An 80-year-old man was referred for a needle biopsy of what looked like a cancerous tumor behind his left knee. Then an ultrasound performed before the procedure turned up something no tumor should have: arterial blood flowing inside the mass.
The lump was not a sarcoma. It was a partially clot-filled aneurysm, a ballooned-out section of a small artery branching off the main vessel behind the knee, according to a case report in the journal Cureus, published June 25, 2026. Pushing a biopsy needle into a blood-filled arterial sac is exactly what the report warns against.
