Clinical Presentation

The patient is a 74-year-old man with a palpable proximal leg mass, paresthesias, and footdrop.

(A–F) A circumscribed, cystic, hyperintense T2 focus (asterisk) extends from the tibiofibular articulation (between arrows) and tracks into the peroneal musculature. A bone infarct is incidentally noted in the fibular head.