Figure 38. Sarcoma invasion of the posterior tibial nerve. Photograph (bottom right) shows the area scanned. A 57-year-old woman complained of a painful mass in the popliteal fossa. US scans in transverse (Tr) and sagittal (Sag) views show an ill-defined hypoechoic mass (arrows) with invasion of the posterior tibial nerve (arrowheads). The mass was identified as high-grade sarcoma.