoma

• Refer all patients with stage III melanoma (sentinel node positive) for PET/CT to detect distant metastases before planning definitive therapy 1
• PET is the most sensitive modality for diagnosing distant metastases in high-risk metastatic melanoma (stages 3-4) 1
• PET detects unrecognized metastases leading to altered management in 10-19% of melanoma patients 1

Colorectal Cancer

• Refer when CT or MRI is equivocal regarding liver metastases and surgical resection is being considered 1
• PET is recommended as first choice when recurrence is suspected but CT is negative, particularly with rising tumor markers 1
• Use PET to rule out distant metastases or secondary cancers that would obviate curative intervention 1

Gynecological Cancers

• Cervical cancer: Refer for response evaluation after treatment, as PET is superior for assessing treatment efficacy and predicts survival 1
• Uterine cancer: Refer when advanced disease is suspected to guide choice between surgical versus systemic treatment 1
• Ovarian cancer: Refer when recurrence is suspected with elevated CA-125 but negative CT and MRI 1
• PET changes management in 22-35% of gynecological cancer cases by detecting local relapse and distant metastases 1

Head and Neck Cancer

• Refer for diagnostic workup and staging, particularly for unknown primary tumors where PET identifies at least 30% of primaries not detected by conventional means 1

When NOT to Refer for PET

Breast Cancer

• Do not refer for routine staging of clinical stage I, II, or operable stage III breast cancer 1
• PET has high false-negative rates for small (<1 cm) or low-grade lesions and low sensitivity for axillary nodal metastases 1
• PET/CT is category 2B (lower recommendation) even for stage IIIA disease 1
• Only consider PET when standard staging studies are equivocal or suspicious in locally advanced or metastatic disease 1

Testicular Cancer (Nonseminoma)

• PET scanning does not contribute and routine use is not recommended for nonseminoma germ cell tumors 1

Early-Stage Cancers Without Symptoms

• Do not refer for routine surveillance imaging in early-stage disease without signs or symptoms of metastatic disease 1

Essential Referral Documentation

When referring for PET scan, the American Urological Association and European Association of Nuclear Medicine require specific information 4:

Patient Demographics and Clinical Context

• Full patient demographics: name, date of birth, medical record number, height, weight 4
• Contact information for patient and referring provider 4
• Clear diagnosis and explicit clinical question the PET/CT must answer 4

Oncological History

• Known tumor sites and any extramedullary disease 4
• Complete treatment history: type and date of last treatment, current medications 4
• Minimum 1-month delay after radiotherapy or surgery to reduce false-positives from inflammation 4

Prior Imaging

• Dates and comprehensive reports from prior imaging (X-rays, CT, MRI, bone scans, previous PET/CTs) 4
• Baseline imaging for comparison when assessing treatment response 4

Specific Clinical Question

• Whether this is initial staging, restaging after treatment, or evaluation for recurrence 4
• Justification for why the scan is medically necessary at this time 4
• The clinical decision that depends on the imaging results 4

Common Pitfalls to Avoid

• Ordering PET for routine surveillance: PET changes management in 38% of cancer patients overall, but only when used for appropriate indications 5
• Ignoring timing after treatment: Wait at least 6 weeks post-chemotherapy to avoid false-positives from inflammation 1
• Assuming PET replaces conventional imaging: PET is an adjunct that complements CT/MRI, not a replacement 1, 2
• Not obtaining tissue confirmation: Positive PET findings in atypical locations or inconsistent with disease presentation require pathologic confirmation 1
• Inadequate clinical information: Approximately 50% of patients need combined viewing of PET and CT data for accurate diagnosis, requiring complete prior imaging documentation 6