Ruptured Baker Cyst on 18 F-FDG PET/CT.

A 78-year-old woman with diffuse large B-cell lymphoma was referred for an 18 F-FDG PET/CT to evaluate therapy response after 6 cycles of R-mini-CHOP. A new 18 F-FDG accumulation was noticed medial in the upper part of the right lower leg, spreading along the medial head of the gastrocnemius muscle. The shaft-bow-looking curvature, arch sign, of 18 F-FDG revealed a fluid collection on CT. This typical pattern and its specific location are indicative of a ruptured Baker's cyst. It can manifest in 1 or both legs.