Teaching Case

A 24-year-old male presented with history of slowly growing right-sided scalp swelling noticed a few months back and left upper and lower limb weakness of 3 days duration. He also gave a history of abdominal pain and distension as well as intermittent leg pain. There was no history of fever or trauma and he had no other relevant medical history.

CT (computed tomography) and MRI (magnetic resonance imaging) of the skull (Fig 1a, 1b,2,3) revealed a large multiloculated lytic expansile lesion in the right parietal bone with smooth enhancing septations and fluid-fluid levels inside the locules. Marked thinning of the inner and outer cortices was seen, with a few areas of defects in the outer cortex. No obvious periosteal reaction, matrix mineralization, or extraosseous soft tissue component was seen. The fluid levels appeared slightly hyperdense on CT, hypointense on T2, and hyperintense on T1 weighted images, corresponding blooming on the susceptibility-weighted image (SWI), suggesting haemorrhage. Intensely enhancing eccentric solid components were seen along the walls of locules which showed facilitated diffusion. Mass effect was seen over the right frontoparietal lobes of the brain with a mild midline shift to the left. There was, however, no evidence of neuroparenchymal infiltration.

Background

Brown tumours (BT) are focal bony lesions caused by bone remodeling in primary (incidence 3%) or less commonly, in secondary hyperparathyroidism (incidence 1.5-7%) [1]. The excess parathyroid hormone stimulates osteoclastic activity, resulting in poor bony trabecular development. The growth of fibrovascular tissue into the marrow space causes microfractures and haemorrhage with hemosiderin deposition [2]. Prior case reports indicate that BT commonly develop in the mandible, pelvis, ribs, and long bones and are rarely seen in skull and para-nasal sinuses [1,3]. 

Clinical perspective

BT manifest clinically as palpable bony swelling, bone pain, or pathological fractures. Osteopenia, subperiosteal/subchondral/intracortical bone resorption, and acro-osteolysis are also common skeletal findings in PHPT (primary hyperparathyroidism). PHPT can have systemic manifestations such as recurrent renal stones, weight loss, polyuria, and gastrointestinal and neuromuscular symptoms [4]. 

Imaging perspective

BT often present as well-circumscribed lytic lesions with cortical thinning but rarely cause frank destruction [5]. Other less common presentations include ill-defined lesions, mixed lytic and sclerotic lesions, adjacent soft tissue involvement, etc.[6]. BT usually contain a mixture of solid, cystic, and hemorrhagic components, hence give inhomogeneous signals in MRI [7]. In T1, degraded blood products show hyperintense signal. Cystic areas may show fluid levels in T2 [3]. In our case, presence of multiple blood fluid levels brings aneurysmal bone cyst (ABC) and telangiectatic osteosarcoma as close differentials. In BT, the enhancing solid components represent fibrovascular tissue which exhibits facilitated diffusion, allowing it to be differentiated from telangiectatic osteosarcoma and metastasis where the solid components show restricted diffusion. In addition, matrix mineralization is seen in telangiectatic osteosarcoma. Primary ABC does not contain enhancing solid components. Giant cell tumours will have solid components, scattered cystic components, areas of T2 hypointensity and blooming with no matrix mineralization making it a close mimicker of brown tumours on imaging. 

On nuclear bone scans, BT show focal high uptake [3]. 

Outcome 

In this case, detection of multiple other lytic bone lesions in the CT abdomen study (Fig 4) taken for abdominal pain evaluation raised the suspicion of BT, and further biochemical workup showed an elevated serum calcium level.  Ultrasonography neck was ordered which revealed an oval hypoechoic lesion in the inferior aspect of left thyroid lobe (Fig 5) suggestive of left parathyroid adenoma. It was then confirmed by nuclear 99mTc-Sestamibi scan (Fig 6). He was diagnosed as a case of primary hyperparathyroidism and started on bisphosphonates. For the scalp lesion, right frontotemporoparietal craniectomy with tumour excision was done. Histopathology showed multinucleated osteoclast-type giant cells in the background with spindle cells, hemorrhage, and hemosiderin deposition. Although histopathology is considered as the gold standard for definitive diagnosis, at times histology can be uncertain as it can resemble a few other giant cell lesions [8].

Excision of the parathyroid adenoma was done. Postoperatively, the patient developed hungry bone syndrome and was managed with calcium supplements. 

Take home message

Radiological features of BT can be variable, and hence need a high index of suspicion for diagnosis. Detailed history, biochemical analysis correlated with a bone scan, and other associated imaging findings of PHPT help clinch the diagnosis.

Written informed patient consent for publication has been obtained.