Predominantly Extraaxial Astroblastoma: Imaging and Proton MR Spectroscopy Features J. Jeffery Baka, Suresh C. Patel, Joseph R. Roebuck, and David 0. Hearshen Summary: The CT, angiographic, MR, and proton MR spectros­ solid nodule to the cyst wall. There was no visible calcifi­ copy findings in a case of astroblastoma, a rare neoplasm of cation or surrounding vasogenic edema. glial cell origin, are presented. Of particular interest is the Left vertebral angiography (Fig. 2A and B) demonstrated predominantly extraaxial location of the tumor. CT and MR a 3-cm tumor stain with neovascularity and arteriovenous demonstrated a complex mass consisting of a solid nodule and shunting in the left occipital region. The blood supply was a peripheral septated cystic component. The extraaxial nature of pial origin and derived from the posterior and middle of the mass was suggested on MR. temporal branches of the left posterior cerebral artery, which were draped around a large, avascular component Index terms: Brain neoplasms, computed tomography; Brain of the mass. neoplasms, magnetic resonance; Brain neoplasms, angiography; Magnetic resonance (MR) (Fig. 3A-C) indicated a prob­ Magnetic resonance, spectroscopy able extraaxial mass that was in contact with the superior aspect of the tentorium. The solid portion of the mass was Astroblastoma is a well known but uncommon hypointense to gray matter on T1-weighted spin-echo and tumor of glial origin ( 1) . Although astroblastic inversion recovery sequences and on proton density­ elements are occasionally identified in specimens weighted spin-echo images and became hyperintense on of glioblastoma and astrocytoma, particularly T2-weighted spin-echo images. Tumor vessels produced punctate signal voids. The signal intensity of the cystic those of the gemistocytic variety, pure astroblas­ component paralleled the cerebrospinal fluid on all pulse tomas are rare ( 1-3). A few series and case sequences. reports ( 1-7) have appeared in the neurosurgery, One-dimensional proton CH) chemical shift imaging pathology, and oncology literature. This report (CSl) was performed in the region of the tumor (Fig. 4A describes the imaging features of a rare presen­ and B). The spectra displayed an increased choline (Cho)­ tation of this uncommon tumor. to-phosphocreatine/creatine (PCr/Cr) ratio, decreased N­ acetyl aspartate (NAA), increased lactate/lipid, and in­ creased myo-inositol (Ins) in those voxels containing pri­ Case Report marily solid tumor, as compared with those containing primarily cyst and normal brain. A 20-year-old woman presented with a 1-month history Gross total resection of the mass was performed. At of sharp left-sided headaches severe enough to wake her surgery, the tumor appeared extraparenchymal and was from sleep. Her only other complaint was difficulty reading, easily separated from the underlying brain. The cyst wall primarily an interpretive problem rather than a visual one; was hypervascular, and the peripherally displaced vessels she stated that it took her an hour to read and comprehend seen on angiography were embedded in the cyst wall. The a single written page. Physical examination revealed a solid component of the tumor was adherent to the tento­ homonymous right superior quadrantanopsia. Her neuro­ rium. logic examination was otherwise normal. The frozen section diagnosis was "extraparenchymous Precontrast and postcontrast computed tomographic neoplasm, probably glial," although atypical meningioma (CT) scans of the brain (Fig. 1) revealed a 7 X 5-cm and hemangiopericytoma were also considered. The final complex left medial occipital mass. Its superomedial com­ pathologic diagnosis, however, was malignant astroblas­ ponent, adjacent to the falx and tentorium, consisted of a toma (Fig. 5). This was confirmed by positive immunoper­ 3-cm enhancing solid nodule, which was sharply demar­ oxidase staining of the neoplastic cells for glial fibrillary cated from the anterior, lateral, and posterior cystic com­ acidic protein, as well as by electron microscopy. Conti­ ponent of cerebrospinal fluid density. There was enhance­ nuity between the solid nodule and the underlying cortex ment of the cyst wall and of septations radiating from the was demonstrated on only one slide, on which the interface Received June 30, 1992; revision requested August 2 1, received September 14, and accepted September 30. Department of Diagnostic Radiology, K-3 Neuroradiology, Henry Ford Hospital, 2799 West Grand Blvd., Detroit, Ml 48202. Address requests for reprints to J. Jeffery Baka, MD. AJNR 14:946-950, Jui/Aug 1993 0195-6108/ 93/ 1404-0946 © American Society of Neuroradiology 946 AJNR: 14, July/August 1993 EXTRAAXlAL ASTROBLASTOMA 947 specific neurologic symptoms ( 1 ,3). Slow growth of the tumor often produces a long history of symptoms prior to presentation, usually 1 to 6 months, but occasionally 2 to 4 years (1). The tumor is most often supratentorial and hemispheric ( 1), although examples have been reported in the cerebellum, brain stem, fourth ventricle, hypothalamus, and pineal region (1 ,6,9). They are often located in the superficial cortical and subcortical tissues (1 ,2), and exten­ sions into the subarachnoid space (5), leptome­ ninges (1 ,2,6), and dura (1) have been described. A predominantly extraparenchymal location ap­ pears to be unusual. Microscopic cortical involve­ ment, as demonstrated in the case presented here, precludes the diagnosis of a primary extra­ parenchymous glial tumor (2). Although most Fig. 1. Contrast-enhanced CT scan demonstrates a mixed astroblastomas are well circumscribed (1 ,2), small cystic and solid left-occipital-region mass. The extraaxial location areas of extension into adjacent neural paren­ of the mass is not readily appreciated. Note the enhancement of chyma can be found ( 1, 7), and poorly defined both the solid tumor nodule and the cyst wall and radiating and infiltrating tumors have been reported (4). septations. Also note the normal calcified pineal gland (the pineal gland is known to be a site of origin of astroblastomas). Lobulated solid tumors measuring from a few millimeters to 8 em are most common (1) , but between tumor and parenchyma was sharp, and no dural small and large cysts are frequent. One of 23 infiltration was seen. Tumor cells were present in the cyst cases reported by Bonnin and Rubinstein had wall and septations as well as in the solid nodule. extensive calcification ( 1) . Microscopically, the tumor consists of charac­ teristic perivascular tumor cells which, when seen Discussion in cross-section, produce a typical pseudorosette Astroblastoma is among the rarest of central pattern reminiscent of ependymoma (Fig. 5) and, nervous system neoplasms (7), representing when sectioned longitudinally, demonstrate a 0.45% of gliomas (8). Usually a tumor of children, papillary or pseudopapillary arrangement (1-7). adolescents, and young adults (1 ,2), it has been Intervascular cells are indistinguishable from the reported in a 67 -year-old patient (3). There is no perivascular cells but are rarified, as opposed to sex predilection (2). Patients present with non- the more compact arrangement of the intervas- Fig. 2. Anteroposterior (A) and lateral (B) views from a left vertebral angiogram demonstrate a moderately hypervascular so lid component of the tumor surrounded by a region of avas­ cularity. Cortical branches (arrows) of the left posterior cerebral artery drape over the avascular portion, suggesting an intraaxial les ion. A 948 BAKA AJNR: 14, July/ August 1993 A Fig. 3. A, T1-weighted spin-echo sagittal image (450/19 [TR / TE]), 8 , T1 -weighted inversion recovery axial image (1500/ 20, inversion time = 600) and, C, T2-weighted spin-echo axial image (2500/50) demonstrate solid and cystic components of the tumor. Note the relationship of the mass to the tentorium in (A), suggesting that the tumor is largely extraaxial. Radiating septa are seen extending from the solid central component to the cyst wall (B and C). cular cells in ependymoma (2), the tumor with mas are associated with a worse prognosis, char­ which astroblastoma is most often confused his­ acterized by one or more recurrences, often with tologically (2,5-7). The tumor cells typically stain increasing degrees of anaplasia as well as con­ positively for glial fibrillary acidic protein, which version to glioblastoma ( 1 ,2). is indicative of a tumor of glial origin (2,6,7, 10). Ultrastructural similarities have suggested to Papillary meningioma and metastatic papillary some that the astroblastoma is derived from the carcinoma can produce a similar histologic pic­ tanycyte, a cell that is intrinsic to the ependyma ture (7). Necrosis is present microscopically in of submammalian vertebrates, but that is also 70% of astroblastomas, regardless of tumor grade found among the ependymal cells of embryonic ( 1 ). and neonatal higher mammals, including humans The natural history of astroblastoma lies some­ (2, 10) . The cells are distinguished from typical where between that of astrocytoma and glioblas­ ependymocytes by long processes that extend to toma (2,3). Two distinct histologic subtypes occur the pial surface of the brain (11). The persistence (1 ,2). Low-grade astroblastomas are associated of tanycytes into adulthood has been demon­ with a favorable postoperative prognosis, with strated in the rodent brain (12); in humans and survival up to 20 years; high-grade astroblasto- other higher mammals, the tanycyte appears to AJNR: 14, July/ August 1993 EXTRAAXIAL ASTROBLASTOMA 949 ho PCr/Cr , ,~ \ ( ~A L;rip _)'~~ 1 6 '-y. J~~ I S ~ I - I ~ 1 3 ~ 1 2 ~ ' ~ 10 ' 2 ' ppm A B Fig. 4. A, voxels used for [1H]CSI. A 2 X 2 X 8-cm voxel was selected with a water-suppressed stimulated-echo acquisition mode sequence (2000/20, TM = 11). Phase encoding in the anteroposterior direction yielded seven 2 X 2 X 1-cm subvoxels labeled 10 through 16. The image corresponds to a 3-mm section through the middle of the stimulated-echo acquisition mode voxel and delineates regions occupied by cyst, solid tumor, and normal brain. Adjacent contiguous slices contain variable amounts of these materials and were used to estimate their relative amounts in each voxel.