01Purpose and principlesWhat the assessment is for and the core concepts behind it.
Positron emission tomography detects pairs of photons produced after a positron-emitting radionuclide decays and annihilates with an electron. In oncological FDG PET, fluorine-18-labelled fluorodeoxyglucose enters cells through glucose transport mechanisms and is phosphorylated but not metabolised normally, causing intracellular trapping over the imaging period. Many cancers show increased glycolysis, but activated inflammatory cells, brain, myocardium, skeletal muscle and some normal organs also take up FDG. PET therefore depicts biological activity with limited specificity; CT provides the anatomical map needed to decide whether a focus lies in tumour, normal tissue or an artefact.
Cancer staging asks whether disease is confined to the primary site, involves regional nodes or has spread distantly, but the useful imaging sequence is cancer-specific. PET-CT can survey much of the body in one examination and may reveal occult nodal or distant disease, define a radiotherapy target or direct biopsy. It does not replace high-resolution local staging such as rectal MRI, brain MRI where indicated, endoscopic ultrasound or pathological nodal assessment. In potentially curable lung cancer, for example, PET-CT informs distant and mediastinal staging, while suspicious mediastinal nodes may still require EBUS, EUS or surgical sampling because false-positive inflammation and false-negative small-volume disease affect treatment.
The examination must be designed for the decision. Preparation suppresses avoidable background uptake, acquisition covers the clinically relevant region, and the CT protocol must be stated because low-dose non-contrast CT may be inadequate for some structural questions. Interpretation compares lesion uptake with expected biodistribution and corresponding morphology, checks for technical misregistration and reviews previous imaging and treatment dates. The report should state the likely stage-relevant findings, important uncertainty, sites suitable for biopsy and whether urgent or additional imaging is needed.
Key points
- 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.
- NICE recommends PET-CT before treatment for people with lung cancer who could potentially receive curative-intent treatment, and uses it within a multimodality nodal-staging pathway rather than as a substitute for tissue sampling.
- Fasting, low insulin state, measured blood glucose, quiet uptake, warmth and avoidance of recent strenuous muscular activity reduce competitive glucose uptake and physiological muscle or brown-fat signal.
- Common false positives include infection, inflammation, healing surgery, radiotherapy change and reactive lymph nodes; common false negatives include small lesions, low-cellularity or low-glycolytic tumours and high background activity.
- SUV is influenced by uptake time, glucose, body-size normalisation, reconstruction and scanner calibration; compare serial scans only when acquisition is sufficiently standardised and interpret the images visually as well.
- Biopsy the most accessible lesion that can establish diagnosis and highest relevant stage, avoiding necrotic centres and recognising that the hottest lesion is not automatically the safest or most representative target.
- A management-changing solitary PET finding needs anatomical correlation and sometimes tissue confirmation because metabolic activity alone is not histology.
02Indications, selection and cautionsWhen it is useful, when urgency changes and important limitations.
Focal uptake corresponding to a structural primary can support localisation and extent, but tumour size, histology, necrosis and partial-volume effect influence intensity; absence of avidity does not exclude malignancy.
Uptake in anatomically relevant nodal stations may upstage disease, yet size and avidity are imperfect; reactive nodes can be hot and microscopic metastases can be below spatial resolution.
Multiple concordant avid lesions in typical organs raise metastatic probability, but a solitary unexpected focus deserves careful CT correlation and confirmation when it would fundamentally change treatment intent.
Brain and urinary tract are normally intense, while myocardium, bowel, muscles, brown fat, marrow and lymphoid tissue vary; symmetry, anatomy and preparation help distinguish normal activity from disease.
Infection, sarcoid-like inflammation, recent intervention, radiotherapy and vaccination activate glycolytic cells and can simulate tumour or nodal spread; history and temporal pattern are essential.
Falling or resolved pathological uptake can precede size reduction, while persistent activity may indicate viable disease; timing and tumour-specific response criteria prevent inflammation being misclassified as failure.
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.
03Method and interpretationA systematic approach to the test and its findings.
Read from the initial assessment onwards. Tests may run in parallel in urgent care; first-line, preferred, confirmatory, definitive and gold-standard labels appear only when the chapter explicitly states them.
- 01
FDG PET with low-dose CTFirst step - Why
- Survey glycolytic activity and anatomically localise it for staging, restaging or response questions in appropriate cancers.
- Interpretation and limitations
- Integrate visual uptake, distribution and CT morphology. Low-dose CT may not fully characterise liver, brain, bowel wall or vascular findings and should not be called equivalent to diagnostic contrast-enhanced CT.
- 02
Diagnostic contrast-enhanced CT - Why
- Define morphology, vascular relationships, organ lesions and complications that need diagnostic CT quality and enhancement.
- Interpretation and limitations
- It may be acquired separately or as part of a protocolled PET-CT. Phase, breath-hold and contrast timing affect fusion and lesion detection, so the report must identify the CT technique actually used.
- 03
Cancer-specific local staging imaging - Why
- Resolve anatomy that whole-body PET-CT does not assess with sufficient local detail, such as rectal wall planes, brain metastases or selected pelvic and head-and-neck structures.
- Interpretation and limitations
- MRI, ultrasound or endoscopic imaging complements PET-CT; a metabolically active primary does not provide a complete local T stage.
- 04
Pathological confirmation - Why
- Establish tumour type or verify an unexpected management-changing lesion when imaging specificity is insufficient.
- Interpretation and limitations
- Choose a safe accessible target representing the highest clinically relevant stage, use PET to avoid necrotic tissue when helpful and remember that treatment should not be diverted by an unverified equivocal focus.
- 05
Mediastinal nodal sampling - Why
- Confirm nodal involvement in potentially curable lung cancer when PET-CT and CT findings affect radical treatment decisions.
- Interpretation and limitations
- EBUS-TBNA, EUS-FNA or surgical staging samples nodes directly; PET avidity selects targets but cannot replace histology where false classification would change curative management.
- 06
Blood glucose and preparation check - Why
- Determine whether competitive glucose, insulin effect or altered muscle and fat uptake may impair scan quality.
- Interpretation and limitations
- Apply the imaging service's validated protocol to glucose and diabetes management. Document deviations because rescheduling may be preferable to a falsely reassuring low-quality scan.
- 07
Tumour-specific tracer PET - Why
- Image cancers whose biology is better represented by a non-FDG target, such as selected prostate or neuroendocrine tumours.
- Interpretation and limitations
- Tracer choice changes normal distribution, sensitivity and eligibility implications; findings cannot be interpreted using FDG assumptions or transferred across tumour types without validation.
04Clinical next stepsHow the result changes management or prompts escalation.
01Potentially curable lung cancerUse PET-CT to guide staging and tissueFirst stepCT shows a resectable lung mass and an enlarged ipsilateral mediastinal node, with no definite distant metastasis.+
- 1Confirm 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.
- 2Interpret 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.
- 3Select the investigation that provides diagnosis and highest relevant stage with least risk; sample suspicious mediastinal nodes with EBUS or EUS where indicated rather than treating PET avidity as histological proof.
- 4If an isolated distant focus would preclude curative treatment but is equivocal or atypical, obtain targeted imaging or tissue confirmation where feasible before abandoning the radical pathway.
- 5Integrate PET-CT, brain imaging where stage indicates, lung function and pathology at the multidisciplinary meeting to decide surgery, radiotherapy, systemic treatment or further staging.
02Lymphoma stagingApply subtype-specific indicationsA patient has a tissue diagnosis of non-Hodgkin lymphoma and staging will determine whether treatment is local or systemic.+
- 1Confirm the exact lymphoma subtype and provisional clinical and CT stage because FDG avidity and guideline recommendations vary across lymphomas.
- 2Use FDG PET-CT to confirm staging in the NICE-specified settings and consider it for other subtypes or stages only when the result is likely to alter management.
- 3Map nodal and extranodal sites, correlate marrow and splenic patterns with clinical data and avoid calling symmetrical reactive or inflammatory activity malignant without context.
- 4Use a standardised baseline when later metabolic response assessment is expected, while following tumour-specific criteria and appropriate post-treatment timing.
03Unexpected solitary focusPrevent false upstagingPET-CT reveals one avid lesion outside the expected drainage pattern that would change a planned curative operation to palliative treatment.+
- 1Review CT morphology, normal variants, recent infection, vaccination, surgery and trauma, and compare prior imaging to estimate whether the focus is metastatic, inflammatory or a second primary.
- 2Ask whether another modality can characterise the site more specifically and whether the lesion is safely accessible for biopsy.
- 3Obtain confirmation when feasible before making an irreversible change in intent, prioritising a target that answers the staging question with the lowest procedural risk.
- 4AlternativeIf biopsy is unsafe or non-diagnostic, record the residual probability and use multidisciplinary judgement, interval imaging or an alternative test rather than converting uncertainty into certainty.
04Response assessmentCompare like with likeA post-treatment PET-CT is requested to determine whether metabolically active cancer persists.+
- 1Check the treatment type and date, expected inflammatory interval, original tumour avidity and whether a comparable baseline study exists.
- 2Use consistent preparation and acquisition where possible, compare visually and semi-quantitatively, and inspect the CT for size, necrosis and new structural disease.
- 3Apply validated tumour-specific response criteria rather than an isolated SUV threshold and describe equivocal uptake at treated sites.
- 4Confirm or follow a discordant result when it would trigger major salvage therapy, biopsy or cessation of effective treatment.
05Risks, monitoring and follow-upComplications, safety checks and further assessment.
- Record fasting, diabetes and insulin management, blood glucose, FDG administration, uptake interval, scan coverage and CT technique so image quality and serial comparability can be judged.
- Compare staging conclusions with pathology and subsequent imaging at the multidisciplinary meeting, particularly when a solitary site changes treatment intent.
- Use tumour-specific timing and criteria for response assessment; recent therapy may produce inflammation, marrow stimulation or immune-related patterns that mimic progression.
- Audit repeat or non-diagnostic scans caused by preparation, motion, injection problems, misregistration or uncontrolled glucose and improve patient instructions and scheduling.
- Track whether biopsy targets selected from PET yielded viable representative tissue and whether imaging-predicted stage was confirmed at surgery or follow-up.
- Communicate urgent structural findings immediately and document the named recipient, advice and planned next assessment.
06Special situationsVariants, exceptions and circumstances that change the usual approach.
FDG uptake is not cancer-specific
Activated neutrophils and macrophages are highly glycolytic, so infection and inflammation can be as avid as tumour and may occur in the same patient.
Resolution limits matter
Small deposits suffer partial-volume loss and can be PET-negative despite malignancy, especially beside intense physiological activity or with respiratory motion.
Attenuation artefact can imitate uptake
Metal, contrast and PET-CT misregistration may create or displace apparent foci; reviewing non-attenuation-corrected images and CT alignment helps identify artefact.
SUV is conditional
Standardised uptake value changes with uptake time, glucose, body composition, scanner calibration and reconstruction, so a decimal value is not a universal biological truth.
Histology determines avidity
Some mucinous, low-grade or low-cellularity tumours show little FDG uptake, while alternative tracers may outperform FDG for selected prostate and neuroendocrine cancers.
The best biopsy target is strategic
Choose a site that safely confirms diagnosis and the most important stage; the most avid focus may be necrotic centrally, inaccessible or irrelevant to the decision.
PET cannot finish every stage
Brain MRI, local pelvic MRI, endoscopic assessment and invasive nodal sampling remain essential in defined pathways even after technically excellent whole-body PET-CT.
07Common pitfallsFrequent interpretation and management errors.
- 01
Requesting PET-CT as a generic cancer screen without a tumour-specific indication or management question.
- 02
Equating every avid focus with metastasis and abandoning curative treatment without correlation or confirmation.
- 03
Calling a non-avid lesion benign despite small size, low-avidity histology or intense adjacent physiological activity.
- 04
Failing to distinguish a low-dose localisation CT from a diagnostic contrast-enhanced CT.
- 05
Using one SUV threshold across scanners, uptake times, body compositions and tumour types.
- 06
Ignoring recent surgery, radiotherapy, vaccination, infection, colony-stimulating treatment or strenuous exercise when interpreting uptake.
- 07
Using PET-CT in place of pathology for suspicious mediastinal nodes when nodal status determines radical lung-cancer treatment.
- 08
Comparing response scans with different preparation or timing without acknowledging reduced quantitative reliability.