Question: A 67-year-old man presented with an 8-year history of progressive right knee swelling, increasing pain, and limited range of motion. MRI was performed (Fig. 1). Ultrasound-guided biopsy was obtained. Follow-up MRI was performed (Fig. 2), which was followed by surgical resection with the histopathologic examination (Fig. 3).

Fig. 1
figure 1

MRI of the right knee including a and b sagittal T2 fat suppressed (FS), c sagittal proton density (PD), and d axial PDFS sequences

Fig. 2
figure 2

Follow-up (20 months) MRI images of the right knee including a sagittal PD, b axial T1, and c sagittal fat-suppressed T1 post-IV gadolinium

Fig. 3
figure 3

Photomicrographs including hematoxylin and eosin (ac), as well as immunohistochemical nuclear staining for STAT-6 (d)