PET scan
18F-FDG ([18F]Fluorodeoxyglucose): glucose analogue that carries a positron-emitting isotope
- Preferentially taken up by metabolically active cells with upregulated cell surface glucose transporters and increased rate of glycolysis
- Unlike glucose, FDG can't be metabolised, so stays inside cells
- Intensity of PET signal are proportional to the accumulation of FDG in metabolically active cells - marker for the uptake of glucose from circulation
- Also taken up by inflammatory tissue, benign lesions
- Low signal in slow-growing tumours or those with low cellularity, such as mucinous carcinomas
- Standardised Uptake Value (SUV): tissue activity measured at a fixed point and normalised to body surface area
- Integrate with CT for anatomic information
- Warburg effect: cancers use aerobic glycolysis (fermentation of glucose to lactate) for a higher proportion of energy generation than normal cells do (which tend to use oxidative metabolism, which is more efficient). This means they use glucose at a much higher rate, and are therefore easily detectable on PET scans. This preference is thought to be because cancer cells need the metabolic intermediaries produced by aerobic glycolysis, and which are not produced by oxidative metabolism.
Incidental detection of CRC
- Occurs in 1-3% of FDG-PET scans, and subsequent colonoscopy detects cancer or polyps in >50%
- The colonoscopy should not be restricted to focus of activity: do the whole colon
- SUV values are not helpful in distinguishing between true or false positive, and not helpful in distinguishing between benign and malignant colorectal lesions