Retrobulbar malignant peripheral nerve sheath tumor in a golden retriever dog: A challenging diagnosis

Abstract

A 9-year-old golden retriever dog was diagnosed with a left retrobulbar mass. Fine-needle aspirations and incisional biopsies resulted in discordant diagnoses: myxosarcoma/myxoma or rhadomyosarcoma, respectively. Immunohistochemistry following exenteration allowed definitive diagnosis of malignant peripheral nerve sheath tumor with fibromyxomatous differentiation. Fifteen weeks after surgery, an aggressive recurrence resulted in euthanasia.

Neoplasia is commonly observed as a cause of orbital pathology in older dogs. Numerous orbital neoplasms have been described in small animals, and can be primary, metastatic, or from local extension of neoplasia of neighboring tissue. Approximately 75% to 95% of these tumors are malignant and are associated with a poor prognosis (1,2).

Canine malignant peripheral nerve sheath tumors (MPNSTs) can occur in the peripheral nervous system (PNS) but also in soft tissues and various organs including the eye or orbit (3–6). These tumors occur in middle-aged to older dogs, particularly in medium- to large-breeds and are characterized by slow growth and rare metastasis (3,4). Lack of specific histological features of these tumors is a common diagnostic challenge in human as well as in veterinary medicine. There is no one specific immunohistochemical marker able to define a PNST, so a panel of markers is needed. Peripheral nerve sheath tumors are mesenchymal tumors positive for Vimentin. Immunoreactivity of S-100 is restricted to a subpopulation of tumor cells, because this marker only labels neoplastic Schwann cells, not the remaining tumor cells. Expression of glial fibrillary acidic protein (GFAP) is variable (4). Documented cases of retrobulbar tumors of nerve sheath origin are scarce in the veterinary literature (5,7–9). Among them, the most recent report was about a 1-year-old pug dog with a retrobulbar pigmented peripheral nerve sheath tumor, whose prognosis was poor (5).

The aim of this work was to describe the clinical and imaging findings of a malignant retrobulbar peripheral nerve sheath tumor, underscoring its challenging histological and immunohistochemical diagnosis. A better characterization of retrobulbar MPNST will allow veterinarians to understand its biological behavior and eventually improve patient management and prognosis.

Case description

A 9-year-old spayed female golden retriever dog was referred to the ophthalmology service of the Veterinary Teaching Hospital of Faculté de Médecine Vétérinaire, Université de Montréal, for evaluation of a left-sided periocular swelling of 2 months’ duration. The patient had been presented to a primary care veterinarian 2 mo before referral for serous discharge and discomfort of the left eye. An elevated nictitating membrane and mild swelling of periocular soft tissues were noticed. A left eye uveitis was suspected and topical treatment with dexamethasone/tobramycin eye drops (Tobradex, Alcon Canada, Mississauga, Ontario), 1 drop every 12 h for 2 wk, and a systemic nonsteroidal anti-inflammatory drug (Firocoxib, Previcox; Mérial, Baie D’Urfé, Quebec), 5 mg/kg body weight (BW), q24h for 5 to 7 d, were prescribed. Because of persistent clinical signs in spite of medical treatment, an ophthalmology consultation was requested.

A complete ophthalmic examination was performed including slit-lamp biomicroscopy (SL-15 Portable Slit Lamp; Kowa Optimed, Torrance, California, USA) and indirect ophthalmoscopy (HC50 L Headband Rheostat; Heine, Dover, New Hampshire, USA). A menace response was present in both eyes. Palpebral, direct, and consensual pupillary light and dazzle reflexes were also present, normal, and symmetric for both eyes. Schirmer tear test I (Color bar Schirmer tear test; Eagle Vision, Denville, New Jersey, USA) values were 25 mm/min and 30 mm/min in the right and left eyes, respectively. Fluorescein staining (MINIMS FLN 2.0; Chauvin Pharmaceuticals, Chauvin Pharmaceuticals, Kingston upon Thames, England) was negative in both eyes. Intraocular pressure measurement was carried out with an applanation tonometer (Tonopen-Vet; Reichert, DePew, New York, USA) following application of a topical anesthetic (proparacaine hydrochloride 0.5%, Alcaïne; Alcon Canada). Values were 7 mmHg in the right eye and 13 mmHg in the left eye. Some abnormalities were diagnosed in both eyes: dark iris, multifocal posterior synechiae, resistance to dilation, pigment deposit on the anterior lens capsule with subcapsular lens opacities, and proteinaceous material in the anterior chamber. These signs were consistent with pigmentary uveitis. Findings specific to the left eye included exophthalmos, moderate resistance to retropulsion, an elevated third eyelid, and conjunctival follicular hyperplasia. No pain was elicited upon opening the mouth and no mass was detected during oral examination. No additional abnormalities were noted on complete physical examination. Results of a complete blood (cell) count (CBC) and serum chemistry panel were unremarkable. The unilateral presentation combined with the absence of pain upon palpation and opening of the mouth, the slow progression of signs, the absence of leukocytosis upon blood analysis, and the age of the dog suggested that an infectious process or abscess was unlikely. Main differential diagnoses included an orbital neoplasia or a salivary mucocele. An immune-mediated myositis could not be ruled out, but was considered unlikely. The patient was placed under general anesthesia so that additional diagnostic tests could be performed.

Magnetic resonance images (MRI) of the head were acquired using a 1.5 T magnet (GE Signa Echospeed HDx; General Electric Healthcare, Mississauga, Ontario). Pre-contrast images were acquired and included the following sequences: sagittal, transverse, and dorsal T2-weighted (T2-w) Fast Spin Echo (FSE), transverse T2-w Fluid Attenuated Inversion Recovery (FLAIR), transverse T1-w FLAIR, and transverse T2*-Gradient Recalled Echo (GRE), transverse T2-w SSFSE (Single Shot Fast Spin Echo). Post-contrast images were acquired following intravenous administration of 0.1 mmol/kg BW of gadobenate dimeglumine (Multihance; Bracco Diagnostics, Monroe Township, New Jersey, USA) and included transverse T1-w FSE (0, 5, and 15 min, respectively, after contrast administration) and delayed dorsal and sagittal plane. A large conic mass infiltrating the left medial retrobulbar region was visualized and associated with a mass effect creating a compression and a rostrolateral displacement of the globe (Figures 1a and 1b). Globe sclera was distorted but remained well-defined and seemed intact. The mass also involved the optic cone and it was unclear if it was only severely compressing the extraocular muscles or if it was invading them. The zygomatic gland appeared normal and unaffected by the process, and there was no evidence of bone lysis or invasion of the calvarium. The mass was well-defined, highly hyperintense on T2w images, moderately hyperintense on T2-FLAIR images, and hypointense and heterogeneous on pre-contrast T1w images. Following administration of contrast medium, the mass showed a strong and well-defined enhancement (Figures 1c and 1d). Based on magnetic resonance images, a neoplastic process appeared most likely. The loculated nature of the mass, with cystic areas and the enhancement pattern on MRI sequences were suggestive of myxosarcoma. Other differential diagnoses included meningioma, malignant lymphoma, carcinoma, or sarcoma.

Figure 1.

Dorsal (a) and sagittal (b) fast spin echo T2W (TR = 5650 ms and TE = 85 ms) MR images of the head. The left retrobulbar mass is well-defined and highly hyperintense to brain parenchyma. The mass infiltrates the left medial retrobulbar region and is associated with a mass effect creating a compression and a rostrolateral displacement of the globe. Transverse T1W-FLAIR (TR = 2403 ms and TE = 26 ms) (c) pre- and (d) post-contrast MR images. The mass is hypointense and heterogeneous on pre-contrast T1W images (c) but shows strong heterogenous contrast enhancement (d). Note the compression of the left globe on both images. The zygomatic gland appeared normal and unaffected by the process (c — white circle).

Immediately following MRI, ultrasound-guided fine-needle aspirates (FNA) and trucut percutaneous incisional biopsies of the retrobulbar mass were obtained with the dog still under anesthesia. A cytology report identified numerous, scattered spindle or ovoid cells, with a high nucleocytoplasmic ratio and basophilic cytoplasm. Mild to moderate anisocytosis and anisokaryosis were also observed. These cells were associated with pink fibrillary material and some hemosiderophages, and mucoid eosinophilic matrix that aligned red blood cells (“windrowing” sign). The findings were consistent with a mesenchymal proliferation with mucin, suggesting a myxoma or a well-differentiated myxosarcoma with chronic hemorrhage.

Histopathologic assessment revealed a poorly demarcated, non-encapsulated mass with low cellularity (Figures 2a and 2b). It was composed of spindle cells in an abundant fibrovascular to myxoid stroma (Figure 2b). Cells had a poorly defined cytoplasm and a round to elongated nucleus. Rare cells with more abundant eosinophilic to amphophilic cytoplasm, consistent with myoblasts, were seen. Anisocytosis and anisokaryosis were moderate. There were up to 2 mitoses per high power field. A presumptive diagnosis of embryonal rhabdomyosarcoma was proposed. To try to corroborate this diagnosis, immunohistochemistry for desmin (Biogenex. Fremont, California, USA; 1:40; no antigen retrieval) was performed. The rare myoblast-like cells were strongly immuno-reactive, but the spindle cells were not; thus, the diagnosis was still uncertain.

Figure 2.

a — Light microscopic image from echo-guided biopsy showing a mass with low cellularity with spindle cells in an abundant fibrovascular or myxoid stroma. Note the spindle cell with ill-defined margins, eosinophilic cytoplasm, and elongated nucleus consistent with muscle cells. b — Alcian blue stain showing abundant blue staining of mucin and pink to red staining of cells nuclei consistent with the myxoid feature of the mass.

Though a definitive diagnosis had not yet been obtained, a high-grade sarcoma was suspected and an oncology consultation obtained. On complete clinical staging, there was no detectable evidence of metastasis or serious concurrent disease based on abdominal ultrasonography, 3-view thoracic radiography, and bilateral submandibular lymph node aspiration and cytology. Various treatment options were discussed, including palliative or radiation therapy with curative intent and various systemic anticancer therapies, but orbital exenteration was recommended as the treatment of choice in light of the localized disease and the opportunity to try to obtain a definitive diagnosis.

Sixteen days after the initial MRI, the patient was placed under general anesthesia and exenteration of the left eye and orbit was performed. An irregular, solid, white neoplasm filled the retrobulbar space. The neoplasm measured approximately 1.5 to 2 cm in diameter with ill-defined borders. It infiltrated and replaced extraocular muscles and orbital fat, and multifocally extended to some of the excision margins.

Histologically, the neoplasm consisted of interlacing streams and bundles of loosely arranged to more tightly packed spindle cells with occasional nuclear palisading (i.e., “storiform pattern”) supported by a fibromyxomatous matrix characterized by very loose and myxomatous features in some areas to less cellular and collagenous in others (Figure 3a). Additionally, neoplastic spindle cells were associated with several small sections of nerves in which Schwann cell proliferations were noticed. Anisocytosis and anisokaryosis were moderate; and up to 6 mitoses per 10 high power fields were counted. A preliminary diagnosis of a malignant peripheral nerve sheath tumor was made based on the histomorphologic features. In view of previous results on the original biopsy and the possibility of a malignant PNST with rhabdomyoblastic differentiation (i.e., Triton tumor), immunohistochemistry for desmin (Biogenex; 1:40; no antigen retrieval) was also performed on the neoplasm. There was strong immunoreactivity that was limited to extraocular muscle cells, either normal or interpreted as atrophic due to neoplastic infiltration; the spindle cells were diffusely negative. A Triton tumor was thus excluded.

Figure 3.

a — Light microscopic image demonstrating spindle neoplastic cells, arranged in a vaguely storiform pattern, separated by a loose fibromyxomatous matrix. Note the admixed hemosiderophages (arrows) that were also observed during cytologic examination. Inset. An infiltrative retrobulbar neoplasm that compresses the posterior segment (asterisk). Hematoxylin and eosin (H&E). b and c — Immunohistochemical characterization of neoplastic cells. Neoplastic cells show strong nuclear and cytoplasmic immunoreactivity for S100 protein (b) as well as cytoplasmic immunostaining for GFAP (c).

The case was referred to Veterinary Pathology Services, Joint Pathology Center (606 Stephen Sitter Avenue, Silver Spring, Maryland, USA) for second opinion consultation. The diagnosis of malignant PNST was supported and confirmed with immunohistochemistry. Immunohistochemical labeling for S100 protein and glial fibrillary acidic protein (GFAP) were performed and both were strongly positive (Figures 3b and 3c).

Seventeen days after surgery, the owner noticed an intermittent green discharge from the left nostril along with occasional sneezing and stertor. Further investigations were declined and a large-spectrum systemic antibiotic (Amoxicillin/clavulanic acid, 15 mg/kg BW, PO, q12 h for 2 wk) and a non-steroidal anti-inflammatory drug (Deracoxib, Deramaxx; Elanco Canada, Guelph, Ontario), 1 mg/kg BW, PO, q24h for 5 to 7 d, were given to treat a possible orbital bacterial infection and/or inflammatory reaction.

After a follow-up with an oncologist, an oral metronomic chemotherapy protocol was chosen by the owner and consisted of the combined administration of an NSAID (Piroxicam; Nostrum Laboratories, Kansas City, Missouri, USA), 10 mg total daily dose PO, a chemotherapeutic agent (Cyclophosphamide; Pharmacia & Upjohn, Mississauga, Ontario), 25 mg total daily dose PO on a 2 days on/1 day off schedule), and a diuretic (Furosemide; Apotex, Toronto, Ontario), 40 mg total daily dose, PO. Fifteen weeks after the exenteration, given the persistent and progressive upper respiratory signs associated with discomfort, a computed tomography (CT) scan of the head (16-slice helical CT scanner, HiSpeed ZXi; General Electric, Mississauga, Ontario) was obtained. Transverse, 1.25-mm thick images of the head were helically acquired and reconstructed using a soft tissue and bone algorithm. Post-contrast images were then acquired with soft tissue algorithm following intravenous administration of 68 mL of Iopamidol (300 mg/mL, Isovue 300; Bracco Diagnostics, Monroe Township, New Jersey, USA). The left orbit was completely occupied by an ill-defined, heterogeneous mass with irregular margins and multiple pockets of fluid (Figures 4a and 4b). This mass involved the orbital muscles, the medial pterygoid muscle, and invaded the left temporal muscle. It was associated with bone lysis of the medial wall of the left orbit, and also infiltrated the left rostral frontal sinus, rostral fossa of the cranial vault, and left sphenoid sinus. A mass effect was also noticed in contact with the olfactory lobe (Figures 4a and 4c). The left nasal cavity and choanes were invaded but without lysis of the palatine bone. A mass effect on the soft palate was described, caudally to left maxillary tooth M2. The left mandibular, retropharyngeal, and parotid lymph nodes had a mild increase in size and irregular margins. With the aggressive nature of the lesion and the animal’s history, a loco-regional recurrence of the PNST was strongly suspected. Given the poor prognosis, the owner elected to stop the chemotherapy. Because of a quick decline in the quality of life and mild behavioral changes, the dog was euthanized 10 d later. No necropsy was performed to confirm the recurrence.

Figure 4.

Transverse CT images (a) pre- (bone reconstruction algorithm) and (b) (detail reconstruction algorithm) post-contrast at the level of the caudal retro-orbital space. Note the bone lysis of the medial wall of the left orbit (a). This mass is characterised by multiple fluid pockets with ring enhancement. Note the extension of the mass into the cranial vault and the mass effect on olfactory bulb (b). Dorsal CT image (detail reconstruction algorithm) (c) post-contrast demonstrating invasion of the left nasal cavity.

Discussion

This case illustrates the marked morphologic variations of canine malignant peripheral nerve sheath tumors and consequently the diagnostic challenge which they present. First, attempts to obtain a diagnosis were made via ultrasound-guided FNA and cytology, which provides a definitive diagnosis in approximately 45% of retrobulbar masses and is especially helpful with exfoliative neoplasia including lymphoma (2,10). Cytology results raised the suspicion of a myxoma or well differentiated myxosarcoma. The cytologic tentative diagnosis was supported by MRI features of the orbital mass consistent with a previous description (11). Myxoma or myxosarcoma are soft tissue tumors of fibroblastic origin that can be distinguished by the presence of an abundant myxoid stroma. They are most frequently located in skin or subcutaneous tissue and around joints, but may occur at any site in the body (12). Orbital myxomas and myxosarcomas have been reported as rare tumors in humans and there are some reports of canine low-grade myxosarcoma localized in the retrobulbar space or involving the orbit (11,13). This presumptive diagnosis was challenged by the histologic results from ultrasound-guided incisional biopsies. Histologic diagnosis was difficult due to the small size of samples, the low cellularity, and the common myxomatous changes shared by many tumor types, including chondrosarcoma, liposarcoma, synovial sarcoma, smooth muscle tumors, embryonal rhabdomyosarcoma, PNST, neurofibroma, and mucinous adenocarcinoma. Based on histomorphology and immunohistochemistry, a diagnosis of rhabdomyosarcoma was considered for the initial biopsy, but was refuted based on results for the second biopsy. Canine rhabdomyosarcomas are uncommon mesenchymal neoplasms of skeletal muscle origin with varying myogenic differentiation. Their common locations are in the larynx and urogenital tract but they have been documented in the head, neck, and face (14,15). Orbital rhabdomyosarcoma seems rare in the dog and a retrospective study of 18 cases of orbital rhabdomyosarcoma has been recently published (16). They are often metastatic and more aggressive in dogs less than 2-years of age, but may be amenable to therapy in older dogs (16,17). Immunohistochemistry for myogenic markers such as myogenin is useful in differentiating rhabdomyosarcomas from other mesenchymal tumors. Myoblast cells express desmin and positive labeling is common to skeletal muscle, cardiac muscle, smooth muscle, and myofibroblast. In the retrospective study of Scott et al (16), all 18 cases demonstrated positive immunolabeling for desmin and 9/18 cases showed cross-striations. In our case, for the second biopsy, the immunoreactivity was limited to extraocular muscle cells but the spindle cells were negative and no cross-striation was observed. This pattern invalidated the first presumptive diagnosis of rhabdomyosarcoma.

Exenteration was performed as part of the therapeutic plan with subsequent histologic assessment to provide a definitive diagnosis. Again, due to overlapping features of their histological patterns, differentiation of soft tissue sarcomas presents a diagnostic challenge. In this case, a final diagnosis of peripheral nerve sheath tumor was reached. Immunohistochemically, PNST are variably positive for vimentin, S100 protein, and GFAP, among other markers (18,19). In human malignant PNST, variable histologic patterns and heterogenous differentiation have been reported (20). The patterns include epithelioid malignant PNST or malignant PNST with divergent differentiation such as rhabdomyoblastic (malignant Triton tumor), cartilaginous, osseous, angiomatous, and glandular or their complex (20). These features have been found in dogs and other species (21–23). In this case, the mass presents variable histologic patterns consistent with what is described in the literature and cells were strongly positive for S100 protein and GFAP; this, combined with the histologic pattern, allowed for a diagnosis of malignant peripheral nerve sheath tumor.

With soft tissue sarcomas, a radical surgical resection with wide tumor-free margins can be curative. For orbital neoplasia, wide tumor-free margins often require radical orbitectomy, an invasive and disfiguring procedure. Another approach is exenteration with subsequent radiation therapy or chemotherapy, especially in cases of incomplete resection or with high-grade tumors (24). In our case, histopathology assessment showed incomplete margins. Radiation therapy was proposed but declined by the owner and an oral metronomic chemotherapy protocol was preferred (25–27). Malignant nerve sheath tumors in dogs commonly recur following surgical excision but rarely metastasize (3,5). In the case reported here, an aggressive loco-regional recurrence was diagnosed 15 wk following surgery but upper respiratory signs had started 3 wk after surgery. Occasionally, with aggressive sarcomas, such as rhabdomyosarcoma in children and young dogs, surgery may not be associated with a favorable outcome (16). Instead, a combination of neo-adjuvant radiation therapy and chemotherapy, with or without surgical intervention, may provide better outcome with longer survival and improved quality of life as documented for children (28). It is possible that the reported tumor behavior was negatively impacted by surgical excision. Indeed, the surgical trauma associated with incomplete margins may have accelerated the local spread of the tumor. Because there was a delay (16 d) between the initial MRI and the surgery, it is also possible that the sarcoma had already progressed in the periorbital tissues in the days leading to surgery.

This case illustrates the importance of combining several diagnostic modalities to define the nature of certain types of retrobulbar neoplasia in dogs and selected immunohistochemistry should be considered in addition to standard stains, especially with soft tissue sarcomas. Exenteration benefits should be carefully assessed in cases of retrobulbar peripheral nerve sheath tumors. CVJ