Although these tumors originate from skeletal muscle, and are commonly reported in large-breed dogs, they can also derive from organs that lack striated muscle, such as the urinary bladder, uterus, cervix and vagina of small breeds.
Multisystemic disease has been reported, with multiple sites affected concurrently. Metastases are also common, with secondary tumors found in localized tissues and lymph nodes as well as distant spread to the spleen, lungs, liver, kidneys and the adrenal glands.
Classification of rhabdomyosarcomas is usually based on location:
- Embryonal rhabdomyosarcoma - most common type, affecting the the orbital and maxillofacial regions, pharynx, trachea and axial skeleton
- Botryoid rhabdomyosarcoma - ureter and urinary bladder - commonly results in chronic renal disease and hydronephrosis - predisposition in young female St. Bernards
- Alveolar rhabdomyosarcoma - mandible, facial, abdominal
- Pleomorphic rhabdomyosarcoma - visceral organs, including the ovaries and prostate
Clinically affected dog present with regionalized tissue swelling and lymphadenopathy associated with localized muscle infiltration. Primary cardiac rhabdomyosarcomas usually present as right-sided congestive heart failure.
Genitourinary (botryoid) rhabdomyosarcomas frequently present as multiple masses within the bladder and reproductive organs. Dogs present with variable degrees of depression, ascites, dysuria and abdominal pain.
Diagnosis is based primarily on histological analysis of biopsied samples.
These tumor consist of neoplastic proliferation of large round cells with abundant eosinophilic cytoplasm and hyperchromatic nuclei and small round cells with less cytoplasm. These cells are usually arranged in a compact sheet.
Immunohistochemically, these neoplastic cells express positively for myosin, nyogenin, desmin and vimentin.
A differential diagnosis would include masticatory muscle myositis, squamous cell carcinoma, lymphoma, leiomyosarcoma, hemangiosarcoma, chemodectoma, neurofibrosarcoma, dysgerminoma and malignant mixed Müllerian tumor.
In metastatic cases, prognosis is usually guarded even with surgical extirpation and chemotherapy. Maxillofacial rhabdomyosarcoma are usually locally very invasive and difficult to treat. Surgical debulking and chemotherapy/radiation therapy may be an option.
Survival times of 1 - 2 years post-diagnosis is common.
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