Thymoma

From Dog
Lateral radiograph of the pleural fluid line (arrow heads) caused by a thymoma (long arrow)[1]
Lymphoepithelial thymoma visualized on postmortem in a Samoyed dog[2]

Thymoma, thymic carcinoma, thymofibrolipoma[3] and thymic hyperplasia are rare neoplasm of the thymus gland of dogs.

These tumors are rare and derive from epithelial components of the thymus, and accordingly classified as either lymphoid and lymphoepithelial thymomas. Dogs usually develop the former type, but lymphoepithelial tumors have been reported[4] .

These tumors are commonly observed in older dogs, easily confused with lymphoma and commonly associated with a number of paraneoplastic syndromes.

Metastatic thymomas occur rarely[5] and ectopic thymic carcinomas has been reported in the cervical region of dogs[6].

Clinical presentation usually involves the presence of a thoracic mass visible on radiographs, and associated lethargy, anorexia, weight loss, intermittent coughing and regurgitation, variable lymphadenopathy, displacement of heart sounds caudally and dyspnea. Previous history of regurgitation, megaesophagus or polymyositis is not uncommon[7].

Thymic carcinoma have been associated with development of vena cava syndrome[8], polymyositis, acquired myasthenia gravis[9], pheochromocytoma, mammary adenocarcinoma or pulmonary adenocarcinoma[10].

Diagnosis is based on presenting clinical signs, radiographic evidence of a well-circumscribed cranial mediastinal mass. Biopsied tissue samples acquired by ultrasound-guided needle biopsy allow for histopathological confirmation of a thymic tumor. Immunohistochemistry of the tissue is usually definitive.

Blood testing is often unrewarding, but a mature T-cell paraneoplastic lymphocytosis has been reported in dogs[11].

A differential diagnoses for a cranial mediastinal mass would include lymphoma (which can occur concurrently)[12], idiopathic mediastinal cysts, ectopic thyroid tumor, hemangiosarcoma[13], parathyroid tumors, lipoma, thymic branchial cyst, granuloma and pulmonary abscesses[14]

Treatment invariably requires thoracotomy and thymectomy[15]. Survival times for the dogs is approximately 2 - 3 years following thymectomy, and recurrence is rare[16].

The prognosis for non-resectable thymoma in a dog with myasthenia gravis and megaesophagus is poor. However, complete thymic resection can result in resolution of megaesophagus[17].

Dogs with thymoma-associated myasthenia gravis may also produce autoantibodies to several neuromuscular antigens, including ryanodine (a skeletal muscle calcium-release channel receptor) and the muscle protein titin[18].

References

  1. Hylands R (2006) Veterinary diagnostic imaging.Thymoma. Can Vet J 47(6):593-596
  2. Akiyama T et al (2009) Lymphoepithelial thymoma characterized by proliferation of spindle cells in a Samoyed dog. J Vet Med Sci 71(9):1265-1267
  3. Morini M et al (2009) Thymofibrolipoma in two dogs. J Comp Pathol 141(1):74-77
  4. Akiyama T et al (2009) Lymphoepithelial thymoma characterized by proliferation of spindle cells in a Samoyed dog. J Vet Med Sci 71(9):1265-1267
  5. Moffet AC (2007) Metastatic thymoma and acquired generalized myasthenia gravis in a beagle. Can Vet J 48(1):91-93
  6. Faisca P et al (2011) Ectopic cervical thymic carcinoma in a dog. J Small Anim Pract 52(5):266-270
  7. Atwater S et al (1994) Thymoma in dogs: 23 cases (1980-1991). JAVMA 205(7):1007-1013
  8. Holsworth IG et al (2004) Use of a jugular vein autograft for reconstruction of the cranial vena cava in a dog with invasive thymoma and cranial vena cava syndrome. J Am Vet Med Assoc 225(8):1205-1210
  9. Stenner VJ et al (2003) Acquired myasthenia gravis associated with a non-invasive thymic carcinoma in a dog. Aust Vet J 81(9):543-546
  10. Aronsohn MG et al (1984) Clinical and pathologic features of thymoma in 15 dogs. J Am Vet Med Assoc 184:1355–1362
  11. Batlivala TP et al (2010) Paraneoplastic T cell lymphocytosis associated with a thymoma in a dog. J Small Anim Pract. 2010 Sep;51(9):491-4
  12. Fukuoka A et al (2004) Thymoma in a dog with a part of granular cell proliferation and concurrent lymphoma cells. J Vet Med Sci 66(6):713-715
  13. Yoon J et al (2004) Computed tomographic evaluation of canine and feline mediastinal masses in 14 patients. Vet Radiol Ultrasound 45:542–546
  14. Day MJ (1997) Review of thymic pathology in 30 cats and 36 dogs. J Small Anim Pract 38:393–403
  15. Mayhew PD & Friedberg JS (2008) Video-assisted thoracoscopic resection of noninvasive thymomas using one-lung ventilation in two dogs. Vet Surg 37(8):756-762
  16. Zitz JC et al (2008) Results of excision of thymoma in cats and dogs: 20 cases (1984-2005). J Am Vet Med Assoc 232(8):1186-1192
  17. Wood SL et al (2001) Myasthenia gravis and thymoma in a dog. Vet Rec 148:573–574
  18. Shelton GD (2002) Myasthenia gravis and disorders of neuromuscular transmission. Vet Clin North Am Small Anim Pract 32(1):189-206