+ Etiology

  • Extraskeletal OSA has a causal association with:
  • Trauma in feline ocular OSA and subcutaneous or intermuscular VAS
  • Retained gauze swabs with jejunal and intermuscular OSA
  • Aflotoxicosis with liver OSA
  • Spirocerca lupi with esophageal OSA
  • Heterotopic ossification (i.e., myositis ossificans)
  • Metaplasia of intraocular fibroblasts or sarcomatous changes of heterotopic bone has been implicated following trauma-associated intraocular FSA, CSA, and OSA
  • 25% of human extraskeletal OSA is caused by either trauma or radiation therapy

+ Sites

  • 80% of extraskeletal OSA involve visceral organs with reported sites including eyes, spleen, liver, heart, adrenal glands, thyroid glands, kidneys, small intestine, larynx, trachea, lungs, retroperitoneal space, greater omentum, muscle and subcutaneous tissue, skin, anal sac, mammary glands, testes, and vagina
  • 80% feline extraskeletal OSA occur in subcutaneous sites and are most likely VAS

+ Metastasis

  • 57% metastatic rate at diagnosis and 85% metastatic rate at necropsy in dogs with extraskeletal OSA
  • 64% metastatic rate to regional lymph nodes
  • However, regional lymph node metastasis is less in mammary OSA than mammary carcinomas (24% v 52%-68%)


+ Signalment

  • Extraskeletal OSA is rare, but has been reported to account for up to 38% of feline OSA and 12.6% of canine OSA
  • Mammary OSA accounts for 1% of all canine mammary tumors
  • Familial breed predispositions: Rottweiler and Saint Bernard
  • Breed predispositions for soft tissue extraskeletal OSA: Beagle and Rottweiler
  • Breed predispositions for mammary extraskeletal OSA: GSD and Miniature Poodle
  • Sex predisposition: female for mammary OSA and none for other types of extraskeletal OSA
  • 75% of dogs with mammary OSA are intact female dogs and 25% are spayed
  • Mean age 11 years
  • 43% dogs < 15 kg

+ Diagnosis

  • Clinical signs are dependent on extraskeletal OSA location
  • Distribution of mammary gland involvement with extraskeletal OSA is 9% in the 1st mammary gland, 12% in the 2nd, 18% in the 3rd, 35% in the 4th, and 26% in the 5th mammary gland
  • 16% (9/57) dogs with mammary OSA have a history of previous mammary tumors and 35% (20/57) diagnosed concurrently with other mammary tumors (i.e., adenoma, ADC, and cystadenoma)
  • Radiographic evidence of calcification present in 31% dogs with extraskeletal OSA
  • Histologic diagnosis of extraskeletal OSA is based on:
  • Uniform morphologic pattern of sarcomatous tissue
  • Production of malignant osteoid or bone by sarcomatous tissue
  • High mitotic index
  • Exclusion of a primary OSA
  • 86% of extraskeletal OSA are poorly differentiated in dogs
  • DDx: myositis ossificans or dystrophic calcification associated with tumors or inflammation

+ treatment

Surgery and chemotherapy (i.e., doxorubicin or cisplatin) extent of surgery did not influence prognosis for dogs with mammary OSA

+ Prognosis

  • MST 74 days overall
  • Prognostic factors: duration of clinical signs, tumor site, tumor size, metastasis, and chemotherapy treatment
  • Clinical signs for > 15 days has a significantly better MST (137-183 days v 2-21 days)
  • Mammary OSA has a significantly better MST than other soft tissue sites (90 days v 26 days)
  • Caudal mammary OSA tended to have a longer MST than cranial mammary OSA (225 days v 90 days)
  • Intra-abdominal soft tissue OSA has a significantly decreased MST compared to skin and subcutaneous OSA (2-50 days v 240-486 days)
  • Tumor size > 15 cm diameter has a significantly decreased MST (0 days v 93 days for tumors < 5 cm and 240 days for tumors 5-15 cm)
  • Metastatic disease significantly decreased MST (15 days v 120 days)
  • Postoperative chemotherapy significantly increases MST (146 days v 33 days)
  • Surgery without chemotherapy increases the risk of tumor-related death by 3.6-fold



T0 No evidence of neoplasia
T1 Tumor confined within the medulla and cortex - Primary Tumor
T2 Tumor extends beyond the periosteum
M0 No evidence of lymph node involvement
M1 Evidence of distant metastasis with site specified - Metastasis