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Neuroradiology Case of the Week

Case 297

Justin Brucker, CC4

Clinical Presentation: Patient is a 15-year-old male with a history of Asperger's and scoliosis who presents with an increase in his chronic back pain. It is localized to the upper and middle spine, and worsens after strenuous physical activity and chiropractic interventions. Physical therapy provides no relief.
     The pain now radiates to his left flank and upper ribs, and is severe to the point of triggering emesis. The patient denies numbness, tingling, weakness, or bowel and bladder incontinence.

Imaging Findings: MRI, Thoracic Spine: There is an intramedullary spinal cord mass extending from the upper portion of T3 to the lower portion of T6. The lesion is large, expansile, and completely occupies the spinal cord. A cystic component is seen along the inferior and posterior borders, and there is a syrinx extending from the superior aspect to C5. On T1-weighted images, the lesion is hypointense to isointense, but there is prominent contrast enhancement of the peripheral margins and to a lesser degree, within several small central foci (Fig. 2). The mass is hyperintense on T2-weighted images. Significant convex-right thoracic scoliosis with an apex at T7 is seen, but no other vertebral abnormalities are present.

Diagnosis: Intramedullary spinal cord tumor: pilocytic astrocytoma

Discussion: Intramedullary spinal cord tumors (IMSCT) only account for 4-10% of all primary central nervous system neoplasms [1]. In adults, the most common IMSCT is an ependymoma, whereas in children the most common type is an astrocytoma [2]. The latter accounts for one-third of all spinal cord gliomas, is more frequently occurring in males [2], but is still very rare. The incidence of a spinal cord astrocytoma is 0.8 to 2.5 per 100,000/year [3], most often in the thoracic spinal cord, followed by the cervical involvement [2].
     Pilocytic astrocytoma is the most common pediatric glioma, accounting for 85% of all pediatric cerebellar neoplasms, and usually occurs within the first two decades of life [4]. It is also the most benign type of astrocytoma (WHO Grade I/IV). There is a strong association between PA and Type 1 neurofibromatosis, with PA appearing in 15-21% of these patients [4].
      Clinical presentation depends on tumor location, but the rate of symptom onset usually reflects PA's slow growth rate. For PA in the spinal cord, persistent back pain is the most common early complaint, followed by sensorimotor disturbances and bowel/bladder incontinence [1,3]. A lack of neurologic symptoms, as our patient had, is a favorable prognostic indicator.
     MRI is clearly the study of choice. Specifically, precontrast T1/T2-weighted images in the sagittal plane, and postcontrast T1-weighted images in the sagittal/axial planes, are extremely useful for evaluating the extent of the lesion and for surgical planning [1,2,4]. Like most intramedullary tumors, there will be some degree of expansion and enhancement, as well as intratumoral and/or polar (60%) cysts [2]. Syrinx is a commonly associated feature, particularly for tumors located higher up in the spinal cord. As described above, PA's are typically isointense to hypointense on T1 with heterogeneous-enhancement, but variably hyperintense on T2 [1,2,4]. Two-thirds of cases will demonstrate a cystic area (often intratumoral) with a mural nodule, either of which may or may not enhance [2]. As one might expect with low-grade gliomas, there is usually limited vasogenic edema.
     Less-utilized imaging modalities include CT and MR spectroscopy. The former may demonstrate a well-defined mass that enhances intensely 94% of the time and may show calcifications, or possibly mild scoliosis, increased distance between pedicles, and bone erosions (more likely for ependymomas). MR spectroscopy has been reported to reveal increased choline:NAA, choline:Cr, and lactate:Cr ratios, without an increase in NAA or Cr [4].
     Macroscopically, PA's of the spine and cerebellum are well-circumscribed masses (though there may be infiltration past the border), with cystic components and a mural nodule. PA's occurring in the region of the optic chiasm and hypothalamus tend to be softer and more infiltrative. Microscopically, PA's possess both glial and piloid tissue types ("biphasic pattern"). The glial tissue contains loose, vacuolated collections of multipolar "protoplasmic" astrocytes and degenerative protein droplets, called "eosinophilic granular bodies" (EGB) [4]. The piloid tissue contains elongated bipolar cells, with fine fibrillary and Rosenthal fibers. Histochemical analysis of our patient's tumor revealed a Ki-67 level (protein-indicator of cell proliferation) of 8-9% in some areas -- considerably higher than expected for most PA's (<5%) [5]. This is associated with a shorter progression free interval and may suggest a mixed or atypical tumor type. However, the biphasic appearance, Rosenthal fibers, and lack of mitotic figures were typical for PA. Some studies report PA cases that were later rediagnosed as a more aggressive variant: pilomyxoid astrocytoma (PMA) [5]. In contrast to PA, PMA's exhibit a monomorphous composition, myxoid background, angiocentric cell clusters, and rarely possess Rosenthal fibers, calcifications, EGB's, protoplasmic cells, or oligodendroglimoa-like patterning [5]. Furthermore, a lack of pseudorosette orientation in our specimen works against a diagnosis of PMA.
     The non-infiltrative nature and benign character of PA makes it very amenable to total resection. In cases where the lesion is surgically inaccessible, radiation therapy and chemotherapy may be therapeutic [3]. Post-operative outcomes for patients are very good, with a survival rate of 94% at 10 years and 79% at 20 years [4] There is a low rate of recurrence and metastatic disease.

References:

1. Castillo M, Mukherji S, Thurnher M. Spinal imaging: overview and update. Saunders. Neuroimaging Clin N Am. 2007 Feb; 17(1): 17-20.
2. Koeller KK, Rosenblum RS, Morrison AL. Neoplasms of the spinal cord and filum terminale: radiologic-pathologic correlation. Radiographics. 2000 Nov-Dec;20(6):1721-49. [Medline]
3. Henson JW. Spinal cord gliomas. Curr Opin Neurol. 2001 Dec;14(6):679-82. [Medline]
4. Koeller KK, Rushing EJ. From the archives of the AFIP: pilocytic astrocytoma: radiologic-pathologic correlation. Radiographics. 2004 Nov-Dec;24(6):1693-708. [Medline]
5. Komotar RJ, Burger PC, Carson BS, Brem H, Olivi A, Goldthwaite PT, Tihan T. Pilocytic and pilomyxoid hypothalamic/chiasmatic astrocytomas. Neurosurgery. 2004 Jan;54(1):72-9; discussion 79-80. [Medline]