A Rare Case of Anaplastic Large Cell Lymphoma of the Knee Joint

INTRODUCTION: Anaplastic large cell lymphoma (ALCL), which makes up only 2–5% of instances of NHL, is a rare and aggressive form of the disease. Anaplastic Lymphoma Kinase (ALK)-positive ALCL is a variant of the illness that is identified by the presence of an ALK gene fusion. The disease is most commonly confined to the nodes, but extranodal spread has been reported. Skin and soft tissue are the most frequently identified locations for extranodal involvement, while joints are extremely rare. We describe a rare case of ALCL affecting the knee joint that is ALK -positive. CASE REPORT: A 51-year-old female presented with pain in her right knee. It was diagnosed as synovitis and treated accordingly. The symptoms did not relieve and a magnetic resonance imaging (MRI) scan was performed that indicated a meniscus tear. She was treated for it; however, the symptoms worsened. She underwent two more scans (1 MRI and 1 PET) that indicated an enhancing polypoidal mass in the knee joint, and two arthroscopic procedures. The histopathology report indicated an ALK-positive ALCL. She was treated with Brentuximab vedotin + CHP followed by involved-site radiotherapy to the postoperative region with margins. The patient has had a complete clinical and pathological response which was assessed after 1 year from the start of the treatment. CONCLUSION: This is the first instance of primary ALK-positive ALCL affecting the knee joint that has been documented, to the best of our knowledge. The case emphasizes the significance of taking ALCL into account when determining the differential diagnosis of knee joint tumors as well as the requirement for a thorough assessment of extranodal involvement.