Teaching Case

A 10-year-old girl was referred by her general practitioner for an ultrasound examination of a hard non-tender mass behind the left ear. The mass was present for about half a year. There was no pain, inflammation, fever or other associated symptoms.

The ultrasound examination showed a sharply delineated hyperreflective mass with diameters of about 9 mm and retro-acoustic shadowing. The surface had a smooth convex shape towards the subcutaneous fat. The structure looked firmly attached to the underlying mastoid-bone and was not compressible. For better demonstration of a possible connection with underlying bone, a CT examination of that specific region was made subsequently without contrast administration.
This showed a mass of compact bone centred over the petrosquamous suture at the mastoid angle. The cortical lining was not continuous with the cortex from the tabula externa. No cancellous bone was noted inside the lesion. There was a thin less radiodense rim between the tabula externa and the medial border of the lesion.
The diagnosis of a mastoid osteoma was made.

A mastoid osteoma is a rare benign osteogenic tumour with only about 150 cases reported in literature until now [4,6,8,9].

Osteomas are frequently seen within the paranasal sinuses and less frequently in the external ear canal. Osteomas arising from the mastoid bone, however, are rare [4].

Osteomas of the skull are classified in 3 subtypes: compact, spongiotic and mixed. Adequate differentiation between them can only be made by histopathological examination. They all have slow growth and remain stable for years [2, 4].

Different aetiological factors have been proposed (previous trauma, chronic infection, surgery, radiotherapy and hormonal factors) without hard evidence for any of them [3, 4, 1].
Mastoid osteomas are most frequently encountered in young female patients.
They are usually asymptomatic, but may enlarge to cause cosmetic deformity. When large, they can cause pain in the auricular region or in the neck. The latter because of irritation of the greater auricular or small occipital nerves.

Although ultrasound can give a hint to the diagnosis, CT with high kernel and osseous window is the key examination. A 3D reconstruction can be performed for better anatomic localisation and proper surgical planning. On CT, the osteoma presents as a well delineated bony and sclerotic bordered lesion. Osteomas can be differentiated from exostoses this way (outer cortex of tabula externa is not continuous with the lesion border in an osteoma). MRI can be used, but is not the modality of choice for proper diagnosis. It is excellent, however, to detect inflammatory tissue around the mastoid osteoma [2].

Treatment of choice is complete resection. It is indicated to perform surgery early in order to prevent voluminous growth and possible risk of complications in the procedure. Recurrence is uncommon and malignant transformation has never been reported in literature [4, 5, 7, 8].