A diabetic patient receives a dose of short-acting insulin subcutaneously one hour before FDG injection because his pre-scan blood glucose was elevated. The subsequent images show intense symmetrical FDG uptake throughout skeletal muscle with poor lesion conspicuity. What best explains this finding?

• A Insulin causes vasoconstriction of tumour feeding arteries, producing a cold defect at the tumour site
• B Insulin accelerates renal clearance of FDG, leaving too little activity in the circulation at acquisition
• C Insulin upregulates GLUT-4 transporters on muscle membranes, diverting FDG into muscle and away from the tumour ✓
• D Insulin competitively inhibits hexokinase phosphorylation inside tumour cells, causing rapid washout

Explanation

Exogenous insulin given close to injection time shifts circulating glucose and FDG into insulin-sensitive tissues, mainly skeletal and cardiac muscle, through GLUT-4 translocation. This lowers available plasma activity and degrades oncological image quality, so scans are usually deferred rather than rescued with insulin. Option B is wrong because muscle sequestration, not urinary excretion, causes the poor contrast. Hexokinase trapping inside tumours is unaffected by insulin.

Reference: Nuclear Medicine: The Requisites (Ziessman), 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.