A 58-year-old man with biochemical recurrence of prostate cancer (PSA 0.8 ng/mL, negative conventional imaging) undergoes 68Ga-PSMA PET-CT. A focus of intense PSMA expression is identified in the right obturator lymph node. Which of the following best explains why PSMA PET-CT detects lesions at this PSA level when conventional imaging cannot?
- A 68Ga emits higher-energy gamma rays than 18F, producing better image resolution
- B PSMA PET uses CT attenuation correction, which conventional MRI lacks
- C PSMA is overexpressed on prostate cancer cells allowing detection of sub-centimetre lesions with molecular specificity independent of tumour size ✓
- D Prostate cancer has higher SUV than other tumours making it more detectable
Correct answer: C. PSMA is overexpressed on prostate cancer cells allowing detection of sub-centimetre lesions with molecular specificity independent of tumour size
Explanation
Prostate-Specific Membrane Antigen (PSMA) is a type II transmembrane protein overexpressed 100–1000-fold on prostate cancer cells. 68Ga-PSMA PET detects metastases based on molecular receptor expression rather than size criteria, enabling detection of lesions <1 cm that are invisible on CT or bone scan. This explains detection sensitivity at PSA levels as low as 0.2–1.0 ng/mL. 18F-PSMA has superior resolution but 68Ga-PSMA is more widely available. SUV is not the distinguishing factor; the molecular targeting specificity is the key advantage.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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