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|LETTERS TO EDITOR
|Year : 2016 | Volume
| Issue : 1 | Page : 176-178
N-acetyl aspartate peak in extra-axial extraosseous chondrosarcoma of the brain on MRI: Unravelling a diagnostic dilemma
Sunitha P Kumaran1, Zarina Abdul Assis1, Sanjaya Viswamitra1, Nandita Ghosal2, Sai Kiran Narayanam3
1 Department of Radiology, Sri Sathya Sai Institute of Higher Medical Sciences, Bangalore, Karnataka, India
2 Department of Pathology, Sri Sathya Sai Institute of Higher Medical Sciences, Bangalore, Karnataka, India
3 Department of Neurosurgery, Sri Sathya Sai Institute of Higher Medical Sciences, Bangalore, Karnataka, India
|Date of Web Publication||11-Jan-2016|
Sunitha P Kumaran
Department of Radiology, Sri Sathya Sai Institute of Higher Medical Sciences, Bangalore, Karnataka
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Kumaran SP, Assis ZA, Viswamitra S, Ghosal N, Narayanam SK. N-acetyl aspartate peak in extra-axial extraosseous chondrosarcoma of the brain on MRI: Unravelling a diagnostic dilemma. Neurol India 2016;64:176-8
|How to cite this URL:|
Kumaran SP, Assis ZA, Viswamitra S, Ghosal N, Narayanam SK. N-acetyl aspartate peak in extra-axial extraosseous chondrosarcoma of the brain on MRI: Unravelling a diagnostic dilemma. Neurol India [serial online] 2016 [cited 2020 Jan 18];64:176-8. Available from: http://www.neurologyindia.com/text.asp?2016/64/1/176/173640
We report a histopathologically proven case of classic chondrosarcoma in a 23-year-old woman, which appeared as a large extra-axial, extra-osseous mass in the right parietal convexity on magnetic resonance imaging (MRI). Apart from its rare incidence and location, one additional and interesting imaging finding was the presence of an N-acetyl aspartate (NAA)-like peak on magnetic resonance spectroscopy (MRS). To the best of our knowledge, this is the first report referring to the significance of NAA-like peak on MRS in an intracranial chondrosarcoma.
Intracranial extraosseous chondrosarcomas are rare and comprise less than 0.15% of all intracranial neoplasms. 75% of intracranial chondrosarcomas originate from the skull base, and rest of these lesions arise from the meninges along the falx cerebri, tentorium, and convexity. Wu and Lapi were the first to report a case of nonskeletal intracranial mesenchymal chondrosarcoma mimicking a meningioma on radiological imaging.
The peculiarity of our case, other than its unusual location, was that there was obvious myxoid degeneration noted within the classical histopathological features of a chondrosarcoma, which explains the NAA peak (2.02 ppm) on MRS.
A 23-year-old woman presented with a history of generalized tonic–clonic seizures (GTCS) and left-sided hemiparesis since 1 year. Brain MRI revealed a well-defined extra-axial dural-based lobulated mass arising from the right parietal convexity, measuring about 65 × 52 × 57 mm, hypointense on T1-weighted images, and heterogeneously hyperintense on T2-weighted images [Figure 1]a and [Figure 1]b. The lesion was causing mass effect over the adjacent brain parenchyma along with a 13mm midline shift towards the left. The mass was seen abutting the superior sagittal sinus, with the normal flow void being maintained. A few punctate areas of blooming were noted on gradient sequences [Figure 1]f, which could represent calcification; and, on postgadolinium scan, there was mild heterogenous enhancement [Figure 1]c. Adjacent dural thickening and enhancement were noted. There was no restricted diffusion on diffusion-weighted imaging (DWI) and apparent diffusion coefficient mapping [Figure 1]d and [Figure 1]e. MRS was done using a two-dimensional multivoxel chemical shift imaging (CSI) with point-resolved spectroscopy sequence (repetition time [TR]/echo time [TE]: 1000/144 ms, number of excitation [NEX]: 1). The volume of interest (VOI) for the chemical shift imaging was rightly positioned within the lesion, and multiple rest slabs were positioned to prevent the contamination of spectral signals from subcutaneous fat signals. Processing of the acquired spectra was carried out using the related functional tools. The single metabolite peak noted included a large peak at 2.02 ppm, which corresponds to the NAA peak [Figure 1]g and [Figure 1]h. Based on the imaging features, the radiological differential diagnosis included a meningioma and a hemangiopericytoma.
|Figure 1: (a and b) T1 axial and T2 coronal MR images showing an extra-axial dural-based lesion (arrow) along the right parietal convexity, which is hypointense on T1 and hyperintense on T2-weighted images. (c) Postcontrast T1 coronal MR image showing a mild heterogenous enhancement within the lesion (arrow). (d and e) Diffusion and ADC images showing no restriction within the lesion. (f) Gradient axial MR image showing areas of blooming (curved arrow) representing calcification within the lesion (g and h). Multivoxel MR spectroscopy in the area of interest showing NAA peak at 2.2 ppm (arrow)|
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The patient underwent a right frontoparietal craniotomy, and tumor decompression was done. At exploration, the tumor was found to be a firm dural-based mass, which, on histopathology, turned out to be a grade I classic chondrosarcoma with interspersed areas of myxoid components. Paraffin section of the chondrosarcoma showed a chondroid matrix ([Figure 2]a, arrow) with foci of myxoid degeneration ([Figure 2]b, asterisk).
|Figure 2: Paraffin section of the chondrosarcoma showing (a) the chondroid matrix (arrow) with foci of (b) myxoid degeneration (asterisk) [hematoxylin and eosin, ×400]|
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On retrospective evaluation of the MRI and MRS, we came to that conclusion that the myxoid components within the lesion gave rise to the NAA peak at 2.02 ppm on MRS. Postoperatively, the patient recovered well, and the follow-up MRI after 6 months showed no residual lesion. The patient was not subjected to adjuvant radiotherapy as this was a grade I tumor.
Chondrosarcoma is a malignant tumor with cells that produce a cartilaginous matrix. Brighter signal on T2-weighted images and absence of the dural tail can differentiate them from meningiomas on MRI. They show a lack of perfusion on MR angiography and appear relatively avascular on digital subtraction angiography, which help in ruling out other differential diagnoses such as hemangiopericytoma and metastases.
NAA characteristically shows a prominent resonance at 2.02 ppm due to the three protons present in the N-acetyl CH3 group. The NAA peak on spectroscopy is suggestive of neuronal integrity, and a decrease in NAA is suggestive of a loss, defect, or displacement of normal neuronal tissue. NAA can increase, and an N-acetyl peak on proton MRS can be seen in patients with Canavan's disease, which is a rare metabolic condition occurring due to the absence of NAA-degrading enzyme. NAA is not specific for neuronal tissue and has also been known to be present in mast cells, the solid part and the cyst fluid of ovarian serous carcinomas, and in the lens of the eye. In our case, the NAA peak may be have been due to the presence of mucin, a hexosamine-rich glycoprotein, within the lesion, as evident on histopathology, which demonstrated the NAA peak at 2.02 ppm. Similar spectroscopic findings of an NAA peak in certain metastatic brain tumors may be due to the presence of other chemical compounds containing N-acetyl groups, such as mucin in cases of mucinous adenocarcinoma; or, it may also be a consequence of the presence of increased concentration of NAA itself in certain extraneuronal lesions such as adenocarcinoma of the ovary. This additional finding of an NAA peak on MRS would aid in the diagnosis of a myxoid chondrosarcoma, which is otherwise rare in this location.
Although extra-axial extraosseous chondrosarcomas are very rare, their occurrence should be kept in mind when on MRI, a T2-hyperintense lesion with little enhancement, with areas of blooming on gradient sequences (which may represent calcification) and no restricted diffusion, is observed along the convexity, irrespective of the patient's age. Tumors showing an NAA peak on MRS suggest an intrinsic biochemical property of the lesion, as seen in our case; following histopathological correlation it is proposed that the NAA peak could have been been due to the myxoid-rich areas within it.
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[Figure 1], [Figure 2]