Question

A 23-year-old male presented to orthopedic clinic with a 6-week history of intermittent pain and increasing swelling in his left popliteal fossa. Physical exam was notable for a firm mobile mass that was mildly tender to palpation (Figs. 1, 2, 3, 4, 5 and 6).

Fig. 1
figure 1

AP (A) and lateral (B) radiographs of the left knee

Full size image
Fig. 2
figure 2

Sagittal and axial MRI images proton density-weighted (A, D), T1 fat-suppressed post-contrast (B, E), and T2 fat-suppressed post-contrast (C, F)

Full size image
Fig. 3
figure 3

Ultrasound-guided biopsy of mass in left popliteal fossa

Full size image
Fig. 4
figure 4

Hematoxylin and Eosin (H&E), 10X

Full size image
Fig. 5
figure 5

Hematoxylin and Eosin (H&E), 40X

Full size image
Fig. 6
figure 6

Immunohistochemical stain for beta-catenin, 40X

Full size image