PET Scan — Functional Metabolic Imaging

PET imaging relies on the principle that malignant neoplastic cells exhibit higher metabolic rates and glucose turnover than healthy surrounding tissues, a phenomenon known as the Warburg effect. The most common radiotracer is fludeoxyglucose F 18 (FDG), a glucose analogue tagged with a positron-emitting radionuclide.

Modern oncological imaging combines PET with computed tomography (PET-CT) in a single gantry. This hybrid approach simultaneously captures precise anatomical detail (CT) alongside biological and metabolic activity (PET), accurately localising sites of increased radiotracer avidity within organs and lymph nodes.

PET scans provide critical staging information that standard imaging might miss, revealing whether a malignancy is confined to an organ or has spread to distant lymph nodes and bones. This differentiation often changes the primary management strategy—distinguishing candidates for curative surgery from those requiring systemic therapies. PET scans are also used mid-treatment or post-treatment to evaluate whether a tumour is metabolically responding, often weeks before noticeable reductions in physical tumour size occur.

PET scans utilise radiotracers administered intravenously. Standard FDG-PET measures glycolysis via standardised uptake values (SUV). Specialised tracers target other biological mechanisms, such as Gallium-68 DOTATATE for neuroendocrine somatostatin receptors and Gallium-68 or Fluorine-18 PSMA agents for prostate-specific membrane antigen. Scans require dietary preparation (fasting and low-carbohydrate intake) to standardise baseline blood glucose.

PET-CT scans are widely employed in staging and monitoring lymphoma (Hodgkin and non-Hodgkin), non-small cell lung cancer, melanoma, colorectal cancer, oesophageal and gastro-oesophageal junction tumours, head and neck carcinomas, and certain gynaecological cancers. Specific non-FDG tracers, such as PSMA, are increasingly used for prostate cancer.

Findings from a PET scan heavily guide clinical choices. Identification of unsuspected distant metastases can prevent futile, invasive surgeries, redirecting care toward systemic chemotherapy or immunotherapy. In radiotherapy planning, PET-CT helps radiation oncologists map target volumes precisely, targeting hypermetabolic tissue while sparing healthy organs. Post-treatment, an absence of tracer uptake ('metabolic complete response') reassures teams that residual masses are likely inactive scar tissue.