Introduction
Meningiomas are non-glial tumors of the central nervous system (CNS), accounting for ~37.6% of all intracranial tumors (1). There is a wide variety of symptoms in cases of symptomatic meningiomas, arising from the compression of nearby structures, straight attacks on or immediate changes in the brain, or due to the barrier of cerebrospinal fluid pathways or vessels (2).
Meningiomas are neoplasms that commonly occur in the brain and spine. Specifically, when occurring in the brain, there are meningothelial cell neoplasms, which commonly attach to the inner side of the dura matter (3).
The most common anatomical locations of meningiomas are falcine (18-22%), convexity (20-34%) and parasagittal (3). Ectopic meningiomas in the sphenoid and middle cranial fossa are more uncommon, and the majority of these occur in the skull of the head and neck area. Other much rarer locations involve the mediastinum, retroperitoneum, lungs, pelvis and extremities (3). Magnetic resonance imaging (MRI) is the diagnostic tool of choice for the study of meningiomas, given its higher contrast differentiation and its general ability to distinguish between intra- and extra-axial lesions (4).
Although the exact diagnosis of meningiomas with standard MRI imaging is, in most cases, easily established, there are unusual depictions of which make the diagnosis challenging. Furthermore, several other malignant and non-malignant neoplasms may mimic meningiomas. Thus, imaging findings can be variable. Intra-diploic meningiomas can exhibit both osteoblastic and osteolytic lesions; thus, possible differential diagnoses on computed tomography (CT) and MRI include fibrous dysplasia metastasis, osteosarcoma and intraosseous hemangioma (5).
In this context, the present study aimed to provide the radiologist or neurosurgeon with a better view of their various potential manifestations in order to be able to distinguish these tumors from the numerous lesions that can imitate their presentation and, thus, ameliorate the surgical planning. The present study describes 7 cases of different types of meningiomas. All the presented lesions had a histological diagnosis of meningioma. Differential diagnoses needs be tailored to the tumor sites and imaging data, although it can mainly include hypervascular tumors.
Discussion
Meningiomas are the most frequent type of brain tumor, accounting for 37.6% of primary brain tumors, with an adjusted yearly incidence of ~8.3 per 100,000 individuals in the USA (1,6). Although multiple risk factors have been identified, the majority of meningiomas develop spontaneously and are of unknown etiology (7). According to the World Health Organization (WHO) classification of the tumors of the CNS, meningiomas can be classified into various subtypes. Specifically, they are divided into grades I, II and III according to their histological characteristics (8). Moreover, the majority of meningiomas are slow-growing benign lesions, although a few exhibit a rapid growth (9).
Almost 98% of meningiomas are classified as non-malignant (WHO grades I or II), whereas 2% of these are classified as malignant. The incidence of meningiomas increases with age (mainly >65 years of age, more frequently affecting the African-American population and females more than males (10).
Meningiomas can be detected on any of the exterior surfaces of the brain and also within the ventricular system, and they originate from the stromal arachnoid cells of the choroid plexus (4). They are the second most common mass lesion of the cerebellopontine angle and can spread through foramina in the skull base (4). Other locations include the optic nerve sheath (0.4-1.3% of cases), the choroid plexus (0.5-3% of cases), the sella turcica, and rarely, outside of the dura with extracalvarial, calvarial, or both calvarial and extracalvarial extension, affecting the temporal bone, mediastinum, mandible and lungs, due to the trapping of the meningocytes or arachnoid cap cells during head trauma (4).
Meningiomas arise from arachnoid meningothelial cells. Intracranially, they are extra-axial masses that typically exhibit iso- to hypointensity on T1-weighted, and iso- to hyper-intensity on T2-weighted images of the cortex, demonstrating avid and often homogeneous post-contrast enhancement. Moreover, there is a strong enhancement that is typically observed following contrast administration (11).
On diffusion-weighted images as well, meningiomas can exhibit various intensities; thus, apparent diffusion coefficient values may differ significantly and may show no diffusion restraint as compared to the brain tissue (12).
Hyperostosis in the underlying bones, dural tails, calcification and linear internal flow voids are also frequently observed in meningiomas. More specifically, the dural tail sign does not represent a specific finding, as it can also be present in some metastases, glial tumors and lymphomas, and the latter are not typically associated with a dural tail to distinguish meningioma from schwannoma in the cerebellopontine angle (13,14).
Peritumoral brain edema can develop when a meningioma becomes large (15). The common MRI signal intensity features comprise an iso-intensity to modest hypointensity T1-weighted sequence in comparison to grey matter and iso-intensity to slight hyper-intensity on the T2 sequence. Following contrast administration, meningiomas traditionally present with avid, homogeneous enhancement with intermittent areas of central necrosis or calcification. Contrast can help to identify en plaque meningiomas. Calcification is typically best demonstrated on a CT scan and an MRI. The dural tail sign is pathognomonic of meningiomas (16).
Meningiomas may lead to changes in bone, which include osteolysis and hyperostosis, and are described in 20% of cases, with the latter being the most common and with the en plaque form. An enlargement of the skull base foramina can also be present (13).
Imaging with a CT scan can achieve a good depiction of the changes in bone associated with meningiomas, which may be appreciated on an MRI as well. Hyperostosis is the benign bone development of the direct tumor attachment to the bone and reactive hyper-vascularity of the periosteum. In 59% of cases, it may be difficult to discriminate against the hyperostosis of en plaque from the primary intraosseous meningioma, which is osteoblastic and may be associated with underlying homogeneous dural enhancement (17,18). The lack of contrast makes meningioma less obvious than other lesions on MRI. The majority of tumors may be identified due to their effects (displacement and edema) on the adjacent brain. Diagnostic issues arise when the meningiomas are small and have a minimal mass effect and minimal or no edema. In these cases, careful attention is required to identify the subtle anatomic distortion and to proceed to intravenous contrast administration, which is the key to correct diagnosis (19).
In multiple myeloma (MM), the accumulation of plasma cells in the CNS or dura is rare. The intracranial manifestation of MM includes either a diffuse leptomeningeal attachment or, less often, a single tumor. The involvement of leptomeninges can be observed as a diffuse enhancement, similar to meningitis, or as focal masses, mimicking meningiomas (20). Epidural involvement can also be observed. Several cases of MM with myelomatous meningeal participation have been described (21,22). The advanced stage of MM, along with circulating plasma cells, indicated that malignant cells spread to the meninges through a hematogenous route. Inappropriately, imaging analyses are often nonspecific, and the differential diagnosis includes carcinomatosis, metastasis, plasmacytoma, dural granulocytic sarcoma and meningitis (23).
In conclusion, the present study provided some examples of types of meningiomas with wide-range potential manifestations and imaging variants where the diagnosis is often challenging. Thus, the reporting radiologist or neurosurgeon needs to be aware of their alternative presences to differentiate these tumors from others that can imitate their appearance.