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(26-19) Focal defect of bone and dura in midclivus is associated with a small ecchordosis physaliphora . (Courtesy R. Hewlett, MD.)

(26-20) Gelatinous-appearing nodule is in front of pons. Incidental finding is physaliphorous ecchordosis. (Courtesy R. Hewlett, MD.)

(26-21) T2WI shows lobulated, well-delineated, hyperintense midline mass indenting the pons. This is physaliphorous ecchordosis.

and found incidentally at autopsy or imaging. They usually lie just in front of the pons and have a thin stalk-like connection to a smaller intraclival component (see below).

Skull base metastases and plasmacytoma are destructive lesions that are usually isointense with brain on all sequences. Predominantly intraosseous meningioma is rare in the skull base. It usually causes sclerosis and hyperostosis rather than a permeative destructive pattern.

CHORDOMA

Etiology

•Arises from cranial end of primitive notochordal remnants

•STAT3 activation

Pathology

•Typically midline

○50% sacrum

○35% sphenooccipital (clivus)

○15% vertebral body

•Three types

○Typical ("classic") with physaliphorous cells

○Chondroid

○Dedifferentiated (< 5%, usually sacral)

Clinical Features

•Any age but peak = 4th-6th decades

•Cranial neuropathies

Imaging Findings

•CT

○Permeative destructive central BOS lesion

○Often contains sequestered bony fragments

•MR

○T1 hypointense, T2 hyperintense

○"Thumb" of tumor extends posteriorly, indents pons

○Variable, usually moderate enhancement

Differential Diagnosis

•Invasive pituitary macroadenoma

•Chondrosarcoma

•Ecchordosis physaliphora

Intracranial Pseudotumors

Ecchordosis Physaliphora

Ecchordosis physaliphora (EP) is a small (usually < 1 cm) gelatinous soft tissue mass that represents an ectopic notochordal remnant (26-20). Ectopic notochordal rests can occur anywhere along the midline craniospinal axis from the dorsum sellae to the sacrococcygeal region. EPs are more common in the spine than the skull and are generally incidental findings at imaging or autopsy.

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 welldelineated hypodense, nonenhancing midline intraclival mass with scalloped sclerotic margins.

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 and hyperintense relative to CSF on T2WI (26-21). EPs do not enhance following contrast administration, and 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

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lesions that restrict on DWI. ECs are more common in the cerebellopontine angle cisterns. Dermoid cysts usually follow fat signal, not CSF.

Textiloma

Hemostatic elements that are introduced into the central nervous system occasionally induce an excessive inflammatory reaction that may be difficult to distinguish from recurrent or residual tumor on neuroimaging studies.

Terminology

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.

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(26-23A) Axial FLAIR shows a hypointense massadjacent to the tumor resection cavity .

(26-23B) The lesion demonstrates solid but heterogeneous enhancement on T1 C+ FS.

(26-23C) Histology (same case) shows amorphous spicules surrounded by blood. This is gelfoam textiloma. (Courtesy B. K. DeMasters, MD.)

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.

Pathology

Most textilomas occur within surgical resection sites or around muslinreinforced 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 (26-23C).

Clinical Issues

Textilomas are uncommon. The highest reported prevalence is following abdominal and orthopedic surgery. Intracranial textilomas are rare with fewer than 75 reported cases.

Textilomas may be asymptomatic or cause symptoms that suggest tumor recurrence.

Imaging

Intracranial textilomas are almost always isoor hypointense on T1WI. Approximately 45% are isoand 40% are hypointense on T2/FLAIR (26-23A). Some "blooming" on T2* may be present. All reported cases of textiloma enhance on postcontrast scans. Ring and heterogeneous solid enhancement patterns occur almost equally (26-23B).

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 the Neuraxis

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

CAPNONs are nonneoplastic, noninflammatory lesions. They are discrete intraor extraaxial masses that contain various combinations of chondromyxoid and fibrovascular stroma, metaplastic calcification, and ossification.

CAPNONs have also been characterized as foreign body reactions with giant cells, tissue ossification, and the formation of lamellar bone or scattered psammoma bodies. The surrounding brain often exhibits inflammatory changes, with gliosis and edema leading to mass effect.

(26-24) NECT (upper L) and bone CT (upper R) show densely calcified mass . FLAIR (lower L) shows that hypointense massis surrounded by edema , doesn't enhance (lower R). This is CAPNON. (Courtesy S. Blaser, MD.)

Positive immunoreactivity to vimentin and epithelial membrane antigen (EMA) are typical. GFAP and S-100 protein are typically negative, helping distinguish CAPNON from astrocytic neoplasms and meningioma.

Intracranial CAPNONs are usually 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.

Most CAPNONs are solitary lesions; multiple lesions have been described but are uncommon.

NECT scans demonstrate a densely calcified leptomeningeal, deep intrasulcal, or brain parenchymal "rock." The temporal lobe is the most common site.

On MR, CAPNONs demonstrate little mass effect, are isointense on T1WI, and are uniformly hypointense on T2WI and FLAIR. Mild "blooming" is seen on T2* GRE. Perilesional edema varies from none to extensive. Enhancement varies from none to moderate. Solid, linear, serpiginous, and peripheral rim-like enhancement patterns have all been reported.

The differential diagnosis of CAPNON includes an ossified vascular lesion—most often a cavernous malformation—and densely calcified neoplasm, such as oligodendroglioma, meningioma, and choroid plexus papilloma with osseous metaplasia. Although cavernous malformations can often be distinguished by their "popcorn" mixed hyperintensity on T2WI, biopsy is usually necessary for definitive diagnosis.

(26-25) Sometimes surgically proven CAPNONs are extremely hypointense on T2WI , incite intense edema , and exhibit rim enhancement . (Courtesy B. K. Kleinschmidt-DeMasters, MD.)

INTRACRANIAL PSEUDOTUMORS

Ecchordosis Physaliphora

•Ectopic notochord remnant

○Dorsum sellae to sacrum

○Physaliphorous cells, mucoid matrix

•Clival defect, sclerotic margins

•Stalk connects to hyperintense prepontine mass

Textiloma

•Foreign body reaction

○Usually to hemostatic elements

○Other = introduced embolic materials

•Iso-/hypointense on T2WI

•Ring, heterogeneous enhancement on T1 C+

Calcifying Pseudoneoplasm of the Neuraxis (CAPNON)

•Chondrocalcific or ossified mass

○Usually in sulcus

•Very hypointense on T2/FLAIR

•Enhancement varies (none to rim-like)