Nerve sheath ganglion

Most comonly seen in the large nerves about the knee - popliteal, peroneal, tibial - at the level of fibular head
Cystic in nature on imaging

Clinical features:
Palpable mass
Neurologic symptoms due to compression

Treatment:
Excision

References:
Murphey MD et al. Imaging of Musculoskeletal Neurogenic Tumors: Radiologic-Pathologic Correlation . Radiographics. 1999;19:1253-1280