Ten-year-old female mixed breed dog (Canis familiaris).This dog presented to the RUSVM Spay Clinic for preoperative evaluation for elective ovariohysterectomy. The owner reported no significant medical history. Physical exam abnormalities included severe tick infestation; mildly but uniformly enlarged peripheral lymph nodes, and multiple mammary masses ranging in size from 1cm to 1.5cm diameter. Masses were present throughout the mammary chain but were concentrated in the caudal glands. Hematologic and chemistry findings included a moderate thrombocytopenia (65 x 109/l, range 200-500); severe lymphocytosis (16.2 x 109/l, range 1-4.8); and significantly elevated plasma protein (10.1 g/dl, range 6- 7.5). The patient was prescribed doxycycline 100mg (8.5mg/kg) q24h for 21 days on the presumptive diagnosis of rickettsial disease. Further diagnostics were strongly recommended but declined. Surgical sterilization was postponed until further workup could be pursued. Nine months later the patient represented to the RUSVM Emergency Service in respiratory distress. The owner indicated the patient’s condition had been deteriorating and elected euthanasia, which was performed with 3ml of pentobarbital (200mg/ml).

Gross Description:  

At necropsy, a 12 x 9 x 8 cm, spheroid, hard mass was within the subcutis of the ventrocaudal abdomen, apparently replacing the left inguinal mammary gland. On cut surface, the mass was mottled pale pink and grey, gritty, and had a peripheral 3 cm cavitated area containing soft gelatinous opaque material. Six smaller firm nodular masses were present in other mammary glands, ranging in size from 0.5 to 2 cm. Similar 0.5-1.5 cm discrete, grey-white, nodular to sessile masses were heavily disseminated throughout the lungs, replacing nearly 90% of the parenchyma. The kidneys, spleen, and liver had a few similar masses, and a similar solitary 0.7cm mass spanned the epicardium and superficial myocard

Histopathologic Description:

Histologically, the large mammary mass and the kidney, spleen, liver, and lung masses consisted of embryonal rhabdomyosarcoma. A section of lung with multiple metastases is submitted for examination. The tumors were composed of large elongate tube-like to fusiform cells admixed with fewer round myoblastic cells and organized into interwoven bundles and streams of within myxomatous stroma. The elongate cells resembled myotubes, often having multiple tandemly aligned oval nuclei and poorly discernable sarcoplasmic cross striations (“strap cells”). Sarcoplasmic cross-striations were accentuated with phosphotungstic acid hematoxylin (PTAH) stain and enhanced by polarized light on preparations stained with Picrosirius red stain. Tumor cells express moderate to marked and diffuse vimentin protein, moderate to marked and segmental desmin protein, but were negative for myoglobin, and smooth muscle actin. When evaluated with electron microscopy, tumor cells varied from round, immature myoblast cells with variable amounts of cytoplasm to multinucleated myotubular developing skeletal muscle cells. The cytoplasm contained moderate amounts of mitochondria, glycogen, and ribosomes. Immature myoblast cells have eccentrically located nuclei with heterochromatin and one or two large nucleoli. The multinuclear tumor cells had indented nuclei with heterochromatin and abundant mitochondria and glycogen. Tumor cells contained myofilaments with Z-band like structures. Unequivocal cell junctions were present in the tumor cells.

Morphologic Diagnosis:  

Lung: Metastatic rhabdomyosarcoma, embryonal

Lab Results:  



Metastatic rhabdomyosarcoma

Contributor Comment:  

Canine rhabdomyosarcomas are histologically classified as embryonal, botryoid, alveolar, and pleomorphic.1 The mixture of round immature myoblastic cells and multinucleated myotubular cells resembling developing skeletal muscle are consistent with the embryonal subtype of rhabdomyosarcoma. In this case, the predominance of the myotubular cells is further indicative of the myotubular variant of embryonal rhabdomyosarcoma.

In humans, embryonal rhabdomyosarcoma is the most common type, and predominantly affects children.7 Similarly, embryonal rhabdomyosarcoma is the most common type diagnosed in dogs, and usually involves dogs under two years of age. However, when excluding the botryoid type of embryonal rhabdomyosarcoma of the urinary bladder, approximately half of the reported cases of canine embryonal rhabdomyosarcoma occur in adult dogs, as observed in this case.1 In humans, embryonal rhabdomyosarcoma is considered an intermediately aggressive variant of rhabdomyosarcoma. Canine rhabdomyosarcoma, in general, is considered an aggressive malignancy having a metastatic rate which appears to approximate that of grade three soft tissue sarcomas. However, there is too little data to make conclusions regarding the prognostic significance of histological subtypes in dogs. The present case would appear to comprise the first report of histologically-confirmed metastasis of embryonal rhabdomyosarcoma in an adult dog (>2 years), but appears to be of intermediate malignancy since the dog survived over nine months following clinical diagnosis of the mammary mass.

Rhabdomyosarcoma has occasionally been reported in dogs to emerge from organs lacking striated muscle, including skull,4 meninges,6greater omentum,8 gingiva,9 spleen, perirenal, and mammary gland.2 In such instances, the neoplasm is thought to arise from stem cells capable of myogenic differentiation. In the present case, the large mammary mass is presumed to be the primary tumor because it was the largest of all masses and was first recognized clinically. Mammary rhabdomyosarcoma is very rare in humans and usually affects adolescent girls, is metastatic rather than primary breast tumor, and is alveolar subtype.5 Similar to the present case, the other reported case of canine mammary rhabdomyosarcoma involved an adult female, and was also of the embryonal subtype.2

JPC Diagnosis:  

Lung: Rhabdomyosarcoma, metastatic, mixed breed dog, Canis familiaris.

Conference Comment:  

The contributor provides an excellent example of a metastatic rhabdomyosarcoma (RMS) in the dog. RMS is a rare, malignant, neoplasm arising from pluripotent mesenchymal stem cells that differentiate toward skeletal muscle.1,3 As a result of their pluripotent origin, RMS has a highly variable age of onset, location, gross, and histological appearance. 3

As mentioned by the contributor, skeletal muscle neoplasms often arise in areas where skeletal muscle is not normally present. Common locations in the canine are the larynx and urinary bladder.3 Laryngeal rhabdomyomas are formally known as laryngeal oncocytomas due to their characteristic eosinophilic, granular, PASpositive cytoplasm. The name was changed to rhabdomyoma due to the presence of myofilaments with Z-bands on transmission electron microscopy (TEM), and positive immune-reactivity for muscle markers.3 In the urinary bladder, RMS is also known as botryoid rhabdomyosarcoma. This most commonly occurs in young (<2 years), large breed bitches, with Saint Bernard dogs overrepresented. Grossly, these botryoid neoplasms occur at the trigone and cause urinary obstruction.3 Interestingly, they have also been associated with hypertrophic osteopathy in dogs.3 In pigs, cardiac rhabdomyomas are benign incidental findings and are thought to be Purkinje cell origin due to expression of protein gene product (PGP) 9.5, a Purkinje fiber marker.3

Almost all RMSs are typically aggressive with local infiltration and distant metastasis to other muscles, liver, spleen, and the lung.3 In this case, in addition to the lung and adjacent mammary tissue, there is metastasis to the kidneys, spleen, liver, epicardium, and myocardium.

RMS in humans and veterinary species has been classified into different categories depending on the degree of differentiation towards skeletal muscle:

• Embryonal: Divided based on cell morphology: myotubular, myoblastic, spindle cell, and botryoid. Embryonal RMS is typically encountered in animals <2 years old, and occur on the face, skull, masticatory muscle, oropharynx, trachea, axilla, scapula, perirenal, tongue, flank, leg, mammary gland, and hard palate. Botryoid RMS occurs in the urinary bladder and uterus. 1,2,3 The mammary gland is theorized to be the primary tumor location in this case.
• Alveolar: Sheets of small, undifferentiated round cells on a fibrous framework arranged in “alveolar-like” structures due to lack of cohesiveness in the center of neoplastic nests. These are usually found in the hip, maxilla, omentum, and uterus. 1,3
• Pleomorphic: Extremely rare and only reserved for RMS that do not display any features of embryonal or alveolar RMS. They are characterized by haphazardly arranged plump spindle cells with marked anisocytosis and anisokaryosis and bizarre mitotic figures. This type is more common in adults.1,3

The use of immunohistochemistry (IHC) to identify vimentin, desmin, α-actins, myoglobin, myogenin, and MyoD1 in conjunction with TEM to identify of sarcomeric structures, Z-bands, and large mitochondria, can also be useful in the diagnosis RMS.1,2,3


1. Caserto BG: A comparative review of canine and human rhabdomyosarcoma with emphasis on classification and pathogenesis. Vet Pathol. 2013; 50(5):806-826.

2. Cooper BJ, Valentine BA: Tumors of muscle. In: Meuten DJ, ed. Tumors in Domestic Animals. Fourth Edition ed. Ames, Iowa: Iowa State Press; 2002:319-364.

3. Cooper BJ, Valentine BA. Muscle and tendon. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, ed. Maxie MG, 6th ed., Vol. 1. Saunders Elsevier, Philadelphia, PA, 2016:241-243.

4. Da Roza MR, De Amorim RF, Carneiro FP, Benatto N, Barriviera M, Miguel MC: Aggressive spindle cell rhabdomyosarcoma in an 11- month-old boxer dog. J Vet Med Sci. 2010; 72(10):1363-1366.

5. Hays DM, Donaldson SS, Shimada H, Crist WM, Newton WA, Jr., Andrassy RJ, et al.: Primary and metastatic rhabdomyosarcoma in the breast: neoplasms of adolescent females, a report from the Intergroup Rhabdomyosarcoma Study. Med Pediatr Oncol. 1997; 29(3):181-189.

6. Illanes OG: Juvenile parameningeal rhabdomyosarcoma in a dog causing unilateral denervation atrophy of masticatory muscles. J Comp Pathol. 2002; 126(4):303-307.

7. Newton WA, Gehan EA, Webber BL, Marsden HB, van Unnik AJM, Hamoudi AB, et al.: Classification of rhabdomyosarcomas and related sarcomas. Pathologic aspects and proposal for a new classification-an intergroup rhabdomyosarcoma study. Cancer. 1995; 76(6):1073-1085.

8. Sarnelli R, Grassi F, Romagnoli S: Alveolar rhabdomyosarcoma of the greater omentum in a dog. Vet Pathol. 1994; 31(4):473-475.

9. Snyder LA, Michael H: Alveolar rhabdomyosarcoma in a juvenile labrador retriever: case report and literature review. J Am Anim Hosp Assoc. 2011; 47(6):443-446.

Click the slide to view.

2-1. Presentation, dog.

2-2. Lung, dog.

2-3. Lung, dog.

2-4. Lung, dog.

2-5. Lung, dog.

2-6. Lung, dog.

2-7. Lung, dog.

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