Monostotic clival fibrous dysplasia

Findings

CT: Expansile, lytic lesion of the clivus is present without cortical disruption. The lesion has a ground-glass appearance. It is lytic centrally and sclerotic peripherally. Bilateral jugular tubercle and right occipital condyle involvement also present.
MRI: Heterogenous (T2 hyper intense and T1 hypo intense) expansile clival lesion. The majority of the lesion demonstrates avid contrast enhancement. The brain parenchyma is normal.

Differential diagnosis:
- Fibrous dysplasia
- Chordoma
- Chondrosarcoma
- Plasmocytoma
- Metastasis
- Lymphoma


Diagnosis: Monostotic clival fibrous dysplasia


Discussion

Monostotic clival fibrous dysplasia is a rare entity (According to Sirvanci et al., only 3 cases were reported in the literature by 2002). Overall, craniofacial fibrous dysplasia represents three percent of bone tumors. Fibrous dysplasia is a developmental disorder felt to be secondary to a genetic mutation. It is not hereditary. It results in abnormal fibroblast proliferation with replacement of normal medullary bone. This causes expansion and structural integrity compromise.

Fibrous dysplasia can be classified as monostotic and polyostotic. The polyostotic form represents 30% of cases and can present as part of Albright-McCune-Sternberg syndrome. The initial presentation in the majority of cases is in the first two decades of life. No racial or gender predilection has been described. Besides craniofacial involvement, other commonly involved areas include the ribs, femurs, tibias, and the pelvis.

Management is almost always conservative with followup imaging to evaluate for stability if warranted (typically CT). Surgical treatment may be warranted if neurological compromise or significant pain develops. Rarely (0.05% of monostotic craniofacial lesions), malignant transformation may occur.


Radiologic Overview

MRI and CT are the best imaging modalities for clival evaluation. On MRI, fibrous dysplasia appears as hypo intense on T1 weighted images. The imaging characteristics on T2 weighted images are variable from hypo intense to isointense to hyper intense. The variability on T2 weighted images is attributed to several factors including (but not limited to) cellularity, cyst formation, and presence of collagen. Avid enhancement after gadolinium administration is typically present.

On CT, a ground glass appearance is associated with fibrous dysplasia. Additionally, a sclerotic border is also present. Smaller cystic, circumscribed areas are present within the lesion. Overall, CT is superior to evaluate for subtle change and internal lesion structure. If nuclear scintigraphy is performed (not necessary), mild, nonspecific activity is usually present in the clivus.