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Adam Schlax School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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 VMD
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Wilfried Mai School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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 Dr Med Vet, PhD
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Lillian E. Duda School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

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History

A 2-year-old 40.0-kg (88.2-lb) castrated male Golden Retriever was referred for evaluation of multiple small masses located in the left caudal maxillary gingiva and a swelling of the left ventral eyelid of 3 weeks' duration. An initial pathology report from the primary care veterinarian suggested a high-grade sarcoma with a mitotic index > 40. The mass was tentatively diagnosed as amelanotic malignant melanoma or fibrosarcoma, and the dog was referred to the oncology department.

At the time of hospital admission, the dog had a good appetite but was lethargic and could not completely open its mouth when yawning. The left nictitans membrane was prolapsed, and a 2-cm-diameter firm swelling was noted ventral to the left eye. Mild exophthalmos was present. A left mandibular lymph node was firm and enlarged. A dark pink mass was observed on the palatal surface of the left maxillary gingiva at the level of the third premolar tooth extending caudal to the last molar. On thoracic radiographs, nodular soft tissue opacities were seen in the lung, suggestive of metastases. To assess the extent of the maxillary mass and confirm pulmonary metastases, CT of the head (Figure 1) and thorax was performed.

Figure 1—
Figure 1—

Transverse CT images of a 2-year-old 40-kg (88.2-lb) castrated male Golden Retriever evaluated for multiple small masses in the left caudal maxillary gingiva and swelling of the left ventral eyelid of 3 weeks' duration. A—Postcontrast transverse CT image (soft tissue algorithm; window width, 300 Hounsfield units [HU]; window level, 40 HU) obtained at the level of the rostral aspect of the zygomatic arch. B—Postcontrast CT image (bone algorithm; window width, 2,000 HU; window level, 300 HU) obtained at a level just rostral to panel A.

Citation: Journal of the American Veterinary Medical Association 248, 7; 10.2460/javma.248.7.755

Determine whether additional imaging studies are required, or make your diagnosis from Figure 1—then turn the page →

Diagnostic Imaging Findings and Interpretation

Scanning was performed with a 16-slice multidetector scanner.a Transverse images were acquired of the head and reconstructed with a bone algorithm and soft tissue algorithm before and after IV administration of iodinated contrast materialb (2.2 mL/kg [1.0 mL/lb]). Transverse images of the thoracic cavity were acquired and reconstructed with a lung algorithm to evaluate for pulmonary metastatic disease. Technique settings were 160 to 200 mA and 120 kVp.

On CT evaluation, a large mass (width, 3.5 cm; height, 3.5 cm; length, 2.7 cm) that is centered at the rostral aspect of the left zygomatic arch and extends into the left orbit, the lateral portion of the left nasal passage, and the left infraorbital canal is apparent (Figure 2). Compression and dorsolateral displacement of the left globe, with possible early infiltration, are caused by the retrobulbar component of the mass. The left zygomatic salivary gland abuts the mass, but it is uncertain whether it is compressed or infiltrated by the mass; the left zygomatic salivary gland has a homogeneous attenuation pattern that is similar to that of the right zygomatic salivary gland. The mass has a heterogeneous attenuation precontrast appearance (range, −7 to 67 Hounsfield units [HU]). Moderate heterogeneous contrast enhancement is evident after IV administration of iodinated contrast material (range, −24 to 186 HU). Multiple hypoattenuating nonenhancing areas are present, consistent with cyst-like changes or areas of necrosis. There is extensive permeative to moth-eaten lysis of the left maxilla from the level of the roots of the maxillary canine tooth to the level of the rostral margin of the temporomandibular joint (Figure 3). Histologic reevaluation of the mass revealed large numbers of spindle-shaped neoplastic cells embedded in an eosinophilic fibrillary extracellular matrix with a mitotic index of 40. Aspirates of the left mandibular lymph node were obtained and revealed a metastatic population of cells. These findings were most consistent with a high-grade sarcoma. Considering the CT and histologic findings, an aggressive infiltrative neoplasia associated with the left maxilla was considered the most likely differential diagnosis.

Figure 2—
Figure 2—

Same CT images as in Figure 1. A—The orbital component of the mass (*) is strongly contrast enhancing and displaces and possibly invades the left globe (open arrow). The mass has heterogeneous attenuation with multiple hypoattenuating, nonenhancing areas with rim enhancement (arrowhead) consistent with fluid (ie, necrosis or cyst-like structures). B—Notice the extensive moth-eaten to permeative lysis of the left maxillary (*), palatine (white arrowhead), and frontal bones (white arrow) as well as the left zygomatic arch (black arrow).

Citation: Journal of the American Veterinary Medical Association 248, 7; 10.2460/javma.248.7.755

Figure 3—
Figure 3—

Volume rendering of the skull from the CT series (viewed from the left) of the dog in Figure 1, demonstrating the rostrocaudal extent of lysis associated with the neoplasm (arrows). Dashed lines depict transverse location of images A and B from Figures 1 and 2.

Citation: Journal of the American Veterinary Medical Association 248, 7; 10.2460/javma.248.7.755

The soft tissue mass is associated with lysis of the left incisive, maxillary, palatine, frontal, and zygomatic bones. Left lateral (width, 2.8 cm; height, 1.8 cm) and medial (width, 1.8 cm; height, 1.4 cm) mandibular lymphadenomegaly is evident, and the nodes are more rounded than typical. The left and right medial retropharyngeal, left parotid, and right mandibular lymph nodes are mildly enlarged. The lymph nodes have a moderate homogeneous contrast enhancement pattern. On thoracic CT (images not shown), there were multifocal soft tissue pulmonary nodules, measuring between 3.0 and 7.2 mm in diameter.

Because of the localization and aggressiveness of the tumor and the young age of the patient, the initial diagnosis on referral of amelanotic malignant melanoma or fibrosarcoma was challenged, and other etiologies such as a juvenile rhabdomyosarcoma were considered. Immunohistochemical staining of the initial biopsy samples was requested. S-100 protein and desmin immunohistochemical staining was performed. Immunohistochemical staining with anti–S-100 protein antibodies (highly sensitive for melanocytic neoplasms) yielded negative results, ruling out melanoma.1 Immunohistochemical staining for desmin yielded positive results, supporting a diagnosis of juvenile rhabdomyosarcoma, although other neoplasms such as leiomyoma and leiomyosarcoma can also stain positive for desmin.2,3 Biopsy specimens were histologically reevaluated; the findings further characterized the neoplasm as the embryonal subclass of juvenile rhabdomyosarcoma. Given the extensiveness of the neoplasm, the exact muscle of origin of the neoplasm could not be determined.

Treatment and Outcome

The patient was given a poor overall prognosis because of the extensive local and metastatic disease. A treatment combination of palliative radiation therapy and chemotherapy was elected. After 3 treatments of palliative radiation therapy over a 2-week period, the dog did not show signs of pain and could fully open its mouth. Radiation therapy was discontinued at that time. The patient was concurrently receiving a rotating schedule of chemotherapeutic agents including vincristine, cyclophosphamide, doxorubicin, and palladia. Two months following initial hospital admission, the patient was euthanized because of progressive lethargy, weakness, and neurologic signs.

Comments

Rhabdomyosarcoma is a rare malignant neoplasm of striated muscle and is classified on the basis of gross and histologic appearance into 4 subclasses: embryonal, alveolar, botryoid, and less commonly pleomorphic.2 In human medicine, alveolar and embryonal rhabdomyosarcoma are found mostly in children under the age of 15 and have been termed juvenile or pediatric rhabdomyosarcoma. In veterinary medicine, alveolar, embryonal, and botryoid rhabdomyosarcoma are typically seen in young animals and are collectively referred to as juvenile rhabdomyosarcoma, although the embryonal form can also be seen in adults.2 Location of rhabdomyosarcoma in dogs is overall variable, but tends to be more specific in juvenile forms. The juvenile botryoid subclass has a characteristic anatomic location in the submucosa of the urinary bladder.2 To our knowledge, only 6 alveolar and 7 embryonal cases of juvenile rhabdomyosarcoma have been documented. Of these, 4 alveolar and 1 embryonal rhabdomyosarcomas were localized to the maxillary gingiva.2

Definitive diagnosis of juvenile rhabdomyosarcoma is challenging. Differentiation from benign neoplasms, such as papilloma, and tumors of odontogenic origin is not diagnostically challenging, but rhabdomyosarcoma shares clinicopathologic features with other more common oral tumors such as malignant melanoma and fibrosarcoma, which make diagnosis difficult.4 Neoplastic cells can undergo varying degrees of striated muscle differentiation and can resemble anything from undifferentiated myoblasts, which can appear round cell in nature, to more differentiated and elongated spindle cells with or without cross striations.2 These characteristics commonly overlap with other neoplasms, both of soft tissue origin and of other histogenesis.2 These neoplasms can rarely be definitively diagnosed by standard histologic evaluation alone.4 Specific immunohistochemical stains are often necessary for a definitive identification. In both humans and dogs, rhabdomyosarcoma consistently stains positive for the cytoskeletal proteins vimentin and desmin and also for muscle and sarcomeric actin.2

Juvenile rhabdomyosarcomas of the oral cavity also behave similarly to other more common oral tumors. They can be locally invasive and cause extensive bone lysis, which appear similarly on radiographic and CT imaging. The CT findings of the dog of the present report were not specific for juvenile rhabdomyosarcoma; however, clinical signs and history, in conjunction with the young age of the patient, should raise the index of suspicion. The mean age of dogs with oral fibrosarcoma and melanoma is reported to be 7 to 9 years old and 12 years old, respectively.5 When a young dog is examined for an aggressive oral tumor with the diagnostic imaging characteristics similar to the dog of the present report, juvenile rhabdomyosarcoma should be included on the list of differential diagnoses and appropriate immunohistochemical staining of biopsy specimens should be requested.

Footnotes

a.

General Electric BrightSpeed Elite Select, General Electric Co, Milwaukee, Wis.

b.

Omnipaque (Iohexol 350), General Electric Healthcare Inc, Princeton, NJ.

References

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  • 2. Caserto BG. A comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol 2013; 50:806826.

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  • 3. Andreasen CB, Mahaffey EA. Immunohistochemical demonstration of desmin in canine smooth muscle tumors. Vet Pathol 1987; 24:211215.

  • 4. Cooper BJ, Valentine BA. Tumors of muscle. In: Meuten DJ, ed. Tumors in domestic animals. 4th ed. Ames, Iowa: Iowa State Press, 2002;319363.

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  • 5. Liptak JM, Withrow SJ. Cancer of the gastrointestinal tract. In: Withrow SJ, Vail DM, Page RL, eds. Small animal clinical oncology. 5th ed. St Louis: Saunders, 2012;381431.

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