Primary lymphomas of bone are uncommon malignancies with involvement of spine, long bones, pelvis, ribs and skull in decreasing order of frequency. We describe an unusual case who presented with recurrent knee effusions turned out to be a high grade B-cell non-Hodgkin lymphoma.

A 34-year-old man is presented with painful recurrent knee effusions without any systemic aberration. Multiple aspirations were carried out to help knee pain and swelling with immediate response, but had negative cytology. Hematological and biochemical parameters were normal. Plain radiographic examination of the knee failed to show any bony abnormalities. An MRI scan of the knee showed extensive marrow oedema of the distal femur with breach in the anterior cortex. Subsequently, open biopsy of the lesion was carried out which confirmed the diagnosis of a high grade B-cell Non-Hodgkin lymphoma. Staging CT-scan of the chest, abdomen and pelvis failed to show any other lesions elsewhere. He was subsequently treated with the help of multi-drug chemotherapy and radiotherapy. At 7 months post treatment, the patient had no relapse and a repeat MRI at 7 months showed reduction in the size of lesion.

This case illustrates the limitations of plain radiograph and the usefulness of MRI in the diagnosis of bony lymphoma. The diagnostic dilemmas while investigating recurrent knee effusions due to primary skeletal Non Hodgkin lymphoma should be resolved by timely MRI scan and histological diagnosis.