A Giant Baker's Cyst in a Patient With Rheumatoid Arthritis: An Unusual Case.

Baker's cysts are fluid-filled swellings in the popliteal fossa commonly associated with degenerative and inflammatory joint diseases, including rheumatoid arthritis (RA), as well as with joint injuries. While they are usually small and asymptomatic, in rare cases, they can enlarge significantly and cause clinically relevant complications. We describe the case of a 69-year-old man with an undiagnosed RA who presented at his first Internal Medicine consultation with a five-week history of progressive swelling in the left popliteal region extending to the mid-calf, accompanied by pain on movement and difficulty walking. Clinical evaluation revealed a large popliteal mass, and subsequent magnetic resonance imaging (MRI) confirmed a well-defined, heterogeneous cyst measuring 24 cm in its longitudinal axis, causing compression of adjacent structures and exerting a mass effect on the medial gastrocnemius muscle without evidence of rupture. This case highlights the potential for Baker's cysts to reach exceptional dimensions, emphasizing the importance of accurate diagnosis for appropriate management and treatment planning.