PET/CT
Combined metabolic and anatomic imaging — the gold standard for cancer staging and treatment response.
What Does PET/CT Show?
PET/CT combines functional metabolic imaging (PET) with anatomic CT to provide both "where" and "how active" information about cancer. FDG (fluorodeoxyglucose) PET detects increased glucose metabolism in cancer cells, inflammatory tissue, and brain. The CT component adds precise anatomic localization. Whole-body PET/CT (78816) surveys from head to toe in a single session.
About PET/CT
PET/CT combines two technologies: PET (Positron Emission Tomography), which maps metabolic activity by tracking a radioactive tracer — usually FDG (fluorodeoxyglucose, a glucose analogue) — and CT, which provides the anatomic framework. The result is a fusion image showing both where lesions are and how metabolically active they are. Cancer cells typically consume glucose at a higher rate than normal tissue, making FDG-avid lesions visible before they are large enough to detect structurally.
When to Use PET/CT — and When Not To
Choosing the right modality is one of the most impactful parts of the prior authorization process. Payers are more likely to approve a study when the chosen modality is clearly the most appropriate tool for the clinical question.
✓ PET/CT is the right choice when…
- Oncology staging and restaging: PET/CT is the gold standard for lymphoma, lung cancer, head and neck cancer, oesophageal cancer, melanoma, and cervical cancer. It detects metabolically active nodal and distant metastases before they are structurally large on CT.
- Treatment response assessment: metabolic response precedes structural response — a tumour can remain the same size on CT while being metabolically inactive after chemotherapy. PET/CT captures this earlier.
- Recurrence detection: elevated tumour markers with negative conventional imaging is a classic PET/CT indication.
- Unknown primary: FDG PET/CT often identifies the primary tumour in metastatic disease from an unknown source.
- Radiation therapy planning: metabolic target volumes improve radiation field design.
⚠ Consider a different modality when…
- Brain tumours: high background brain glucose metabolism limits standard FDG PET. Specialised amino acid tracers and MRI perfusion/spectroscopy are preferred.
- Prostate cancer: FDG is not well suited — PSMA PET (a different tracer) is now preferred for prostate cancer recurrence.
- Low-grade or mucinous tumours: not reliably FDG-avid — CT or MRI with pathology correlation needed.
- Too soon after treatment (within 6–8 weeks of chemotherapy or radiation): inflammatory FDG uptake creates false positives.
- Routine surveillance when guidelines support CT or clinical monitoring alone.
+ Technical strengths, limitations & patient preparation
Strengths
- Detects metabolically active disease before structural change on CT
- Whole-body survey for distant metastases in a single session
- Distinguishes viable tumour from scar tissue after treatment
- Gold standard staging for multiple cancer types per NCCN guidelines
- Identifies unknown primary in metastatic disease
Limitations
- Higher radiation dose than CT alone
- FDG uptake non-specific — inflammatory and infectious conditions also show increased uptake (false positives)
- Not suitable for all cancer types (prostate, low-grade tumours)
- Blood glucose must be controlled — hyperglycaemia significantly degrades image quality
- High cost; limited availability outside major centres
- Results unreliable within 6–8 weeks of chemo or radiotherapy
📋 Patient Preparation
Fast 4–6 hours before scan. Blood glucose checked on arrival. FDG injected; patient rests quietly for ~60 minutes before scanning. Diabetic patients need individualised preparation.
Common Indications
Clinical scenarios in which PET/CT is typically ordered:
- Initial staging of most solid cancers (lymphoma, lung, colorectal, head and neck, melanoma, esophageal, cervical)
- Restaging after treatment — assess treatment response
- Detection of recurrence when conventional imaging is equivocal
- Characterization of solitary pulmonary nodule — benign vs. malignant
- Identifying unknown primary in metastatic disease
- Radiation therapy planning
- Cardiac viability assessment (FDG cardiac PET)
- Select neurologic indications (Alzheimer's workup, epilepsy)
Contrast Protocols
When to order each protocol for PET/CT:
Prior Authorization Overview
✓ Key Approval Criteria
- Specify exact indication: initial staging vs. restaging vs. recurrence vs. treatment response
- Reference the histologic cancer diagnosis and stage
- For restaging/response: specify what treatment was administered and timing
- Nodule characterization: document size and CT characteristics per Fleischner/Lung-RADS
- Non-oncologic indications (cardiac, neurologic) have separate specific criteria
⚠ Common Denial Pitfalls
- Cancer types where PET/CT is not guideline-supported (prostate cancer — PSMA PET is now preferred)
- PET/CT too soon after chemotherapy or radiation — FDG uptake changes are unreliable within 6–8 weeks of treatment
- Ordering PET/CT for surveillance when CT or clinical monitoring is the guideline-supported approach
- Not documenting blood glucose level at time of scan (hyperglycemia affects image quality)
Coverage by Payer
Select your payer for specific prior authorization criteria and policy details.
Aetna
General guidance available
View general guidance →Cigna (eviCore)
General guidance available
View general guidance →UnitedHealthcare
General guidance available
View general guidance →Anthem BCBS (Carelon)
General guidance available
View general guidance →Humana
General guidance available
View general guidance →Frequently Asked Questions
Related Procedures
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