PMC6190812_01
PET/MRI and MRI of the left knee. Notes: Axial (A) and coronal (B) PET/MRI show high uptake of 18F-FTC-146 in the intercondylar notch (red arrows: maximum standardized uptake value = 2.04). By comparison, the intercondylar notch of the right knee is normal (white arrow: maximum standardized uptake value = 0.17). MRI shows small joint effusion, with no synovitis, and an amorphous, mass-like in the intercondylar notch, initially presumed to be a ganglion cyst or localized pigmented villonodular synovitis/fibrous lesion. Sagittal (C) and coronal (D) MRI (T2-weighted with fat saturation) of the left knee, acquired approximately 3 years before PET/MRI study, show abnormal high-signal amorphous, mass-like but equivocal lesion in the intercondylar notch (white arrows). This had been overlooked or regarded as clinically insignificant.
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The scan revealed focal increased uptake of the radiotracer in the inter-condylar region of the left knee (Figure 1A, B). While this lesion had been present in MRIs obtained in previous years (Figure 1C, D), it had not been noticed or commented upon.
Hematoxylin and eosinstain section of the left-knee mass. Notes: The mass (pale tan tissue measuring 0.4 x 0.4 x 0.1 cm) was biopsied anteriorly from between the anterior cruciate ligament and the posterior cruciate ligament. This was one of three masses removed during knee arthroscopy, synovial biopsy, and synovectomy. The section shows mature adipose cells with overlying synovial lining. The infiltration and presence of inflammatory cells in this specimen is consistent with an inflamed synovial lipoma.
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Biopsy showed mature adipose tissue with synovial lining, revealing the mass to be an intraarticular synovial lipoma (Figure 2).