First recurrence of small cell lung cancer presenting as subacute monoarticular arthritis

We describe a case of recurrent small cell lung cancer presenting as an acute monoarticular arthritis. This patient had recently undergone comprehensive review and surveillance imaging under a local oncology unit, 18 months after undergoing chemoradiotherapy for limited disease small cell lung cancer. He had presented to the emergency department on multiple occasions and been managed as an outpatient for a provisional diagnosis of spontaneous haemarthrosis in the setting of rivaroxaban therapy. Subsequent investigation revealed occult fracture of the distal femur with joint effusion, secondary to isolated metastatic disease from recurrent small cell lung cancer. This case demonstrates the importance of reconsidering differential diagnoses when a patient's symptoms do not respond to appropriate treatment as expected. It also highlights the limitations of surveillance protocols and the influence that recent specialist input can have on diagnostic processes.