Identifying benign and malignant lesions of the skull on brain computed tomography (CT)

It is essential to look carefully at the skull on all head computed tomography (CT) imaging scans, as well as the brain. Unexpected benign and malignant skull diseases are common; you should be aware of the appearance of benign lesions so that unnecessary imaging or procedures are avoided.

Let’s look at identifying features of the following seven types of skull lesions on CT:

1. Sclerotic (osteoblastic) lesions
2. Lytic skull lesions
3. Mixed sclerotic and lytic lesions
4. Arachnoid granulation
5. Osteoma of bone
6. Hemangioma of bone
7. Paget’s disease

Bone lesions can be evident on CT when they are of higher attenuation than the skull due to excessive bone formation. These are called sclerotic (osteoblastic) lesions. When the bone appears destroyed, making the lesion appear of lower attenuation than the normal skull, the lesion is described as osteolytic.

Cases 1 and 2: Osteoblastic lesions

First, let’s examine CT scans from two patients with unsuspected skull abnormalities. The abnormalities are of higher attenuation than the skull.

One case features a right-sided skull fracture and a well-marginated lesion with homogeneous dense bone that appears expanded. This appearance of bone is called ground-glass, and it is typical for a benign process in the bone called fibrous dysplasia.

The other case involves a 66-year-old man with a sclerotic bone lesion that was noted along with other similar lesions in the skull. When single, a benign bone island would be a consideration. But, when you see multiple lesions, you should consider the diagnosis of osteoblastic metastases. This is the typical appearance of metastatic bone disease from prostate cancer, but in women, treated breast cancer metastases can also have this appearance.

Case 3: Chordoma

Rather than added bone, our next case features a destructive or osteolytic lesion on CT in the patient’s clivus. At the same level on T2-weighted magnetic resonance imaging (MRI), a soft tissue abnormality with high-signal intensity extends beyond the skull.

These findings are typical for a chordoma, which is a primary malignant bone tumor that frequently involves the clivus (but can also be found in the spine).

Case 4: Multiple myeloma

Our next case features a bone-filtered CT image which shows two lytic lesions in the skull. On CT, there is no evidence of a dense rim around the lesions; they just fade into the skull at the edges. This appearance is consistent with metastatic disease of the skull, which will require further evaluation. This patient proved to have multiple myeloma.

Case 5: Arrested pneumatization

In our next case, the patient has inhomogeneous bone at the skull base with areas of both low and high attenuation. Unlike the previous cases, the lesion is not purely lytic or osteoblastic in character.

This represents a benign finding called arrested pneumatization that is typically seen in this location. The low attenuation areas are not really lytic since they do not represent bone destruction; they are regions of fat intermixed with bone. If you have any questions about the diagnosis, consider a follow-up CT or MRI.

Case 6 and 7: Arachnoid granulations

Keep in mind that not all lytic skull lesions are due to malignancy. In our next case, a lesion was noted in the skull of a patient with cancer. It resembles a lytic bone lesion, except that you can see a thin rim of bone around the lesion and the skull is not expanded. For this reason, an MRI was requested rather than a biopsy.

The T2-weighted MRI, in this case, shows that the skull lesion resembles the signal of cerebrospinal fluid seen elsewhere in the brain. The post-contrast MRI demonstrated no enhancement, which would be expected with a metastatic lesion. These are typical findings for arachnoid granulations and require no follow-up. Note it is adjacent to a dural sinus.

Our next case features erosions of the inner table of the skull on either side of the midline. This is also a manifestation of the bone changes seen with arachnoid granulations. Note the smooth margin and proximity to the superior sagittal sinus, and also notice that there is only peripheral enhancement visible on MRI.

Case 8: Osteoma of the skull

Our next example features a 65-year-old patient. Focal increased attenuation was noted on the patient’s skull x-ray, and a CT was requested since a malignancy is always considered in a patient of this age.

The axial bone-filtered CT image showed very dense bone projecting from the outer table of the skull with the same attenuation as the skull cortex. This is the typical appearance of an osteoma of the skull and requires no further follow-up.

Case 9: Benign capillary hemangioma

An MRI with contrast from another 65-year-old patient demonstrated a single enhancing skull lesion that was of concern, and the possibility of a metastatic lesion was mentioned in the imaging report. A CT scan was obtained since bone-filtered CT images are very helpful whenever characterizing a skull lesion.

The bone-filtered CT demonstrated a fine internal structure at the site of the abnormality rather than bone destruction, which would be expected with metastatic disease. This is the typical appearance of a benign capillary hemangioma of bone and illustrates how CT and MRI findings should be consolidated when considering skull lesions.

Case 10: Cavernous hemangioma

Our next patient also has a hemangioma of the skull. While this hemangioma has an internal structure on CT, it is more expansive and demonstrates radiating or spoke-wheel spicules of bone. This is called a cavernous hemangioma and is less common than capillary hemangiomas. And it is just as well, since these can grow and require surgery to reduce the mass effect.

Case 11: Paget’s disease

On our next scan, you can see that the patient’s skull is expanded on the left side with mixed high and low attenuation. While it is reasonable to consider fibrous dysplasia, you should consider Paget’s disease in an adult when you see skull expansion with both sclerotic and lucent bone. Magnetic resonance imaging in such cases can be helpful since it is common to see preserved fatty marrow in the region of the Pagetic bone.

Integration of the findings on MRI and CT are frequently necessary to characterize a lesion in the skull. After reviewing this article, you should be better equipped to identify several types of benign and malignant skull lesions on CT. Of course, you first have to find the lesion before you can identify what type it is, so look carefully at the skull on all head CT scans.

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