Intraosseous benign notochordal cell tumor.
Abstract: * Intraosseous benign notochordal cell tumor is a presumably benign, intravertebral lesion of notochord origin, which can be found incidentally on removal of the vertebra for unrelated lesions or during an autopsy. The notion of a larger benign notochordal cell tumor that is macroscopic, occasionally symptomatic, and seen radiologically was recently introduced, and described as giant vertebral notochordal rest, giant notochordal hamartoma of intraosseous origin, and benign chordoma. Because of the location, size, and similar histologic and immunohistochemical characteristics, larger benign notochordal cell tumors are easily misdiagnosed as the malignant notochord-type of tumor/chordoma, with critical implications. In this review, we attempt to shed light on the major differences between benign notochordal cell tumors and chordoma and other related notochord lesions, such as notochordal vestiges of the intervertebral disk and ecchordosis physaliphora.

(Arch Pathol Lab Med. 2010;134:283-288)
Article Type: Clinical report
Subject: Spinal cord tumors (Diagnosis)
Spinal cord tumors (Development and progression)
Spinal cord tumors (Care and treatment)
Spinal cord tumors (Patient outcomes)
Authors: Amer, Hoda Zeinab M.
Hameed, Meera
Pub Date: 02/01/2010
Publication: Name: Archives of Pathology & Laboratory Medicine Publisher: College of American Pathologists Audience: Academic; Professional Format: Magazine/Journal Subject: Health Copyright: COPYRIGHT 2010 College of American Pathologists ISSN: 1543-2165
Issue: Date: Feb, 2010 Source Volume: 134 Source Issue: 2
Topic: Canadian Subject Form: Spinal cord tumours; Spinal cord tumours; Spinal cord tumours; Spinal cord tumours
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 230245851
Full Text: Intraosseous benign notochordal cell tumor (BNCT) is a presumably benign intravertebral lesion of notochord origin, which can be found incidentally on removal of the vertebra for an unrelated cause or during an autopsy. In rare cases, a BNCT can present as a larger size lesion that is symptomatic and detected on advanced imaging, mainly magnetic resonance imaging (MRI). These larger BNCTs, in particular, must be differentiated from chordoma, which is a malignant neoplasm of notochord origin. Imaging studies on chordomas usually demonstrate extensive bone destruction, soft tissue invasion, and contrast enhancement on MRI, which are not seen in BNCTs. Histologically, they both show the characteristic physaliphorous cell; they stain for vimentin, S100, epithelial membrane antigen, and low molecular weight cytokeratins; and they are negative for high molecular weight keratins. Features such as atypia, mitosis, pleomorphism, necrosis, field to field variability, syncytiae formation, an intercellular myxoid matrix, and a higher MIb-1 (proliferative) index will favor a diagnosis of chordoma. Benign notochordal cell tumors have a favorable prognosis; however, their exact neoplastic nature and relationship to chordoma has yet to be clarified in further studies.

Other notochordal lesions include notochordal vestiges of the intervertebral disk and ecchordosis physaliphora (EP). Notochordal vestiges of the intervertebral disk are an incidental microscopic finding rarely found in adults. They can be differentiated from BNCTs by their location in the intervertebral disk and by the presence of an intercellular matrix, which is consistently absent in BNCTs. Ecchordosis physaliphoras are likewise incidental findings that only rarely form mass lesions at the base of the skull, where they must be differentiated from chordoma, which can rarely occur extraosseously at this location. Radiologically, EPs do not enhance on contrast medium, and on microscopy, they lack the features, such as atypia, mitosis, pleomorphism, syncytiae formation, and the high MIB-1 index, characteristic of chordoma. Both notochordal vestiges and EPs show the characteristic physaliphorous cells and demonstrate similar immunohistochemical staining pattern.

BACKGROUND

The notochord forms after the third gestational week of development, when it has a role in inducing the formation of the neural plate and vertebral column and provides a structural support for the development of the axial skeleton. During the process of development, the notochord regresses in the region of the vertebral bodies and intervertebral disks to eventually disappear entirely at term, replaced by water, collagen type II, and aggrecan as the nucleus pulposus. (1,2) Notochordal remnants have occasionally been found in the intervertebral disk where they are known as rests or vestiges, to disappear by the age of 1 to 3 years. (3-5) They rarely persist into adulthood. (2) Extravertebral ectopic notochordal rests have been reported as incidental findings, mainly at the base of the skull. (6) These lesions were first described in 1856 by Luschka7 and named ecchondrosis/ecchordosis physaliphora spheno-occipitalis. (8,9) In 1894, they were found in 2% of autopsy cases, and the concept of a notochordal origin was introduced. (10) In 1923, these extravertebral notochordal rests were found in 1.5% of a series of 200 autopsies. (9) A more recent autopsy review found EP in 6 of 1600 autopsies (0.38%). (11) They are occasionally symptomatic, forming mass lesions. (12) Intraosseous and intravertebral notochordal remnants are a novel entity. These were first described as an autopsy finding in 1982 and documented with the name of ecchordosis physaliphora vertebralis. (13) Since then, they have been observed numerous times as microscopic remnants in incidental autopsy findings (14) or upon removal of the vertebra for unrelated lesions. (4,13,14) They are described as being less than 1 cm, requiring no treatment or follow-up, and as not being seen radiographically. (4,13) The notion of a larger intravertebral notochordal lesion that is macroscopic, occasionally symptomatic, and seen radiologically was recently introduced. The first such lesion was described in 1996 by Darby et al (15) and initiated much debate as to whether the lesion represented a giant notochordal rest, comparable to those found in the intervertebral disk and EP, or rather a vertebral chordoma, the malignant notochord-type tumor. The term giant notochordal rest was preferred by some. (16) It was argued, however, that they are histologically and immunohistochemically unlike vestiges and embryologic notochord rests, and therefore, this term would be a misnomer. (17,18) A chordoma or benign chordoma was also suggested but discouraged in light of the benign behavior and as of yet questionable neoplastic nature. (16,19) Giant notochordal hamartoma of intraosseous origin was presented, suggesting the possibility that these lesions are hamartomatous, growing from infancy to early adulthood, and thereafter attaining a stable growth pattern. (4) This term was also debated because these lesions are formed of immature tissue, unlike hamartomas, which are an overgrowth of mature cells and tissues. (17,19) Finally, the term intraosseous benign notochordal cell tumor was introduced17 to include smaller lesions found incidentally, including that of the previously described EP vertebralis, and those that have become large enough to be detected on advanced imaging or have become symptomatic, presumably lesions that grew from their smaller sizes. (20)

CLINICAL FEATURES

Benign notochordal cell tumors are usually incidental microscopic findings found on autopsy. (13) A recent autopsy series found BNCTs in 20% of 100 cadavers, in patients ranging in age from 7 to 82, with a distribution of 11.5% of the clival region, 5% of cervical vertebrae, 2% of lumbar vertebrae, 12% of sacrococcygeal vertebrae, and 0% in the thoracic vertebrae. (14)

Larger BNCTs are also seen in a variety of ages and in both sexes, (4,5,16) with the youngest age described thus far, to our knowledge, in a 13-year-old boy.16 Larger BNCTs have been found incidentally during work-up for trauma, neck stiffness from degenerative joint disease, and following a fall or a motor vehicle accident.4,20 When symptomatic, the most common complaint of a BNCT is that of back pain. This back pain can be chronic, lasting several years, or of acute onset of a few months. (4,5,15,16) Coccydynia was described, often manifesting as pain on pressure with associated neuro-muscular manifestations.18 Other symptoms, depending on the location of the lesion, include limitation of movement, numbness, pain of fingers, neck, upper back, buttock and shoulder stiffness. (5,16,20) One patient was unique in having Tourette syndrome and intestinal malrotation, suggesting that embryologic maldevelopment may be involved. (16)

RADIOGRAPHIC STUDIES

The smaller BNCTs are not seen radiologically because they are incidental microscopic findings. Larger BNCTs, however, will commonly show nondiagnostic or normal radiographic findings, (16) and a normal bone scan. (4) Computed tomography often shows a sclerotic lesion. Magnetic resonance imaging is the most helpful. On T2-weighted images, these lesions have homogenous high-signal intensity and on T1-weighted images, homogenous low-signal intensity. (4,5,16,20) They are well circumscribed, involving their respective vertebrae without extension or destruction beyond the vertebra. They can involve up to 50% of the vertebral body (5) or almost the entire body. (4) The surrounding bone may be normal or show mild trabecular thickening. Radiographically, the differential diagnoses include hemangioma, atypical infection, lymphoma, plasmacytoma, Paget disease, or metastatic disease. A computed tomography-guided biopsy is suggested. (4)

GROSS PATHOLOGY

Macroscopic BNCTs have been described up to 4.0 cm in size. (4,18) They are seen as grey-white and well delineated, (5,16) filling up the vertebra and taking its shape. (4) Occasionally mucoidlike tissue can be seen grossly within the intertrabecular spaces of the lesion. The surrounding bone is intact and uninvolved, without visible destruction. (4,16)

HISTOPATHOLOGY

Benign notochordal cell tumors are sharply demarcated lesions. (5,16) The trabecular bone is preserved, with moderate thickening of some of the lamellar bone trabeculae, cement lines, and appositional new bone formation or reactive sclerosis. (5,17,19) In between the bony trabeculae, and filling the intertrabecular spaces, are nests or sheets of clear cells that resemble fat on low power. (4,5) They have clear cytoplasm and eccentrically located, round or oval nuclei. (4,16,17) Occasionally, the cells may be large, with polymorphic shapes. (17) Smaller granular cells are also seen, interspersed with eosinophilic globules of condensed cytoplasm. (5,16) These less-vacuolated eosinophilic tumor cells show centrally located, enlarged nuclei and, occasionally, small nucleoli. (17) Admixed among the cells are "bubbly" cells containing multiple clear, cytoplasmic vacuoles, indenting the nucleus, which are known as physaliphorous cells. Signet ring cells can rarely be seen. Focally, the cells may fuse to form microcystic regions containing an eosinophilic colloidlike material. (16,17) Occasionally deposits of hyalinized material, mimicking cartilage tissue, can be seen. (17) There is no intercellular stroma, (5,16) and extracellular myxoid matrix is lacking.19 There is no mitosis, syncytiae, fibrous bands, and minimal to no nuclear pleomorphism or necrosis. (5,4,17) They show a uniform cell proliferation, without marked variability from field to field. (16) Fat cells and islands of hematopoietic bone marrow are uniformly distributed and interspersed in between. (4,19) (Figures 1 and 2).

ANCILLARY TESTS

Mucicarmine, alcian blue (pH 2.5 and 1.0), and periodic acid-Schiff stain the cytoplasm of the granular cells and physaliphorous cells, but the clear cytoplasm of the large cells fail to stain. (16,17) The cytoplasmic globules seen in the more eosinophilic granular tumor cells are positive for periodic acid-Schiff and resistant to diastase. The aforementioned cystic spaces containing eosinophilic, colloidlike material are positive for both Alcian blue and periodic acid-Schiff. (17)

Immunohistochemistry is confirmatory and helpful for these lesions: They react strongly to cytokeratin, such as AE1 AE3 and CAM 5.2 (Figure 3); however, they are nonreactive to high molecular weight cytokeratin (34[beta[E12). (16) Cytokeratin 18 is reactive in the physaliphorous cells, a supposed, but questionable, distinction from fetal notochordal tissue (see below). (2,19,21) They are immunoreactive to epithelial membrane antigen, S100, and vimentin. (20) They are nonreactive to p53, and they show a low (<1%) Ki-67 (MIB-1) proliferation index. (4,16)

DIFFERENTIAL DIAGNOSIS

The striking histologic similarity of the embryonal notochord, notochordal vestiges of intervertebral disks, and EPs, BNCTs, and chordomas render their various diagnoses difficult, especially on biopsy. (3,4) They all commonly contain univacuolated and physaliphorous cells and have similar immunohistochemical staining patterns, (16) such as positivity for vimentin, S100, epithelial membrane antigen, and low molecular weight cytokeratins and negativity for high molecular weight keratin (34pE12). The clinical and radiologic picture must be assessed and incorporated in each case (Table). (4,16)

Intervertebral vestiges, which are persistent notochords within the intervertebral disks in adults, (2) should be differentiated from BNCTs and the malignant chordoma. The first main difference is their location because they are present within the center of the intervertebral disk, rather than within the vertebral body. Grossly, they are described as a central, cysticlike space in the annulus fibrosus. (2) Histologically, they are composed of cords or strands of notochordal cells embedded within a myxoid background. They exhibit small, pyknotic, round nuclei, with evenly condensed chromatin and eosinophilic cytoplasm with small vacuoles. They lack prominent nucleoli or mitotic figures (17) (Figure 4). The presence of an intercellular myxoid matrix differentiates them from BNCTs. They also, unlike BNCTs and chordomas, do not form sheets of cells. (2) Originally, it was thought that these notochordal vestiges did not express CK18 by immunohistochemistry as shown in a study (20) of 27 notochordal vestiges, 34 BNCTs, and 3 classic chordomas. However, this was challenged by another study (2) in which fetal notochordal remnants, notochordal remnants in an adult intervertebral disk, and chordomas were positive for CK18. In addition, CK18 was positive in examination of notochords from 32 embryos and fetuses by yet another group of investigators. (21)

Ecchordosis physaliphora are ectopic, extraosseous notochordal lesions of embryologic origin, similar to notochordal vestiges of the intervertebral disk. (7,8,9) They are found intracranially on the dorsum of clivus, in the posterior cranial fossa. (12,22) They usually present extradurally and occasionally have been seen to traverse into the intradural, subdural, and subarachnoid spaces along the axial skeleton while maintaining a connection to similar cells within the clival area by a pedicle. (22) They can be found at any level from the dorsum sellae to the sacrococcygeal region.18 They are incidental findings found to be in 2%, (10) 1.5%,9 and more recently in 6 of 1600 autopsies (0.38%). (11) They very rarely (at least 8 cases described so far) form mass lesions that are symptomatic, causing neurologic deficits, such as hemihypoaesthesia and contralateral hemiparesis, diplopia, and visual disturbances, cerebrospinal fluid fistula, and even sudden death due to subarachnoid bleeding. (22-25) Because of their location, they are not among the differential diagnoses of BNCTs, but they should be distinguished from chordoma, which has been known to occur rarely, extraosseously, at this site. (23) On MRI, EP appears as hypointense on T1-weighted images and hyperintense on T2-weighted images without enhancement on contrast media, (6) though some variability occurs (intermediate on T1, and low intensity on T2), depending on the relative amounts of water and gelatinous matrix. (22) An associated possible bony defect can be seen. This is different from chordoma, which consistently enhances on contrast media and shows extensive bony and soft tissue destruction. (22) Grossly, EPs are described as nonadherent soft, jellylike excrescences, up to 3.5 cm. (22) They are similar histologically to intervertebral notochordal vestiges or rests and share the same immunohistochemical profile as the other notochordal lesions. Immunostaining for the proliferation marker MIB-1 is low (less than 1%), (2,12) and malignant features characteristic of chordoma, such as atypia, pleomorphism, and necrosis, are not seen. (6,12) From a treatment standpoint, the rare symptomatic cases of EP have been treated by surgery, although firm principles of management have not been determined. Of the cases described thus far, 2 cases, to our knowledge, resulted in mortality from complications of subarachnoid hemorrhage. (24,25)

The main differential diagnosis of BNCT is chordoma, a low-grade malignant neoplasm, a diagnosis that carries critical implications and consequences. Chordomas can occur almost anywhere in the axial skeleton, although they follow a similar anatomic distribution with BNCTs, with the base of skull and sacrococcygeal vertebrae the most common locations. (14,20,26) They are destructive lytic lesions, usually with soft tissue extension, causing much morbidity due to spinal cord involvement and local invasion to surrounding structures. (27) Distant metastasis to lung, bone, soft tissue, lymph node, liver, and skin has been reported in up to 43% of patients. (26) The main clinical presentation is that of pain, whereas neurologic deficits vary based on the location of the lesion. The overall median survival time with chordoma has been estimated to be approximately 6 years. (26) They are large at presentation, often up to 25 cm, and can involve more than one vertebral body. Nonosseous extradural or intradural chordomas have been reported. (23)

Radiographically, chordomas are similar to that of notochord rests in that the MRI shows hypointense or isointense images on T1-weighted images, whereas the T2-weighted images are of high signal intensity. (4) Chordomas, however, generally enhance after application of contrast medium. (6) Also, the extensive lytic and destructive nature of these lesions is evident, often with soft tissue invasion. (4,27) This is best demonstrated on lateral views. The surrounding bone shows reactive sclerosis. Intratumoral calcification has also been described. (4)

Grossly, chordomas are irregular lesions that destroy bone to extend to the surrounding soft tissue structures. They are described as having a mucogelatinous, lobulated appearance, glisten, and are a grayish tan to bluish white. (27)

Microscopically on low power, chordomas show a distinct lobularity, with fibroblast-rich fibrous septae. In between, an intercellular myxoid matrix is seen, with the classic physaliphorous cells present either individually or forming irregular cords and syncytiae. The cells show pale and bubbly cytoplasm with fine, intracytoplasmic septations, or abundant eosinophilic cytoplasm with fewer to absent intracytoplasmic bubbles. There is moderate to severe atypia, with atypical nuclei, and typical and atypical mitotic figures are found. (4,16,19) Typically, there is marked variability from field to field. (16) Destruction of bone trabeculae is evident, and necrosis can be seen. (4,19) Fat and the normal bone marrow elements are not seen within the tumor but, rather, are apparent at the edge of the lesion and interface with normal bone. (4) They are cellular lesions, with variable amounts of pleomorphism, hyperchromatism, and mitosis, indications of their proliferative activity and metastatic capability (28,29) (Figures 5 and 6). Chordomas dedifferentiate in less than 5% of cases to show features of high-grade spindle cell sarcoma, acquiring a more aggressive clinical course. (30)

Chordomas stain similarly to BNCTs and other notochordal cells for low molecular weight cytokeratin, such as AE1 AE3, and CAM 5.2, and they are nonreactive to high molecular weight cytokeratin (34[beta]E12). (16) Cytokeratin 18 is variable; in a study of 16 cases, 9 (56.3%) were positive. (31) They are immunoreactive to epithelial membrane antigen, S100, and vimentin.20 p53 overexpression and increased Ki-67 (MIB-1) indices are expressions of proliferative activity. (28) Chordomas must be treated aggressively by surgery and radiotherapy. (30)

Other lesions that can be confused with BNCTs include normal fatty bone marrow and metastatic clear cell carcinoma of the kidney. In the former, careful observation, as well as immunohistochemistry, will easily distinguish them because epithelial markers are consistently negative in fatty marrow. The latter may be distinguished radiologically by its lytic nature and increased signal intensity on T1-weighted MRIs and histologically by showing a characteristic alveolar pattern with vascular fibrous septa. Metastatic clear cell carcinoma of the kidney is only rarely positive for S100.20

[FIGURES 1-6 OMITTED]

TREATMENT

Unless symptomatic, it is generally accepted that BNCTs may be simply observed with long-term follow-up. (16,18) This follow-up consists of serial MRI and computed tomography scans for lytic changes and destruction of trabeculae or extension to soft tissue, which are indications of a chordoma. (4,5) If symptomatic, a vertebrectomy with fusion of the bodies and support by posterior pedicular instrumentation is usually sufficient to provide pain relief. (16,18)

PROGNOSIS

The neoplastic potential of BNCTs, especially the larger BNCTs, and their link to the malignant notochordal-type tumor chordoma, has prompted much controversy. (4,32) It has been suggested that BNCTs represent precursor lesions to chordomas or slow-growing chordomas. (4,15,32) This is especially arguable when BNCTs are found juxtaposed, or in the vicinity of, chordomas. In 2007, one study identified BNCTs in 7.3% (6 of 82) of sacral/coccygeal resections performed for primary chordoma. (32) In 2002 a case report (19) described a chordoma seen adjacent to, and apparently arising from, an otherwise benign notochordal lesion in the coccyx. The same case also showed 2 discrete BNCTs in the sacrum. In 2005, a report (33) described 2 cases that showed tumors exhibiting both extraosseous infiltrative and intraosseous components. The intraosseous component consisted of what appeared to be a BNCT sharply juxtaposed to a proliferation of chordoid cells with minimal nuclear atypia in an intercellular myxoid matrix. The extraosseous component consisted of solid nests or cords of atypical chordoid cells similarly seen within a myxoid matrix. In the latter, the nuclei were described as slightly larger than those of BNCTs, however, without a large lobular configuration or wide variety of nuclear atypia typical of classic chordomas. They were, thereby, designated as incipient chordomas, to signify either miniature chordomas or prechordomatous intermediate conditions. The presence of incipient chordoma in the vicinity of a BNCT strengthened the supposition that chordomas can arise from a preexisting BNCT, through a transformation process, acquiring infiltrative capabilities. (33) Arguing against this is the generally benign nature of BNCTs, found more often incidentally or at postmortem, their long stable behaviors up to 10 years without surgery, (4) and their benign postoperative course, without pain or symptomatology, for up to 10 years. (18) Therefore, it may be concluded that the exact neoplastic nature and malignant potential of these "benign" notochord cell tumors has yet to be elucidated.

CONCLUSIONS

Benign notochordal cell tumor, chordomas, notochordal vestiges of the intervertebral disk, and ecchordosis physaliphora are all lesions of notochordal origin that share similar histologic and immunohistochemical characteristics. Correlation of clinical, radiologic, and histopathologic studies is needed for accurate diagnosis and management.

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Hoda Zeinab M. Amer, MD; Meera Hameed, MD

Accepted for publication May 21, 2009.

From the Department of Pathology and Laboratory Medicine, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, Newark.

The authors have no relevant financial interest in the products or companies described in this article.

Reprints: Hoda Zeinab M. Amer, MD, Department of Pathology and Laboratory Medicine, University of Medicine and Dentistry of New Jersey, New Jersey Medical School, 150 Bergen St, UH E155, Newark, NJ 07103 (e-mail: hodazamer@gmail.com).
Benign Notochordal Cell Tumor (BNCT) Differential Diagnosis

Characteristic                     BNCT

Clinical picture     Asymptomatic; back pain; neurologic
                     manifestations

Location             Intravertebral body; mainly base
                     of skull and sacrococcygeal region

Radiographic         T1 low signal, T2 high signal; no
finding (MRI)        bone destruction or soft tissue
                     involvement

Gross                Well demarcated; [less than or
                     equal to] 4 cm; grey-white

Architecture         Nests or sheets; microcystic regions
                     with colloidlike material

Cells                Physaliphorous cells; clear cells
                     resembling fat; eosinophilic
                     cells with cytoplasmic globules

Fat, hematopoietic   Uniformly dispersed
cells

Vimentin, EMA,       2+
S100, AE1/AE3,
CAM 5.2

34[beta]E12          --

CK18                 2+

Ki-67                <1%

Treatment,           Surgery; favorable prognosis with
prognosis            follow-up

Characteristic                   Chordoma

Clinical picture     Back pain; neurologic manifestations

Location             Intravertebral body; mainly base of
                     skull and sacrococcygeal region;
                     extraosseous locations possible

Radiographic         T1 low signal, T2 high signal, contrast
finding (MRI)        enhancement; bone destruction,
                     soft tissue infiltration

Gross                Irregular mass; up to 25 cm; muco-
                     gelatinous; grey-tan, bluish-white

Architecture         Lobulated with fibrous bands; irregular
                     cords and syncytiae; inter-cellular
                     myxoid matrix; necrosis

Cells                Atypical, pleomorphic, physaliphorous
                     cells, pale and bubbly, or
                     eosinophilic; mitosis

Fat, hematopoietic   At margin of lesion
cells

Vimentin, EMA,       2+
S100, AE1/AE3,
CAM 5.2

34[beta]E12          --

CK18                 [+ or -]

Ki-67                2+

Treatment,           Surgery, chemotherapy, radiotherapy;
prognosis            poor prognosis with risk of
                     dedifferentiation and metastasis

Characteristic       EP/Fetal Vestiges

Clinical picture     Asymptomatic; neurologic manifestations

Location             EP--mainly extradural, intracranial,
                     at base of skull
                     Fetal vestiges--intervertebral disk

Radiographic         EP--variable, usually T1 low signal,
finding (MRI)        T2 high signal

Gross                EP--jellylike excrescence;
                     [less than or equal to] 3.5 cm

                     Fetal vestiges--cystlike space in
                     annulus fibrosus

Architecture         Cords or strands; intercellular
                     myxoid matrix

Cells                Physaliphorous cells, eosinophilic
                     with pyknotic nuclei

Fat, hematopoietic   NA
cells

Vimentin, EMA,       2+
S100, AE1/AE3,
CAM 5.2

34[beta]E12          --

CK18                 [+ or -] (a)

Ki-67                <1%

Treatment,           EP--surgery; favorable prognosis
prognosis

Abbreviations: BNCT, intraosseous benign notochordal cell tumor;
CK, cytokeratin; EMA, epithelial membrane antigen; EP, ecchordosis
physaliphora; MRI, magnetic resonance imaging; NA, not
applicable.

(a) Please refer to text for further elaboration.
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