Synopsis
Use FDG PET-CT selectively for cancer staging and response assessment, understand tracer biology and preparation, distinguish physiological or inflammatory uptake from malignancy and act appropriately on management-changing findings.
- Most oncological PET-CT uses fluorine-18 fluorodeoxyglucose, a glucose analogue taken up by glucose transport pathways and trapped after phosphorylation, so signal reflects glycolytic activity rather than tumour cells specifically.
- The PET component maps tracer distribution and the CT component localises uptake, detects structural lesions and corrects attenuation; diagnostic contrast-enhanced CT and low-dose localisation CT are not interchangeable.
- Use PET-CT only where the tumour type and clinical decision support it: its value lies in changing stage, biopsy target, treatment field, operability or response classification.
Key red flags
Focal vertebral uptake with cortical destruction, collapse or epidural soft tissue and new pain or neurological symptoms requires urgent assessment for spinal instability or cord compression.
A new intensely avid lesion at a site that would change curative to palliative intent should be confirmed when feasible if inflammation, a second primary or benign uptake remains plausible.
Diffuse or focal uptake after recent surgery, radiotherapy, vaccination, infection or granulomatous disease can mimic spread and must be interpreted with timing and anatomy.
Severe hyperglycaemia, recent insulin, strenuous exercise or inadequate resting conditions can degrade biodistribution and lesion conspicuity, making apparent negative staging unreliable.
CT evidence of vertebral collapse, epidural extension, impending long-bone fracture, airway narrowing or visceral complication can be more urgent than the metabolic staging question and needs direct escalation.
Investigation priorities
Survey glycolytic activity and anatomically localise it for staging, restaging or response questions in appropriate cancers.
Management branches
CT shows a resectable lung mass and an enlarged ipsilateral mediastinal node, with no definite distant metastasis.
- Confirm that curative treatment remains plausible and arrange PET-CT as part of the staging pathway, reviewing symptoms, histology if known, diabetes and recent infection or procedures.
- Interpret distant sites first because a confirmed metastasis changes intent, then correlate mediastinal nodal uptake with CT stations and evaluate the primary and possible synchronous lesions.