11 Neoplasms of muscle

Primary neoplasms of the striated muscle are rare in veterinary species. Although usually found in muscle, they can arise in unexpected locations devoid of striated muscle, such as the bladder.

11.1 Rhabdomyoma

As it’s name suggests, a rhabdomyoma is a benign tumour of striated muscle. It is seen most frequently in the hearts of pigs, particularly the red wattle breed. They are typically incidental findings. They present grossly as circumscribed, smooth-surfaced, nodular masses embedded in the myocardium. A similar neoplasm occasionally arises on the larynx of dogs. Laryngeal rhadomyomas may cause respiratory difficulty or altered bark. They are typically minimally invasive and tend not to metastasize.

11.2 Rhabdomyosarcoma

These are the malignant counterparts to rhadomyomas. They are most common in the dog. Counterintuitively, they occur more frequently at sites that normally lack skeletal muscle versus those that don’t. There are a variety of subtypes, however, whether there is any prognostic significance to differentiating them is uncertain. They all tend to be locally invasive and metastatic. Prognosis is poor for all subtypes. Grossly, the tumours appear as pale, white to tan, firm masses, often with areas of necrosis. The botryoid (“cluster of grapes”) subtype occurs in the bladder as a polypoid mass. The histologic appearance of these tumours is quite variable, though occasionally elongated, variably striated, myotube-like cells known as “strap cells” are present, which is suggestive of rhadomyosarcoma. Immunohistochemistry is usually required to definitively diagnose these tumours.

11.3 Non-muscle primary tumours of muscle

Granular cell tumours occur in the tongue of dogs and cats. They are composed of densely packed round cells with PAS positive granules. The supporting mesenchyme of muscle can occasionally produce a (usually) malignant neoplasm. Hemangiosarcomas can arise in the muscles of dogs and horses, and aspiration of these large, intramuscular masses typically only reveals hemorrhage. Like their splenic cousins, they metastasize frequently.

11.4 Secondary tumours

Muscles are occasionally infiltrated by local neoplasms. Infiltrative lipomas are characterized by relatively well differentiated adipocytes crawling and invading through myofibers. They are highly invasive and require excision. Other neoplasms, such as subcutaneous mast cell tumours, lymphoma, hemangiosarcomas, and soft tissue sarcomas can invade into muscle. Metastasis to muscle is uncommon but does occur.