A novel meningioma with tyrosine‐rich crystals in a 6‐year‐old Great Dane

Abstract A 6‐year‐old female spayed Great Dane was evaluated for acute onset cluster seizures. Magnetic resonance imaging (MRI) identified a mass in the olfactory bulbs with a large mucoid component caudal to the primary mass. The mass was removed via transfrontal craniotomy and histopathology revealed a tyrosine crystalline‐rich, fibrous meningioma with a high mitotic index. Repeat MRI at 6 months showed no detectable tumor regrowth. The dog is clinically normal with no seizures at the time of publication 10 months after surgery. This meningioma subtype is rare in humans. This unique meningioma occurred in a dog of younger age and uncommon breed for intracranial meningioma. Biological progression of this tumor subtype is unknown; however, growth rate might be slow despite the high mitotic index.

There was no history of seizures, abnormal medical history, or potential toxin exposure. In retrospect the owners noted some mild lethargy in the weeks to months before the seizures began.
On physical examination, the dog was tachycardic (140 beats/min) and mildly tachypneic (40 breaths/min). The rectal temperature was normal. Abnormalities were not detected on general physical examination. After initial recovery from the seizures, the dog had a normal neurological examination other than mild cervical hyperesthesia.

| Immunofluorescence identification of tyrosine hydroxylase crystals
Paraffin-embedded canine brain tissue was sectioned at 5-μm thickness and mounted onto polyionic slides. Slides were deparaffinized and tissue sections were immunofluorescently labeled on a fully automated Leica Bond RX m robotic staining system. Epitope Immunofluorescence staining for TH within tissue sections of an animal with unremarkable brain pathology ( Figure 3A) show minimal F I G U R E 2 (A) Photomicrograph of the forebrain mass. The neoplasm is highly cellular and is composed of interlacing bundles of plump spindle cells with mitotic activity (blue arrow), and embedded multifocal tyrosine crystals (red arrows). Scale = 100 μm. (B) Higher magnification of the mass demonstrates multifocal extracellular crystalloid material exhibiting floret-like arrangement with many 2 to 3 μm petals (P) that surround 30 μm elongated central core (C). Scale = 100 μm.
TH + puncta, where positive staining is indicative of non-nucleated red blood cells bordered by epithelial cells, apparent by the elongated DAPI + nuclei. TH staining within animal tissue sections revealed multifocal extracellular tyrosine-rich crystalline structures that were apparent in multiple Â60 high magnification imaging planes, confirming crystalline-like diffraction patterns of intensity, that were not bordered by epithelial cells (Figure 3B).

| Outcome
Administration of prednisone was gradually reduced and discontinued by 3 weeks after surgery and phenobarbital (1.8 mg/kg PO every 12 hours) administration was continued long-term. Brain MRI was repeated 6 months after craniotomy (Figure 4). There was no detectable tumor regrowth. An area of bilaterally symmetrical T2W hyperintensity was identified in the rostral olfactory bulbs which was expected to be consistent with normal postoperative change but could represent early recurrence of the mucoid component of the mass. The dog is clinically normal with no seizures at the time of manuscript submission (10 months postcraniotomy).

| DISCUSSION
This case report describes a meningioma subtype exhibiting tyrosine rich crystals. Only 3 tumors of this subtype are reported in humans, including 2 intracranial and 1 spinal tyrosine-rich meningioma. [16][17][18] Clinical signs were apparent for 6 months to 2 years in these humans before diagnosis. This subtype exhibits magenta-colored extracellular deposits that appear "petal-shaped" on electron microscopy. These eosinophilic crystals have blunt ends that are radially arranged and are presumed to be tyrosine-rich crystals based on their appearance and positivity with Millon staining, which indicates the presence of tyrosine residues. 16,17 In 1 patient, the crystalline material was unevenly distributed with more numerous depositions occurring in areas with degenerative changes characterized by fibrosis, myxoid stromal change, and increased chronic inflammatory infiltrates. 18 Clinical outcome and long-term follow-up of the 3 humans with tyrosine-rich meningiomas was not reported. [16][17][18] The presence of tyrosine-like crystalloid material in meningiomas does not yet have prognostic relevance in humans because of their scarcity in the literature.
Canine and human meningiomas share many histologic and imaging similarities, and previous studies have attempted to apply WHO classification of meningiomas to dogs. [2][3][4]9 No correlation between histologic grade and prognosis has been identified. 4 Evaluation of immunohistochemical staining patterns to better classify meningiomas has been performed; however, further studies are needed to evaluate clinical relevance. 19 Treatment of intracranial meningiomas can consist of surgical resection, radiation, and less commonly administration of chemotherapies such as hydroxyurea. [7][8][9][10][11][12][13][14][15]20 There are conflicting reports regarding survival time with some studies stating no significant difference in outcome between dogs with surgical resection alone compared with surgery with chemotherapy, radiation, or a combination of these therapies and another showing prolonged survival time with postoperative radiation. 9,[11][12][13][14][15] One study examined the influence of tumor cell proliferation receptors on effectiveness of radiation therapy and found that meningiomas with reduced tumor proliferation index were 9 times more likely to be controlled by radiation, which might explain improved survival times with radiation for some meningiomas. 21 The dog in this case report received surgical resection alone and will be F I G U R E 3 Immunofluorescence staining of tyrosine hydroxylaserich crystals. Immunofluorescence staining of tyrosine hydroxylase (cyan) and cellular nuclei (DAPI, blue) in tissue with unremarkable brain pathology (A) and within biopsy tissue section of case study patient (B). Scale bars in low magnification montage images = 200 μM and scale bars in high magnification insets = 10 μM. monitored with recheck examinations and repeat MRIs as clinically indicated. This dog was younger than the median age for dogs with meningiomas and in a breed that has not been commonly reported, [5][6][7][8][9] and it is possible that this tumor has a different biological pattern than other canine meningiomas. It is also important to note that despite the high mitotic index observed on histopathology, there are no neurological signs or MRI evidence of regrowth in the follow-up period to date.