GCTs account for approximately 5% of all bone tumours and 20% of all benign bone tumours. There is a slight female preponderance for these tumours and most cases are diagnosed when patients are between 20 and 44 years old [2]. GCTs typically occur at the distal femur, the proximal tibia, the distal radius, and the sacrum and are particularly frequent at the long bone epiphyses [1]. Anecdotal case reports of cranial GCTs describe tumours that arise from sphenoid, temporal, and occipital bones of the skull and frontal sinus [3,4,5]. In our patient, the tumour likely arose from the left temporal fossa, specifically the left temporal bone, as was seen intra-operatively.
GCTs develop by endochondral ossification. Sphenoid and temporal bones also develop from endochondral ossification, which can explain the preponderance of cranial GCTs arising at these bones [4]. Although GCTs are benign tumours, they have varying degrees of aggressiveness. Pulmonary metastases have been reported. Three cells types comprise GCTs: multinucleated giant cells, mononuclear cells with a round morphology resembling monocytes or macrophage-like cells, and mononuclear stromal cells that may show round, oval, or spindle-shaped nuclei [6]. Mononuclear stromal cells are neoplastic cells that may show cellular atypia in malignant counterparts of GCTs, although malignant GCTs are very uncommon. In the histopathological examination conducted for this patient, all three cell types were observed.
The clinical presentation of GCTs depends on the tumour location. Temporal bone GCTs can cause hearing loss and facial paresis [7, 8]. These tumours grow slowly and thus patients typically have a long-standing history.
GCTs of the long bones have no characteristic features on radiographs, although they often have a radiolucent appearance at the epiphyseal end of the bone [1]. However, for cranial GCTs, skull radiographs are not commonly taken given that these images are typically not useful for the investigation of a space-occupying lesion or, as in the case of this patient, hearing loss. Matushita et al. [5] described a frontal sinus GCT in which the radiograph showed osteolytic lesions in the frontal sinuses with surrounding wall destruction, a finding that suggested an aggressive lesion that could be a malignancy or osteomyelitis, but was not specific for GCT. Cross-sectional imaging of the brain also produces non-specific findings for GCT. Most previous studies described CT imaging showing an expansive lesion causing bone destruction [4, 5, 7, 9]. Such lesions have variable signal intensity and enhancement on MRI, depending on the amount of necrosis, soft tissue, or bony components present [4, 7]. However, CT and MRI are useful methods to evaluate the extent of bony destruction and intracranial involvement, respectively.
As GCTs have no characteristic imaging features, many differential diagnoses are often considered including eosinophilic granuloma, plasmacytoma, metastatic lesion, fibrous dysplasia, brown tumour of hyperparathyroidism, or osteomyelitis [4, 7,8,9]. In our patient, cystic teratoma was considered since teratomas can have variable signal intensity on MRI that reflects varying components of these tumours. Based on the hearing loss for this patient as well as lesion density and signal behaviour on MRI, atypical location of a cholesterol granuloma could also be included in the differential diagnosis. This range of differential diagnoses exemplifies the difficulty in recognizing intracranial GCTs based on imaging findings. Thus, the final diagnosis of GCTs should rely on histopathological examination to demonstrate the presence of multinucleated giant cells, mononuclear cells resembling monocytes, and spindle-shaped stromal cells.
Complete surgical resection is the treatment of choice for GCTs occurring in the long bones [7, 8]. However, complete surgical resection can be difficult to achieve for cranial tumours, as demonstrated for this case. Prognosis for GCTs depends on the extent of the surgical resection [9]. Adjuvant treatment with radiation therapy has been used but remains controversial as this method does not seem to alter outcomes [7]. GCTs can have malignant potential, typically lung metastases. Our patient had significant residual disease that required further resection and subsequent referral for radiation therapy. He declined both treatments. Follow-up imaging through 4 years after initial treatment showed that the residual tumour was stable and there was no further progression of symptoms.
