Miscellaneous Tumors and Tumor-Like Conditions

Main Text

Preamble

Some important neoplasms that affect the calvaria, skull base, and cranial meninges are not included in the 2021 5th edition WHO classification of CNS tumors. This chapter covers several of these intriguing tumors as well as tumor-like lesions that do not easily fit into other sections. Although infections, granulomatous disease, demyelinating disorders, and vascular diseases (among others) may sometimes mimic CNS neoplasms, they are treated separately in their own respective chapters.

We begin with extra cranial tumors and tumor-like conditions. These lesions mostly arise within the calvaria or skull base. We then turn our attention to an interesting group of intra cranial lesions that all mimic neoplasms, i.e., they are pseudotumors. These tumor-like lesions may arise within the meninges, CSF cisterns, or brain parenchyma. Some lesions may involve multiple compartments and can be intracranial, extracranial, or a combination of both.

Extracranial Tumors and Tumor-Like Conditions

Fibrous Dysplasia

Benign fibroosseous lesions of the craniofacial complex are represented by a variety of intraosseous disease processes. These include bone dysplasias, the most common of which is fibrous dysplasia (FD).

Terminology

FD is a benign, dysplastic fibroosseous lesion that is also known as fibrocartilaginous dysplasia, osteitis fibrosa, and generalized fibrocystic disease of bone. FD can occur in one (monostotic FD) or multiple bones (polyostotic FD).

Etiology

General Concepts

FD is a developmental lesion with local arrest of normal structural/architectural development. Abnormal differentiation of osteoblasts results in replacement of normal marrow and cancellous bone by immature “woven” bone and fibrous stroma.

Genetics

Recent studies have demonstrated that FD is a neoplastic—not dysplastic—lesion. Activating mutations of the GNAS gene result in overexpression of the c-Fos protooncogene, which contributes to the initiation and progression of FD.

Pathology

Location

Although FD occurs throughout the skeleton, it is most commonly found in craniofacial bones, long bones, and spine.

Virtually any bone in the head and neck can be affected by FD. The skull and facial bones are the location of 10-25% of all monostotic FD lesions. The frontal bone is the most common calvarial site, followed by the temporal bone, sphenoid, and parietal bones. Involvement of the clivus is rare. The orbit, zygoma, maxilla, and mandible are the most frequent sites in the face (33-1).

Size and Number

FD lesions range in size from relatively small (< 1 cm) to massive lesions that involve virtually an entire bone. Altered osteogenesis may occur within a single bone (“monostotic FD”) or multiple bones (“polyostotic FD”). Monostotic FD accounts for ~ 60-80% of all lesions;polyostotic FD occurs in 20-40% of cases.

Polyostotic FD with endocrinopathy is known as McCune-Albright syndrome (MAS) and occurs in 3-5% of cases. The classic MAS triad consists of multiple FD lesions, endocrine dysfunction (typically precocious puberty), and cutaneous hyperpigmentation (“café au lait” spots).

Gross Pathology

FD is a well-defined, tan to whitish gray mass (33-2). Depending on the relative amount of fibrous vs. osseous content, texture varies from firm and rubbery to “gritty.” Prominent cyst formation can be present in older lesions.

Microscopic Features

Fibrous and osseous tissues are admixed in varying proportions (33-3). In the early stages, pronounced osteogenesis with thin osteoid anastomosing trabeculae rimmed with osteoblasts is seen. A stromal fibroblastic element with variable vascularity is interspersed between irregular, curvilinear trabeculae of immature “woven” bone that resembles Chinese letters.

Almost 60% of cases demonstrate different stromal patterns admixed with the usual fibroblastic elements. These include focal fatty metamorphosis (20-25%), myxoid stroma (15%), and calcifications (12%). Cystic degeneration occurs but is uncommon.

Clinical Issues

Epidemiology

FD is rare, representing ~ 5-7% of all benign bone tumors. It is the second most common pediatric primary skull lesion (after dermoid cysts).

Demographics

Although FD can present at virtually any age, most patients are younger than 30 years at the time of initial diagnosis. Polyostotic FD presents earlier; the mean age is eight years. With the exception of FD as part of MAS, which affects female patients more than male patients, there is no sex predilection.

Presentation

Symptoms of craniofacial FD depend on lesion location. Painless osseous expansion with calvarial or facial asymmetry is common. Proptosis and optic neuropathy are common in patients with orbital disease. Conductive hearing loss and facial weakness are typical in patients with temporal bone FD. Mandibular FD typically presents with “cherubism.”

Polyostotic FD may cause “leontiasis ossea” (lion-like physiognomy) or complex cranial neuropathies (secondary to severe narrowing of the neural foramina).

Natural History

Disease course varies. Monostotic lesions do not regress or disappear, but they generally stabilize at puberty. In contrast, polyostotic FD generally becomes less active after puberty, although long bone deformities may progress, and microfractures may develop.

Patients with MAS are more likely to have pain, pathologic fractures, more bones involved, more aggressive disease, and more complications than those with polyostotic or monostotic FD.

Malignant sarcomatous transformation is a rare and challenging complication of craniofacial FD, occurring in < 1-9% of all FD cases, and has been described in both the monostotic and polyostotic forms.

Treatment options for FD are limited. Recurrence is very high following curettage and bone grafting. Radiation therapy is generally avoided, as it may induce malignant transformation. Intravenous bisphosphonate therapy has been used to ameliorate the disease course with some reported success.

Imaging

General Features

Most craniofacial lesions are monostotic. However, skeletal survey or whole-body MR is recommended to detect asymptomatic lesions in other bones that would indicate polyostotic disease or MAS.

Imaging findings depend on disease stage. In general, very early lesions are radiolucent and then undergo progressive calcification, resulting in a ground-glass appearance. Mixed patterns are common.

CT Findings

Nonaggressive osseous remodeling and thickening of the affected bone are typical. NECT shows a geographic expansile lesion centered in the medullary cavity. Abrupt transition between the lesion and adjacent normal bone is typical.

Bone CT appearance varies with the relative content of fibrous vs. osseous tissue. FD can be sclerotic, cystic, or mixed (sometimes called pagetoid). A pattern with mixed areas of radiopacity and radiolucency is found in almost 1/2 of all cases (33-4A) (33-5). The classic, relatively homogeneous ground-glass appearance occurs in 25%. Densely sclerotic lesions are common in the skull base. Almost 1/4 of all FD cases have some cystic changes, seen as central lucent areas with thinned but sclerotic borders.

MR Findings

Signal intensity on MR is quite variable, and the diagnosis of FD should not be based on MR alone. FD lesions may show evidence of cortical destruction &/or soft tissue extension and still be benign. Recent studies have shown FD is homogeneously hypointense on T1WI in 60% of cases, although almost 20% have some hyperintense foci.

Signal intensity on T2WI is also quite variable. Moderate hypointensity is characteristic of ossified &/or fibrous portions of the lesion (33-4B). Active lesions may be heterogeneous and may have hyperintense areas on T2 or FLAIR (33-5D). Cysts appear as rounded high-signal foci.

Enhancement following contrast administration varies depending on the lesion stage and ranges from no enhancement to diffuse, avid enhancement in active lesions (33-4C). Slightly over 50% of cases exhibit at least some enhancement.

Secondary aneurysmal bone cyst-like changes are relatively common, seen in 5-6% of cases. In these cases, fluid-fluid levels are present in 50%.

Nuclear Medicine

FDG PET and Ga-68 PET/CT show increased metabolic activity in one or more sites and can mimic metastatic disease.

Differential Diagnosis

The major differential diagnoses for craniofacial FD are Paget disease and ossifying fibroma (OF).

Paget disease typically occurs in older adult patients and usually involves the calvaria and temporal bone. A cotton wool appearance is typical on digital skull radiographs and bone CT.

OF may mimic the cystic monostotic form of FD. OF has a thick, bony rim with a lower density center on bone CT and generally appears more mass-like and localized. Diffuse sclerosing osteomyelitis of the mandible may also resemble FD.

Intraosseous meningioma is another differential consideration. Intraosseous meningiomas are more common in the calvaria than in the skull base and facial bones. A strongly enhancing en plaque soft tissue mass is often associated with the bony lesion. A mixed sclerotic-destructive skull base metastasis may mimic FD. In most cases, an extracranial primary site is known.

The DDx of FD includes rare fibroosseous disorders that can affect the craniofacial bones. These include osteitis deformans, florid osseous dysplasia, focal cementoosseous dysplasia,and periapical cemental dysplasia.

Facial bone changes associated with hyperparathyroidism and renal osteodystrophy may present with a classic ground-glass appearance on both conventional radiography and CT. However, in contrast to FD, these changes are generalized and diffuse.

Paget Disease

Terminology

Paget disease (PaD) of bone is a metabolic bone disorder characterized by bone resorption followed by compensatory bone formation.

Etiology

The initial abnormality is an increase in osteoclast activity followed by rapid compensatory bone formation. Lamellar bone is replaced by weak “woven bone” that is more susceptible to fractures and deformities.

Genetic alterations occur in both classic PaD of older adults and the uncommon familial Paget-like bone dysplasias that arise during childhood. Mutations involved in osteoclast differentiation affect function of the RANKL molecular pathway, a membrane protein on osteoblasts that induces differentiation and activation.

Pathology

PaD follows a well-established sequence of events. First, osteoclast expansion generates lytic lesions. A combination of bone resorption with new disorganized bone formation then ensues. In the final phase, bone formation predominates, leading to bone enlargement and sclerosis.

Location, Size, and Number

The skull (both calvaria and skull base) is affected in 25-65% of patients and is often asymptomatic (33-6). In contrast to FD, PaD is more commonly polyostotic (65-90% of cases).

Gross Pathology

The pagetoid skull shows diffuse thickening (33-7). Patches of fibrovascular tissue initially replace fatty marrow.

Microscopic Features

In the early lytic stage, active PaD is characterized by cellular fibroosseous lesions with minimally calcified osteoid trabeculae. Increased vascularity is common. Osteoblastic rimming is present together with osteoclastic resorptive lacunae. Osteoclasts are numerous and larger than normal; they also have increased numbers of nuclei.

In the inactive stage, bone turnover and excessive vascularity decrease and the trabeculae coarsen.

Clinical Issues

Epidemiology

PaD is common, affecting up to 10% of individuals over the age of 80 years. It is especially prevalent in the United States, the British Isles, Canada, Australia, and some parts of Western Europe. PaD is rare in Asia and Africa.

Demographics

Classic PaD is a disease of older adults. Most patients are 55-85 years of age with < 5% of cases occurring in patients under the age of 40 years. There is a moderate male predominance.

Juvenile PaD, a.k.a. idiopathic hyperphosphatasia, is an autosomal-recessive bone dysplasia. It begins in infancy or early childhood and is characterized by long bone widening, acetabular protrusion, pathologic fractures, and skull thickening.

Presentation

Presentation varies with location, and all bones of the craniofacial complex can be affected. Patients with calvarial PaD may experience increasing hat size. Cranial neuropathy is common with skull base lesions, most commonly affecting CNVIII. Patients may present with either conductive (ossicular involvement) or sensorineural hearing loss (cochlear involvement or bony compression).

Markedly elevated serum alkaline phosphatase is a constant feature, whereas calcium and phosphate levels remain within normal range.

Natural History

In the extracranial skeleton, osseous expansion with progressive skeletal deformity is typical. Osseous weakening leads to long bone deformities and fractures. In comparison, craniofacial PaD generally has a more benign course and may remain asymptomatic for many years.

Two neoplastic processes are associated with PaD: Giant cell tumor (benign) and sarcoma (malignant). Giant cell tumor is an expansile intraosseous mass that usually occurs in the epiphyses and metaphyses of long bones in patients with longstanding polyostotic PaD. Giant cell tumors that arise secondarily in pagetoid bone are rare. Just 2% occur in the skull, where the most common site is the sphenoid bone. Involvement of the calvarial vault is rare.

The most feared complication in PD is sarcomatous transformation. Malignant transformation to osteosarcoma occurs in 0.5-1.0% of cases and is generally seen in patients with widespread disease. The humerus, femur, and pelvis are the most common sites; sarcomatous transformation in the craniofacial bones is rare.

Most pagetic osteosarcomas are high grade and have already metastasized at the time of diagnosis. Only 15% of patients survive beyond two or three years.

Treatment Options

Bisphosphonates reduce bone turnover and have been effective in many cases of PaD.

Imaging

General Features

Imaging findings in PaD vary with disease stage. In the early active “lytic” phase, radiolucent lesions develop in one or multiple bones. In the calvaria, this condition is termed osteoporosis circumscripta and is characterized by round osteolysis that may cross suture lines.

In the “mixed” phase, enlarged bone with mixed lytic and sclerotic foci and confluent nodular calcifications follows (the cotton wool appearance). The final inactive or quiescent stage is seen as dense bony sclerosis.

CT Findings

In early PaD, bone CT shows well-defined lytic foci (osteoporosis circumscripta) (33-9). Mixed areas of bony lysis and sclerosis then develop, producing the cotton wool appearance (33-8). Varying degrees of dense bony sclerosis eventually develop (33-11).

In severe cases, the softened expanded skull base can produce basilar invagination.

MR Findings

The MR features of PaD depend on the phase of the disorder. Multifocal T1-hypointense lesions are intermixed with foci of residual yellow fatty marrow (33-10A). Signal intensity on T2WI is often heterogeneous (33-10B). Patchy enhancement on T1 C+ can occur in the advancing hypervascular zone of active PaD (33-10C).

Nuclear Medicine

Bone scans detect the reaction of bone to the disease and not the destruction of bone. The active stage of PaD shows markedly increased uptake on Tc-99m bone scans with well-delineated margins between the affected and normal regions of bone. Nonactive pagetic lesions in the late “burnt-out” disease stage may not be detected.

18F-NaF PET/CT can also demonstrate high uptake in PD, mimicking metastatic disease.

Differential Diagnosis

FD may appear very similar to craniofacial PaD. However, PaD occurs mostly in older adults and does not have the typical ground-glass appearance that often characterizes FD.

Sclerotic metastases may resemble PaD, but no trabecular coarsening or bony enlargement is present. The early lytic phase of PaD may resemble lytic metastases or multiple myeloma; neither enlarges the affected bone.

Aneurysmal Bone Cyst

Terminology

Aneurysmal bone cysts (ABCs) are benign, expansile, multicystic lesions that typically develop in childhood or early adulthood. At least 70% of ABCs are primary lesions; the rest arise secondarily within a preexisting benign tumor, such as giant cell tumor or osteoblastoma.

Pathology

The most common overall ABC location is the metaphysis of long bones (70-80% of cases) with the vertebrae (generally the posterior elements) being the site of 15% of lesions.

The craniofacial bones are a relatively uncommon location. Lesions can occur in the jaws (maxilla, mandible), petrous temporal bone, basisphenoid, and paranasal sinuses. ABCs of the skull and orbit are rare, accounting for < 1% of all cases.

ABCs consist of blood-filled cavernous spaces with intracystic hemorrhages of variable ages. Multiple variably sized cysts are separated by septa lined by endothelium, spindle-shaped fibroblasts, and scattered multinucleated giant cells.

Clinical Issues

ABCs represent 5% of all primary bone tumors and are the second most common pathologically proven bone tumor of childhood. About 70% occur in the first two decades with a slight male predominance. Symptoms vary with location. Many lesions are asymptomatic or present with slowly progressive swelling.

Treatments for symptomatic ABC are curettage, cryosurgery, and bone graft. Recurrence rates are high, varying from 20-50%. Preoperative embolization may be helpful in selected cases.

Imaging

NECT scans show an eccentric lesion with expanded, remodeled, ballooned (“aneurysmally dilated”) bone surrounded by a thin sclerotic rim (33-12). Multiple cystic spaces with fluid-fluid levels are present (33-13).

MR shows a multicystic lesion with a hypointense rim surrounding multiple fluid-filled spaces. Hemorrhages of varying ages with fluid-fluid levels are a prominent imaging feature, as are smaller cysts (“diverticula”) that project from larger lesions. The surrounding rim and fibrous septa enhance following contrast administration (33-13).

A rare manifestation is so-called solid variant ABCs. These typically lack the characteristic radiographic features and have a wide differential diagnosis, including Ewing sarcoma, Langerhans cell histiocytosis, osteosarcoma (OS), metastasis, and giant cell tumor.

Differential Diagnosis

Some ABCs may have a phase of relatively rapid growth and can be mistaken clinically for a more aggressive lesion. The most important imaging differential diagnosis of ABC is telangiectatic OS, which may have fluid-fluid levels that resemble those of ABC. Incomplete margination, soft tissue mass, cortical destruction, and significant solid portions should suggest telangiectatic OS instead.

Giant cell tumor and osteoblastoma are associated with secondary ABC, and both show significant solid components.

Intracranial Pseudotumors

Ecchordosis Physaliphora

Ecchordosis physaliphora (EP) is a small (usually < 1 cm), gelatinous soft tissue mass that represents an ectopic notochordal remnant (33-15). Ectopic notochordal rests can occur anywhere along the midline craniospinal axis from the dorsum sellae (33-14)to the sacrococcygeal region. EPs are generally asymptomatic and incidental findings at imaging or autopsy. A few reported cases have presented with CSF rhinorrhea of variable duration.

Histopathologically, EPs consist of physaliphorous cells imbedded in a myxoid matrix. The cells are characterized by large mucin-containing intracytoplasmic vacuoles. Necrosis and mitoses are absent.

Imaging features of EPs are quite characteristic. CT demonstrates a well-delineated hypodense, nonenhancing midline intraclival mass with scalloped sclerotic margins (33-17).

The key imaging feature of EP that distinguishes it from other similar-appearing lesions is the presence of a small pedicle or stalk that connects the clival lesion to an intradural component in the prepontine cistern. Best demonstrated on MR, EPs are hypointense to brain on T1WI (33-16A)and hyperintense relative to CSF on T2WI (33-16B). EPs do not suppress on FLAIR nor enhance following contrast administration (33-18). Follow-up studies show no change in lesion size.

The major differential diagnosis of EP in the basisphenoid bone is clival chordoma.Chordomas are permeative destructive lesions. Other prepontine cistern lesions that can mimic EP include arachnoid, neurenteric, epidermoid, and dermoid cysts. Arachnoid cysts are much more common in the cerebellopontine angle cisterns and behave exactly like CSF on all sequences.

Neurenteric cysts are often slightly off-midline and somewhat lower, adjacent to the pontomedullary junction. Epidermoid cysts (ECs) are irregular, somewhat frond-like lesions that restrict on DWI. ECs are more common in the cerebellopontine angle cisterns. Dermoid cysts usually follow fat signal, not CSF.

Intracranial Foreign Body Granuloma

Retained surgical elements, such as cotton balls and Gelfoam, may induce an excessive inflammatory reaction that may be difficult to distinguish from recurrent or residual tumor on neuroimaging studies.

Terminology

Intracranial foreign body granuloma (FBG) is a rare inflammatory reaction to retained surgical materials that can appear months to years following the initial procedure.

FBG (a.k.a. textiloma) refers to a mass created by a retained surgical element (inadvertently or deliberately left behind) and its associated foreign body inflammatory reaction. The terms gossypiboma, gauzoma, and muslinoma refer specifically to retained nonresorbable cotton or woven materials.

Etiology

Hemostatic agents can be resorbable or nonresorbable. All classes of resorbable and nonresorbable agents may produce textilomas as an allergic response.

Resorbable agents include gelatin sponge, oxidized cellulose, and microfibrillar collagen. Nonresorbable agents include various forms of cotton pledgets, cloth (i.e., muslin), and synthetic rayon. Although bioabsorbable hemostats are often left in place, nonresorbable agents are typically removed prior to surgical closure. Any of these materials may induce an inflammatory reaction, creating a textiloma.

Rare FBGs have been reported following endovascular coil embolization, mechanical thrombectomy, or use of hydrophilic polymers.

Pathology

Most FBGs occur within surgical resection sites or around muslin-reinforced aneurysms. Histologic examination typically shows a core of degenerating inert hemostatic agent surrounded by inflammatory reaction. Foreign body giant cells and histiocytes are often present. Each agent exhibits distinctive histologic features, often permitting specific identification (33-19B).

Clinical Issues

Intracranial FBGs are rare with around 100 cases reported in the literature. Most reported cases following brain tumor resection were diagnosed within the first 3 postsurgical months and were often identified incidentally during routine follow-up imaging.

Imaging

Intracranial FBGs are located adjacent to the resection cavity and are almost always iso- or hypointense on T1WI. Approximately 45% are iso- and 40% are hypointense on T2/FLAIR (33-19A). Some “blooming” on T2* may be present. All reported cases enhance on postcontrast scans (33-20). Ring and heterogeneous solid enhancement patterns occur almost equally. Central diffusion restriction is common.

FBGs occurring after cerebral embolization procedures may show multiple foci of FLAIR hyperintensity and contrast-enhanced nodular lesions in the affected vascular distribution.

Differential Diagnosis

The major differential diagnosis is recurrent neoplasm or radiation necrosis. Residual or recurrent tumor can coexist with textiloma. If present, T2 hypointensity helps distinguish textiloma from neoplasm or abscess. Definitive diagnosis typically requires biopsy and histologic examination with both routine stains and polarized light.

Calcifying Pseudoneoplasm of Neuraxis

Terminology

Calcifying pseudoneoplasm of the neuraxis (CAPNON) is a rare nonneoplastic lesion of the CNS. Calcifying pseudoneoplasms are also known as fibroosseous lesions, cerebral calculi, brain stones, brain rocks, and crudoma.

Etiology

The precise origin of CAPNONs is unknown. They have been attributed to a reactive proliferative process induced by infection, inflammation, degeneration, or injury, although many are discovered incidentally on imaging studies without an identifiable etiology.

Pathology

CAPNONs are discrete nonneoplastic, noninflammatory lesions. Most are solitary; multiple lesions have been described but are uncommon. CAPNONs can occur anywhere in the CNS, including the spine. Most are extraaxial, but intraparenchymal and even intraventricular CAPNONs do occur.

Grossly, CAPNONs are well-demarcated, white, firm/gritty, intra- or extraaxial masses. They contain various combinations of chondromyxoid and fibrovascular stroma, metaplastic calcification, and—in some cases—even ossification with the appearance of mature bone.

CAPNONs are often surrounded peripherally by palisading spindle/epithelioid cells (“cortical layer”) resembling arachnoid cells. Positive immunoreactivity to vimentin and epithelial membrane antigen (EMA) are typical. GFAP and S100 protein are typically negative, helping distinguish CAPNON from astrocytic neoplasms and meningioma.

Rarely, CAPNONs exhibit characteristics of foreign body reactions with giant cells, tissue ossification, and the formation of lamellar bone or scattered psammoma bodies. The surrounding brain may exhibit inflammatory changes with gliosis and edema leading to mass effect.

Clinical Issues

Although age at diagnosis ranges from 2 to 90 years, CAPNONs affect mostly middle-aged adults and are rare in the pediatric population. There is a slight male predominance.

Presentation is site dependent. Some intracranial CAPNONs are asymptomatic and discovered incidentally on imaging studies, although seizures and headache have been reported. A few cases have been reported in association with meningioangiomatosis and neurofibromatosis type 2.

Imaging

CAPNONs are heavily calcified, well-delineated, leptomeningeal (extraaxial) (33-23)or parenchymal (intraaxial) masses resembling fibroosseous lesions. NECT scans demonstrate a heavily calcified leptomeningeal, deep intrasulcal, or brain parenchymal “rock”(33-21). The calcifications can be solid or partial, peripheral or central.

On MR, typical CAPNONs demonstrate little mass effect, are iso- to hypointense on T1WI, and are uniformly hypointense on T2WI and FLAIR. Mild “blooming” is seen on T2* GRE and SWI (33-23).

Perilesional edema varies from none to extensive in some cases of intraaxial CAPNONs (33-22). Enhancement varies from none to moderate. Solid, linear, serpiginous, and peripheral rim-like enhancement patterns have all been reported.

Differential Diagnosis

The differential diagnosis of CAPNON includes an ossified vascular lesion—most often a cavernous malformation.Although heavily calcified cavernous malformations can often be distinguished by their “popcorn” mixed hyperintensity on T2WI, blood-fluid levels, and hemosiderin rim, biopsy is usually necessary for definitive diagnosis. Residua of infection, such as calcified tuberculous granuloma or neurocysticercosis, can also resemble CAPNONs.

CAPNON can mimic densely calcified neoplasms, such as oligodendroglioma, meningioma, and polymorphous low-grade neuroepithelial tumor of the young (PLNTY). The rare intraventricular CAPNON can resemble meningioma or a choroid plexus papilloma with osseous metaplasia.

PLNTYs often have dense central tumoral calcifications and closely mimic CAPNONs. While CAPNONs generally affect middle-aged and older patients, they can occasionally be seen in children and young adults where they may mimic PLNTY.

Metaplastic Dural Ossification

Terminology

Physiologic calcifications (“plaques”) of the intracranial dura are common, age related, and generally of no clinical significance. In contrast, osseous metaplasia of the CNS dura involves new bone formation, is relatively rare, and should not be mistaken for neoplastic disease.

Etiology

The precise etiology of dural osseous metaplasia is unknown. As dura is derived from multipotential mesenchymal cells, activation from trauma, hemorrhage, inflammation, or age-related degeneration might lead to the aberrant formation of bone tissue. Systemic disorders, such as hyperparathyroidism, vitamin D intoxication, and chronic renal failure, have been reported in some cases.

Pathology

Location

Osseous metaplasia of the dura is most common in the spine, where chronic arachnoid inflammation from surgery or trauma can lead to intrathecal ossification (“arachnoiditis ossificans”).

In contrast to the spine, dural ossification with new bone formation involving the cranial meninges is rare. By far, the most common site is the falx cerebri; the second most common site is the convexity dura followed by the tentorium cerebelli.

Two types of falx ossification have been identified. In some cases, ossification begins in and is in close proximity to the interior surface of the calvarium with direct continuity to the falx lesion. In others, the ossification is located intracranially without any continuity with the calvarium.

Size and Number

Dural ossifications can be solitary or multifocal. Multifocal lesions with numerous confluent or scattered foci present within the dura are more common. Lesions vary in size from a few millimeters to rare cases > 10 cm in length and width up to 1-2 cm (33-24).

Gross Pathology

Sheets or nodules of hard white material with a shell of dense mature cortical bone surrounding normal-appearing marrow are typical. Falx ossifications often have a linear medial border with a slight convex or irregular lateral border. In other locations, the ossification may be nodular, irregular, wavy, or even round in appearance.

Microscopic Features

Osseous tissue complete with bone marrow elements is present. The osseous tissue is bounded by rows of spindle cells continuous with adjacent connective tissue cells of the falx.

Clinical Issues

Fatty marrow development in the falx occurs in 0.5-1.0% of the population and is usually an incidental finding of no clinical significance. Patients are generally middle aged to older (45-85 years).

Imaging

Most cases of intracranial dural ossification are discovered during routine clinical imaging and are incidental findings of no clinical significance.

CT Findings

Membranous bone formed in the intracranial dura behaves like and resembles bone elsewhere. An outer shell of dense cortical bone surrounds an inner core of marrow. These range in size from small solitary or multifocal lesions (33-25A)to large, bizarre-appearing bony structures that can extend along much or all of the affected dura (33-25). A thin, hypodense line often separates the metaplastic ossification from the inner table of the skull (33-25A).

MR Findings

The central marrow-containing portion of the ossified mass is hyperintense on T1WI (33-26B). The bony cortex is hypointense on T2WI, and the marrow is slightly hyperintense relative to brain. The lesions “bloom” on T2* sequences (33-26C). Enhancement is usually minimal or absent on T1 C+ FS, although hemopoietically active marrow nodules may enhance. Slight thickening with continuous linear enhancement of the adjacent dura is common (33-26C).

Differential Diagnosis

Physiologic dural calcifications are hyperdense, flat plaques that exhibit a laminar pattern of calcification on NECT. No fat or internal marrow cavity is present.

Hyperostosis frontalis interna is a benign nodular, continuous overgrowth of the frontal bone that is seen primarily in older postmenopausal females. In contrast to metaplastic dural ossification, the bony overgrowth is continuous with the inner table of the skull.

Dural osteoma is a focal lesion with dense, solid calcification. Macroscopic calcification in meningioma is common; osseous metaplasia is rare. A densely calcified meningioma (brain “stone”) usually exhibits some enhancement on T1 C+ FS.

Calcifying pseudoneoplasm of the neuraxis (CAPNON)is rare and can be intra- or extraaxial. Extraaxial CAPNONs tend to be rounded, focal lesions with dense calcification, not osseous metaplasia, and located within a sulcus rather than directly adjacent to the dura.

INTRACRANIAL PSEUDOTUMORS

Textiloma

• Foreign body reaction (Gelfoam, gauze, etc.)

• Iso-/hypointense on T2WI

• Ring, heterogeneous enhancement on T1 C+

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