Skull Base Bone Lesions I: Imaging Technique, Developmental and Diffuse Bone Lesions

Introduction

The skull base is a bony-cartilaginous structure, oriented in the axial plane, providing a barrier between the intracranial compartment and the extracranial head and neck. It is pierced by several neurovascular foramina, which provide a crossroad for disease to spread between these two compartments. Most tumors affecting the skull base have their origin outside the skull base proper, mainly from the suprahyoid neck, and secondarily affect the skull base by direct or perineural extent. Primary and secondary bone tumors tend to be forgotten. The skull base division into anterior, central, and posterior, so useful in the differential diagnosis of extrinsic skull base lesions, does not apply when lesions originate from the fibrocartilaginous elements of the skull base proper as they are common to all skull base compartments. Therefore, when facing a lesion arising from bone the same rules for the differential diagnosis of bone lesions seen elsewhere in the body should be applied.

Except for metastases, bone tumors of the skull base are overall rare and often a diagnostic dilemma. Benign tumors and bone dysplasias can behave aggressively, and some malignant tumors can look misleadingly benign. Although imaging features can be quite helpful in the differential diagnosis, they are not very familiar to radiologists who do not routinely deal with skull base lesions and only a scant specific literature is available on this subject. Moreover, secondary/metastatic tumors account for the majority of bony skull base lesions, with primary skull base bone tumors being rather rare. To simplify the approach, it is useful to divide bone lesions of the skull base into three major groups: those related to developmental anomalies and embryonic remnants, primary and secondary bone tumors, and diffuse or multifocal bone lesions ( Table 15.1 ).

This chapter reviews the imaging technique and focuses on developmental lesions originating from embryonic remnants trapped within the skull base during embryonic development and diffuse bone lesions, including bone dysplasias and infection; the following chapter addresses primary and secondary bone tumors.

Imaging Technique

Imaging of bone lesions in the skull base obeys the same principles applied to bone lesions elsewhere. To the exception of plain radiographs, which, in most centers, are no longer performed to investigate the skull base, and ultrasound imaging, with obvious limitations in the assessment of this anatomic region, the same imaging techniques (CT, MRI, bone scintigraphy, and fludeoxyglucose [FDG]-PET CT) are routinely used for diagnosing, staging, and follow-up of patients with skull base bone lesions. Volumetric CT acquisition parallel to the skull base with axial, coronal, and sagittal thin, millimetric, reconstructions on both soft tissue and high-resolution bone algorithm should be obtained. Contrast-enhanced imaging is performed to assess the pattern of vascularization of a given lesion, as well as to determine its relationship with major adjacent vessels and assess their patency. It provides a map of the bony anatomy, required for surgical planning, and allows three-dimensional (3D) reconstructions to help planning skull base reconstruction or grafting on the basis of 3D modeling and 3D printing. CT is the imaging technique with the highest sensitivity to depict calcification and ossification, critical in the assessment of bone or cartilage matrix-producing tumors and in depicting different types of periosteal reaction. The pattern of bone involvement and the evaluation of the transition zone, between tumor and normal bone, are also best depicted on CT. Still, for some authors, CT remains the most accurate technique in the diagnosis of fibroosseous lesions, which may show misleading imaging findings on other techniques, such as MR and bone scintigraphy. Moreover, as most bone lesions heal by ossification and sclerosis, CT is very useful to assess tumor response to treatment.

MRI is the modality of choice to evaluate the full extent of a bone tumor, including the extent of bone marrow involvement, presence of soft tissue components, meningeal and intracranial extent, involvement of cranial nerves and vessels, and relationship with adjacent soft tissues of the supra-hyoid neck. T1-weighted (T1W) images show clearly abnormal replacement of the fatty bone marrow, whereas T2-weighted (T2W) and short-tau inversion recovery (STIR) images clearly depict tumor matrix and any associated bone marrow edema. Contrast-enhanced fat-suppressed T1W images evaluate tumor enhancement and can clearly differentiate enhancing lesions from the surrounding fatty marrow. MR is also used to assess tumor response to chemo and radiation treatment by differentiating necrosis from viable tumor. This information can be derived from contrast-enhanced fat-suppressed T1W images, diffusion-weighted images (DWI), and dynamic contrast-enhanced perfusion-weighted images (DCE-PWI). DWI is very useful to support the diagnosis of small round cell tumors that tend to show low mean apparent diffusion coefficient (ADC) values and to differentiate bone malignancies from infection. Moreover, in the case of skull base osteomyelitis, it is routinely used to depict complications such as abscesses or subdural empyema, also featured by restricted diffusion. Whole-body DWI can depict diffuse and multifocal abnormalities and is being increasingly used for staging multiple myeloma and metastatic bone disease. MR angiography (MRA) and MR venography (MRV) may be required to assess the patency of any adjacent arteries and rule out dural sinus invasion or thrombosis, respectively. Bone scintigraphy using ^99m Tc-MDP (methylene diphosphonate) and ¹⁸ F-NaF PET-CT are very sensitive techniques in the detection of osteoblastic activity, whereas ¹⁸ FDG-PET depicts tissues with high glucose metabolic rates. These functional techniques have the advantage of imaging the entire skeleton and therefore are useful to detect diffuse and multiple lesions. ⁶⁷ Ga-single-photon emission CT (SPECT) and ^99m Tc MDP-triphasic SPECT should be the choice for the early diagnosis and to monitor treatment response in patients with skull base osteomyelitis.

Bone scans can be used to depict bone-forming tumors, including osteogenic primary tumors and sclerotic metastasis, as well as any bone healing process. Tumors causing rapid bone destruction with no compensatory bone formation are missed by this technique, with the most striking examples being multiple myeloma and bone metastases from thyroid cancer. The flare phenomenon, a diffuse increase in ^99m Tc-MDP uptake after treatment of bone metastases persisting until 6 months after treatment, is a well-known limitation in the assessment of tumor response and should not be mistaken for disease progression.

Imaging is also widely used to direct biopsies to relevant tumor areas avoiding cystic/necrotic regions. In skull base lesions, most biopsies are performed endoscopically.