Teaching Case

A 49-year-old lady presented with insidious onset of left knee pain for half a year. There were no relevant medical records or history of trauma. The pain was not mechanical in nature and occurred mainly during the night, waking her up. She did not have any fever or constitutional symptoms.

Radiograph of left knee was performed as initial investigation showing a small faint radio-lucent area at the distal metaphyseal region of the left knee (Fig. 1). It did not show joint involvement or association with a periosteal reaction. Cortex was apparently preserved.

Thereafter CT of the left knee showed an irregular lobulated lytic lesion with incomplete sclerotic border at the anterolateral femoral condyle, surrounded by small lytic areas. The lesion extended inferiorly breaking the cortex. Mild surrounding bony sclerosis was associated but no definite periosteal reaction.

MRI of the left knee with contrast was then performed. It showed a lobulated cystic type lesion at the anterolateral femoral condyle with intermediate signal intensity on T1W images and high signal intensity on T2W images, surrounded by bone marrow oedema. After gadolinium injection, the lesion showed enhancing component. No definite extra-osseous component.

Overall features suggested an aggressive bone lesion.

The diagnosis was done after bone biopsy.

The differential diagnosis included: chondroblastoma because of its epiphyseal location and surrounding bone marrow oedema. However, chondroblastoma presents at a young age. Brodie's abscess could be in the differential diagnosis due to the the presence of lytic area breaking the cortex. However, no periosteal reaction and soft tissue swelling was associated and the laboratory test was normal. Primary bone lymphoma was included; despite its multiple radiological appearance the most frequent pattern is lytic destructive [5].

In view of indeterminate nature, biopsy in a tertiary bone tumour centre was offered. The histology result was Rosai Dorfman disease.

Rosai Dorfman or sinus histiocytosis with massive lymphadenopathy [1, 2] (SHML) is a rare benign idiopathic proliferative disease that involves phagocytic histiocytes. The exact cause is unknown but disordered immune regulation and viral infections are thought to be involved.

Due to the wide disease spectrum, radiographic features can be variable. Nodal involvement is appreciated as lymphadenopathy on plain film and cross sectional imaging. Isolated bone involvement as the only manifestation of SHML is extremely rare [3], with less than 50 cases reported in the literature. After all, multifocal osseous involvement as the sole manifestation of Rosai-Dorfman disease had been reported [4].

Radiological appearance is variable. Although usually presented as a lytic enlarging lesion, blastic or mixed appearance have been described. Aggressive features like cortical breakage, wide zone of transition and incomplete sclerotic border are associated.

In this case, with the progressive history of increasing pain and vague lesion detected on radiograph, CT and MRI were offered, which found a lytic lesion with aggressive imaging features as described above. Bone biopsy was performed and pathologically showed large histiocytes embedded among a mixed inflammatory cell infiltrate. The histiocytes showed moderate to large amount of pale eosinophilic, occasionally vacuolated cytoplasm, oval or round nuclei and showed immunoreactivity for S100 protein. Overall features were suggestive of Rosai Dorfman disease.

Curettage of the lesion under computer navigation and cementation of the bone cavity was performed. Post-operative MRI showed residual disease, but MRI follow up for 2 years showed no significant interval progression of the disease.

As take home message, given the clinical history of progressive increase in pain, we should maintain a high index of suspicion to investigate the cause and do not hesitate to offer CT and MRI with contrast if tumour or atypical infection is suspected. Bone tumour biopsy should be performed at a tertiary bone tumour centre for better operative planning.