23-year-old female patient presented with complaint of gradually increasing swelling over the left temporal region and hearing loss in the left ear during the last 2 months. The patient had no significant previous illness history. On clinical examination bony hard swelling was palpated in the left temporal region with obliteration of the left external auditory canal. The patient was sent for CT scan.
Multi-slice Computed Tomography (CT) without intravenous contrast showed large, expansile, lytic mass lesion involving the left parietal and temporal bone with internal solid-cystic (predominantly cystic) soft tissue component giving soap bubble appearance. The cystic part showed internal fluid-fluid level. It caused mass effect on the left cerebral hemisphere with compression of the left lateral ventricle and mild midline shift towards the right side. There was non-visualisation of the left external auditory canal and the left middle ear, suggesting the destructive nature of the mass (Fig. 1, 2).
On multi-slice CT with intravenous contrast (CECT) solid part showed intense enhancement. The cystic part showed peripheral enhancement with internal fluid-fluid level. (Suggest coexistent aneurysmal bone cyst) (Fig. 3).
On Magnetic Resonance Imaging (MRI) the solid part appeared isointense on T1-weighted image and hypointense on T2-weighted image due to haemosiderin or fibrosis. The cystic part appeared hyperintense on T2-weighted image with internal fluid-fluid level (Fig. 4, 5). On the diffusion-weighted image (DWI) no diffusion restriction was seen (Fig. 6).
Based on imaging the diagnosis of giant cell tumour with coexisting aneurysmal bone cyst was made which was confirmed by histopathology.
Giant cell tumours (GCT) or osteoclastomas are common benign bone tumours arising from the metaphysis of the long bones after the closure of growth plates . They most commonly involve the distal femur and proximal tibia .
GCT of the skull is very rare . It usually arises from sphenoid and/or temporal bones [3, 4]. It may be due to the fact that the skull base and part of the temporal bone is formed by endochondral ossification same as the epiphyses of the long bones where GCTs are primarily found .
Microscopically they show osteoclastic giant cells and mononuclear cells with frequent mitotic figures . GCTs are low-grade tumours only 5-10% are malignant . Sarcomatous transformation may be seen in radiotherapy in the treatment of inoperable tumours . Coexistence of aneurysmal bone cysts is found in 14% of cases which show areas of haemorrhage due to presence of thin-walled vascular channels .
The most common presenting complaint is pain, localised swelling and tenderness. Other signs and symptoms are related to its location .
On CT, GCT is relatively well-circumscribed, expansile lesion causing cortical thinning and 40% of tumours have “soap bubble” appearance representing reactive trabecular bone formation by appositional bone growth. Cranial GCT tend to be aggressive and usually manifest as purely lytic lesions with cortical breakthrough and soft-tissue extension .
On MRI T1-weighted imaging a low signal solid component is seen, which shows post-contrast enhancement. On T2-weighted imaging heterogeneous high signal with variable low signal areas due to haemosiderin or fibrosis are seen. Fluid-fluid levels can be seen if aneurysmal bone cyst component is present. Oedema may be seen in adjacent bone marrow [8, 9].
Treatment is curettage and packing with bone chips or polymethylmethacrylate (PMMA) or wide local excision. In 40-60% of cases local recurrence occurs from periphery of the lesion. Use of intraoperative adjuncts such as thermocoagulation, cryotherapy or chemical treatment of the lesion margins reduces recurrence rate to 2.5-10% . Donosumab is used as an adjuvant treatment to promote tumour necrosis .
Take home message: As GCT of skull bone is very rare, it has to be differentiated from other lytic bone lesions. Imaging plays a crucial role in diagnosis.
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