Temporal Bone Tumors

Introduction

For the evaluation of a suspected tumor of the temporal bone, Computed Tomography (CT) and Magnetic Resonance Imaging (MRI) are used.

CT is used to evaluate the osseous invasion by the tumor. The aspect of bone invasion can provide additional information regarding the type of tumor.

MRI is considered to have a superior contrast resolution. Various pulse sequences can be used. The use of intravenously administrated gadolinium is highly recommended to see the enhancement of the tumor, soft tissues, meninges, and surrounding vascular structures.

The best way to evaluate a tumoral lesion of the temporal bone is MRI—using various pulse sequences—to detect, delineate, and differentiate the tumor in combination with a high-resolution CT (HRCT) in bone algorithm or cone beam CT (CBCT) to see the osseous delineation and eventual osseous lysis and/or sclerosis of the lesion.

High-Resolution CT/Cone Beam CT

Multidetector CT (MDCT) uses a combination of a fan beam X-ray configuration and multiple detector rows to acquire slices and to subsequently reconstruct a volume dataset. From this dataset, multiple reconstructions can be made in different planes with various thicknesses.

CBCT uses a combination of a cone beam X-ray configuration and a flat panel detector to directly acquire a volume dataset from which slices can be reconstructed in different planes and with different fields of view. CBCT delivers high-resolution bone images (usually in slice thickness of 0.1 mm) at a dose that is mostly substantially lower than that of MDCT.

CT is best acquired without intravenously administrated iodinated contrast by means of a high-resolution bone algorithm to see the exact bony delineation of the tumor.

If access to MRI is not available, CT can be performed using intravenously administrated iodinated contrast medium. The dataset should in those cases be acquired in soft tissue as well as high-resolution bone algorithm to allow the evaluation of the soft tissues as well as the bony details of the temporal bone.

Multiplanar reconstructions can give additional information regarding the exact extension of the tumor and the invasion of vascular structures, nerve channels, and the membranous labyrinth.

Magnetic Resonance Imaging

MRI of the temporal bone should be performed using a dedicated multichannel head coil and a high-field-strength MRI system. The authors use a combination of a multichannel head coil with small surface coils placed inside. Depending on the region to be evaluated, the head coil or the small surface coils can be switched on. Both temporal bones should be imaged to compare both sides. The entire skull base should be included. Evaluation of intracranial extension can also be done similarly.

An MRI of the temporal bone should start with an entire brain examination using a T2-weighted fast spin echo (FSE), a turbo spin echo (TSE), or a fluid-attenuated inversion recovery (FLAIR) sequence to exclude associated brain pathologic processes. A heavily T2-weighted sequence, usually a submillimetric three-dimensional (3D) TSE/FSE T2-weighted sequence, is performed to evaluate the fluid content and the signal intensity characteristics in the membranous labyrinth. Thin-slice (2 mm maximum) spin echo T1-weighted sequences are obtained before and after intravenous administration of gadolinium. A complementary axial submillimetric 3D T1-weighted gradient echo sequence is also scanned, to generate the maximum number of slices through the temporal bone. Fat saturation techniques should be applied in one direction after contrast administration.

The acquisition matrix should be at least 512 × 512 with a field of view of 20 cm or less.

Magnetic resonance angiography (MRA) sequences are obligatory in case of a suspected glomus tumor or in the evaluation of a patient with pulsatile tinnitus. A 3D time-of-flight (TOF) MRA sequence before and after gadolinium administration is mandatory to see the small vessels in various vascular malformations and to evaluate the high- and/or low-velocity status of these vessels.

Diffusion-weighted imaging (DWI) sequences, preferably a non-echo planar DWI sequence (non-EP DWI), should definitely be added in case of a suspected tumoral or pseudotumoral lesion of the temporal bone. Using DWI, differentiation between cholesteatomatous (congenital or acquired) lesions or epidermoid cysts can be easily made. In case of lesions with a high cellularity (e.g., lymphoma), DWI can give a clue to the diagnosis by showing a strong diffusion restriction reflected by a very low apparent diffusion coefficient (ADC) value.

Tumor Extension

In the temporal bone, specific attention should be paid to tumoral invasion along the facial nerve. Spinocellular and basocellular carcinomas of the external auditory canal (EAC) as well as adenoid cystic carcinomas of the parotid gland have the propensity to spread along the course of the facial nerve ( Fig. 6.1 ). The stylomastoid foramen should always be included in the scanning volume to evaluate its fatty content with the centrally located hypointense facial nerve. Adenoid cystic carcinomas even have the propensity for skip metastases, so careful evaluation of the entire course of the facial nerve is mandatory.

Cholesteatoma and Keratosis Obturans

Cholesteatoma and keratosis obturans have been regarded in the past as the same disease process or variations of the same underlying condition. The terms have been used interchangeably in patients presenting with an accumulation of exfoliated keratin within the bony EAC. However, these are two distinct disorders with their own clinical presentations, physical and pathologic findings, and treatment.

Cholesteatomas are most frequently found in the middle ear and mastoid cavity. In rare cases, they originate in the EAC. Typically, this lesion presents in the older age group, is usually unilateral, and is accompanied by a history of chronic ear discharge and otalgia. Several possible causes, such as prior surgery and trauma, have been mentioned. Congenital and acquired cholesteatomas have been reported in cases of congenital stenosis or atresia ( Fig. 6.2 ). Usually external ear canal cholesteatoma requires surgical intervention to remove the cholesteatoma sac and adjacent necrotic bone. Imaging findings are nonspecific, showing a soft tissue mass in the EAC with associated bony erosion on CT. Usually the margins of the bony erosion are sharp and regular. In case of associated periostitis, the margins can become irregular. In these cases, differential diagnosis from a malignant tumor is difficult. It is known that accumulated keratin in a cholesteatoma sac causes clear hyperintensity on a b 1000 DWI MR image ; thus non-EP DWI sequences are mandatory to make the diagnosis.

Keratosis obturans commonly occurs in a younger patient group, often in patients with a history of sinusitis or bronchiectasis. Chronic ear discharge is rare. Symptoms are acute severe otalgia and conductive hearing loss. Contrary to cholesteatoma, keratosis obturans is often bilateral. Keratosis obturans more often causes diffuse widening of the EAC and less bone erosion.