A wide variety of benign and malignant tumors can originate in the sinonasal region, including hamartomas (1, 2). Esthesioneuroblastoma (ENB) or olfactory neuroblastoma (ONB) is an uncommon type of neuroectodermal tumor that is specific to the sinonasal tract (3). It accounts for about 3-6% of all intranasal tumours (4, 5). ENB is derived from the specialized olfactory epithelium, which lines the superior portion of the nasal cavity. The origin of ENB is generally confined to the distribution of olfactory epithelium, specifically the cribriform plate, the superior turbinate, and the superior one-third of the nasal septum (6). As a result of its origin in the superior nasal cavity, the tumor typically invades the superomedial orbit and the anterior cranial fossa; the tumor invades the latter through the cribriform plate of the ethmoid bone. Bone invasion by any malignant sinonasal tumor is via osteolysis and is observed as a lytic pattern on CT imaging. Rarely, the osteoblasts are involved in bone invasion resulting in exuberant bone formation. In the English literature, we found only four cases of hyperostotic ENB causing bone invasion (7, 8). Regenbogen et al. (7) first used the term, "hyperostotic" ENB. In our case, bone infiltration and dominant hyperostosis, mimicking fibrous dysplasia were depicted on three imaging modalities. The indolent course of hyperostotic ENB may be potentially related to this predominant pattern of bone infiltration. However, the indolent course of hyperostotic ENB cannot be clearly determined due to the limited number of reported cases in the literature.

A 65-year-old male initially presented at another institution with mild visual disturbances, loss of taste, and proptosis. CT imaging was suggestive of fibrous dysplasia. No treatment was administered at that time. Serial imaging performed over three years showed slow progression of the lesion? with no significant clinical deterioration. The patient was subsequently referred to our institution. CT, MRI and FDG-PET imaging were performed. CT (Fig. 1A, B) showed expansile, diffuse hyperostosis of the ethmoid bones, nasal septum, superomedial orbital walls and anterior frontal skull base. There was an extensive intracranial and extracranial enhancing soft tissue thickening along the hyperostotic bones. MRI (Fig. 1C, D) showed hypointense T1 and T2 signals of the bone marrow that corresponded to the hyperostotic bones on CT, with moderate diffuse enhancement. The rind of soft tissue mass along the hyperostotic bones showed hypointense to isointense T1 and T2 signals and diffuse enhancement. Intracranial rim enhancing small cysts were observed at the tumor-brain interface. Multi-focal extracranial mucoceles were also identified. FDG-PET (Fig. 1E, F) showed a corresponding diffuse hypermetabolic uptake in the soft tissues and bones. Endoscopic biopsies of the mass and bone confirmed the diagnosis of ONB with osseous tumor infiltration (Fig. 1G, H). The patient opted for non-surgical treatment and expired in about 2 years.

The typical imaging features of ENB have been described previously (9-11). CT usually shows an isodense to hyperdense expansile enhancing soft tissue mass with osteolytic bone erosion, and a speckled pattern of calcification within the tumor matrix is rarely observed. Occasionally, small foci of calcification may be seen (12, 13). MRI also shows non-specific features, the mass typically shows homogeneous enhancement, and hypointense to isointense signal on T1- and isointense to hyperintense signal on T2-weighted images (9). The heterogeneous appearance is due to cystic degeneration, necrosis and hemorrhagic foci. The presence of well-defined cysts at the tumor-brain interface is a relatively specific diagnostic indicator of ENB (14, 15). The soft tissue mass in the coronal plane of cross-sectional imaging can show the following features: 1) localization in the superior nasal cavity and ethmoid labyrinth, 2) limited erosion of the cribriform plate and/or medial/superior wall of the orbits, and 3) a large bulky extension into the anterior cranial fossa and/or orbits. The Kadish staging system divides tumors into 3 groups: group A lesions are limited to the nasal cavity; group B lesions involve the nasal cavity and paranasal sinuses; and group C lesions extend beyond the nasal cavity and paranasal sinuses. On the other hand, TNM staging provides more detailed information on the invasion of the thats sound correct skull base and orbits (16).

Our case was classified as Kadish stage C. There was diffuse sinonasal, orbital, and frontal skull base hyperostosis and a diffuse rind of soft tissue mass along the margins of the hyperostotic bone. Typically, an ENB is observed as an osteolytic bony defect in the anterior skull base, which provides an easy conduit for bulky intracranial extension. Our case was a variant of ENB, and hence a large lytic bone defect was not observed and hyperostosis was the predominant finding. The hyperostotic ENB had a homogeneous ground-glass appearance, truly mimicking fibrous dysplasia. The mucocele formation indicated the chronicity of the process with obstructive sequelae and chronic rhinosinusitis. As expected, the hyperostotic tumor was hypointense on T1- and T2-weighted images. Diffuse enhancement of the involved bones indicated tumor infiltration. Diffuse hypermetabolism in the bone-soft tissue tumor complex on FDG-PET imaging corresponded to the biopsy-proven tumor in the hyperostotic bones and surrounding soft tissue. According to our experience as well as that described in the previous reports, large, bulky intracranial tumors are characterized by the presence of cysts at the tumor-brain interface. However, in this case, the cysts were present despite a smaller tumor bulk (i.e., sub-centimeter intracranial soft tissue thickening).

The previous three reported cases were of females while our patient was a male. The case reported by Schiro et al. (17) had a long clinical history of about seven years. Our case also had a long indolent course of about three years. The chronicity is manifested by secondary obstructive and mucosal stigmata in the sinuses. The current case had more extensive hyperostosis. FDG-PET findings of hyperostotic ENB have not been described previously, but FDG-PET may provide excellent details for tumor mapping.

The following are different potential causes of hyperostosis associated with tumors: hypervascularity, non-invasive irritation of the bone, hormone-mediated osteoblast stimulation, bone formation by the tumor, and most importantly, direct tumor invasion. Hyperostosis adjacent to skull-base meningiomas particularly sphenoid and peri-clinoid meningiomas ("hyperostotic meningiomas") is generally considered as a marker of bone invasion and necessitates surgical intervention (18). Hyperostosis associated with meningiomas is usually localized and proportional to the size of the tumor. However, the rare variant of ENB that was observed in our case showed more extensive hyperostosis.

This case report describes a rare variant of ENB, which truly mimics anterior skull base and sinonasal fibrous dysplasia. Ground-glass appearance of the bone matrix is a characteristic feature of fibrous dysplasia. Tehranzadeh et al. (19) compared fibrous dysplasia with Paget's disease of the skull using CT imaging. Ground-glass appearance, absence of cortical thickening, involvement of the paranasal sinuses and asymmetric findings were the specific features of fibrous dysplasia. The other less specific features which still favored the diagnosis of fibrous dysplasia included the following: orbital, nasal cavity, sphenoid or maxillary sinus involvement, soft tissue thickening, and cystic lucency of the hyperostotic bones. More symmetric bone involvement and cortical thickening were observed in Paget's disease, but ground-glass appearance, sino-nasal involvement, cystic lucencies or soft tissue thickening were not seen in Paget's disease. Only 3 cases of fibrous dysplasia showed soft tissue attenuation, which were proved to be fibrous stroma on histological examination. Chronic sinusitis can result in localized or diffuse sinonasal hyperostosis. The specific features of chronic sinusitis include retracted margins, sinus opacification, irregular hyperostosis and post-operative changes.

Like in our case, it may be very difficult to differentiate fibrous dysplasia from hyperostotic ENB on imaging. The latter condition can show most of the imaging features of fibrous dysplasia especially on CT. Enhancing soft tissue thickening along the hyperostotic bones and cysts at the tumor-brain interface are highly suggestive of hyperostotic ENB. FDG-PET was helpful for detecting the presence of the tumor as well as for mapping the extent of tumor in the bones.