What to think about an ossified falx cerebri?

Clinical History

A Caucasian 67-year-old man with a history of prostatic cancer was admitted to our centre in order to reevaluate his disease, due to the recent increasement of PSA (prostatic specific antigen) value.

Imaging Findings

A total body CT with intravenous iodinated contrast agent was performed in order to re-stage the disease.
The chest evaluation did not report pulmonary suspicious nodules; abdominal CT is negative except for enlarged para aortic lymph nodes at the left renal vessels level and close to the left common iliac vessels.
There are also diffuse and multiple osteoblastic bone metastases (with a "ground glass" aspect) in the entire spine, pelvis and sternal manubrium.
In the absence of brain metastases, cerebral CT shows a thickness of the falx cerebri with a density similar to bone tissue of the adjacent skull (figure 1c).

Discussion

The falx cerebri is a large crescent shaped “sickle like” fold of the dura mater that lies in the midline between the two cerebral hemispheres. Dura of the brain is composed of two layers: the periosteal dura mater (outer), attached to the inner surface of the skull, that serves as the periosteum and is composed by osteoprogenitor cells, fibroblasts, collagen bundles, blood vessels; and the meningeal dura mater (inner) that contains fibroblasts, fine collagen fibers, small blood vessels.
The falx cerebri ossification is a well-known phenomenon [1]; it consists of two cortical layers, like the calvarial bone, with the presence (in the 0.7% of the population) of fatty marrow in the middle [2].
Two types of falx cerebri ossification are described [3]:
Type A: The ossification begins and is in close proximity to the interior surface of the calvarium and continues to the falx-like osteophyte.
Type B: Ossification is located intracranially without any continuity to the calvarium.
The most frequent ossifications are localized in the frontal region of the falx, and more rarely are located in the middle area and in the posterior part [3]. In our case, we found a type B ossification located in the frontal region.
Since the falx cerebri is derived from multipotential mesenchymal cells, these can be stimulated by degeneration, irritation, haemorrhage or trauma, leading to the formation of bone tissue [4].
Falx cerebri ossification is usually an incidental finding, without clinical significance.
This new-formed bone behaves like membranous bone elsewere in the body and can be the site of metastatic involvement, myelometaplasia, leukemic infiltration, falcine osteosarcoma and calcified meningioma [1, 5, 6].
Endocrine disorders, basal cell nevous syndrome, myotonic dystrophy, brachyolmia are associated with an higher incidence of falx ossification [3, 8, 9]. In literature, there is often an overlap in the use of the terms "calcification" and "ossification" [1, 3]: in our opinion it is important to distinguish between the two terms because these entities have different aetiologies and consequently different outcomes. In fact, the calcification is a pure calcified deposit. On the other hand, ossification, as already described, needs greater consideration, because it is formed by normal bone tissue, and so can potentially be the seat of all the pathological processes of the normal bone tissue.
Therefore X-ray investigation is sufficient to identify falx anomalies, while CT study can discriminate between calcification and ossification. In case of doubt, MRI allows detection of all bone components, including demonstration of bone marrow represented by fat signal.

Differential Diagnosis List

Final Diagnosis

Falx cerebri ossification

URL:	https://www.eurorad.org/case/14270
DOI:	10.1594/EURORAD/CASE.14270
ISSN:	1563-4086