"Masson's pseudoangiosarcoma" in a popliteal aneurysm: tumor or thrombus? Cause or effect?

A 44-year-old male presented to our vascular surgery unit with a partially thrombosed 22 mm popliteal aneurysm giving rise to progressive distal ischemia. Microscopy of embolus retrieved from the posterior tibial artery at bypass surgery revealed the unexpected finding of intravascular papillary endothelial hyperplasia (Masson's pseudoangiosarcoma).