Felis ISSN 2398-2950


Contributor(s): Philip K Nicholls, Marion O'Leary, Isabelle Desmas-Bazelle


  • Thymoma is an uncommon (17/3284 feline neoplasms in one study) tumor. It is the second most common mediastinal tumor in cats.
  • Disease due to space occupying effects (lung compression) and paraneoplastic syndromes including myasthenia gravis, polymyositis, myocarditis, dermatitis and megaesophagus.
  • Diagnosis: thoracic imaging (radiography, ultrasonography, computed tomography (CT), magnetic resonance imaging (MRI), direct sampling (cytology, histopathology), and testing for related effect, eg serum levels of anti-acetylcholine-receptor antibody.
  • Treatment: surgery, may be combined with radiotherapy.
  • Prognosis: fair to guarded.



  • Neoplastic transformation of thymic epithelium.
  • No clear risk factors or etiological agents.
  • Distinct from thymic lymphosarcoma Mediastinal lymphoma Thymus: neoplasia, which is more common, and mostly in younger cats.


  • Neoplastic transformation causes uncontrolled cellular proliferation and a mass lesion.
  • Different histologic types have been described: epithelial, lymphocyte-rich, clear cell.
  • Direct effects of space-occupying lesion include compression of lungs, caudal displacement of heart, and compression / displacement of trachea, leading to clinical signs such as dyspnea and coughing.
  • Ectopic thymoma has been reported in the cervical region.
  • Thoracic effusion may compound dyspnea.
  • Immune disorders sometimes associated with thymoma, with anti acetylcholine receptor antibodies causing neuromuscular junction blockade and flaccid paresis (myasthenia gravis Myasthenia gravis - regurgitation, head and neck ventroflexion). Thymoma accounts for more than 25% of all reported feline myasthenia gravis cases.
  • Malassezia dermatitis Malassezia dermatitis is associated with internal neoplasia including thymoma in cats. Non-inflammatory alopecia and paraneoplastic pemphigus have also been described. paraneoplastic syndromes Cutaneous paraneoplastic syndromes may occur at presentation, later in the course of the disease, and even after tumor removal.
  • Metastases are rare but rate as high as 20% have been reported.


  • Signs reported for 2 weeks - 3 years prior to presentation.


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Further Reading


Refereed papers

  • Recent references from PubMed and VetMedResource.
  • Fournier Q, Bavcar S, Philbey A W, Smith S, Varjonen K (2019) A previously undescribed cutaneous paraneoplastic syndrome in a cat with thymoma. Vet Dermatol 30(4), 342-e98 PubMed.
  • Rick T, Kleiter M, Schwendenwein I, Ludewig E, Reifinger M et al (2019) Contrast-enhanced ultrasonography characteristics of intrathoracic mass lesions in 36 dogs and 24 cats. Vet Radiol Ultrasound 60, 56-64 PubMed.
  • Rohrer Bley C, Meier V, Schneider U (2018) Dosimetric benefit of adaptive radiotherapy in the neoadjuvant management of canine and feline thymoma-An exploratory case series. Vet Comp Oncol 16(3), 324-329 PubMed.
  • Spugnini E P, Menicagli F, Pettorali M, Baldi A (2017) Ultrasound guided electrochemotherapy for the treatment of a clear cell thymoma in a cat. Open Veterinary Journal 7(1), 57-60 PubMed.
  • Griffin M A, Sutton J S, Hunt G B, Pypendop B H, Mayhew P D (2016) Video-Assisted Thoracoscopic Resection of a Noninvasive Thymoma in a Cat with Myasthenia Gravis Using Low-Pressure Carbon Dioxide Insufflation. Vet Surg 45(S1),O28-O33 PubMed.
  • Nagata N, Miyoshi T, Otake Y, Suzuki H, Kagawa Y et al (2016) Temporal deterioration of neurological symptoms and increase of serum acetylcholine receptor antibody levels after thymectomy: a case report of a cat with myasthenia gravis. J Vet Med Sci 78(12), 1893-1896 PubMed.
  • Patterson M M, Marolf A J (2014) Sonographic characteristics of thymoma compared with mediastinal lymphoma. J Am Anim Hosp Assoc 50(6), 409-413 PubMed.
  • Hill P B, Brain P, Collins D, Fearnside S, Olivry T (2013) Putative paraneoplastic pemphigus and myasthenia gravis in a cat with a lymphocytic thymoma. Vet Dermatol 24(6), 646-649 PubMed.
  • Lara-Garcia A, Wellman M, Burkhard M J, Machado-Parrula C, Valli V E et al (2008) Cervical thymoma originating in ectopic thymic tissue in a cat. Vet Clin Pathol 37(4), 397-402 PubMed.
  • Zitz J C, Birchard S J, Couto G C, Samii V F, Weisbrode S E 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 PubMed.
  • Patnaik A K, Lieberman P H, Erlandson R A et al (2003) Feline cystic thymoma: a clinicopathologic, immunohistologic, and electron microscopic study of 14 cases. J Feline Med Surg (1), 27-35 PubMed.
  • Mauldin E A, Morris D O & Goldschmidt M H (2002) Retrospective study: the presence of Malassezia in feline skin biopsies. A clinicopathological study. Vet Dermatol 13 (1), 7-13 PubMed.
  • Kaser-Hotz B, Rohrer C R, Fidel J L et al (2001) Radiotherapy in three suspect cases of feline thymoma. J Am Anim Hosp Assoc 37 (5), 483-488 PubMed.
  • Smith A N, Wright J C, Brawner W R Jr. et al (2001) Radiation therapy in the treatment of canine and feline thymomas: a retrospective study (1985-1999). J Am Anim Hosp Assoc 37 (5), 489-496 PubMed.
  • Day M J (1997) Review of thymic pathology in 30 cats and 36 dogsJSAP 38 (9), 393-403 PubMed.
  • Forster-Van Hijfte M A, Curtis C F & White R N (1997) Resolution of exfoliative dermatitis and Malassezia pachydermatis overgrowth in a cat after surgical thymoma resection. J Sm Anim Pract 38 (10), 451-454 PubMed.
  • Galloway P E, Barr F J, Holt P E et al (1997) Cystic thymoma in a cat with cholesterol-rich fluid and an unusual ultrasonographic appearance. J Small Anim Pract 38 (5), 220-224 PubMed.
  • Malik R, Gabor L, Hunt G B et al (1997) Benign cranial mediastinal lesions in three cats. Aust Vet J 75 (3), 183-187 PubMed.
  • Gores B R, Berg J, Carpenter J L et al (1994) Surgical treatment of thymoma in cats: 12 cases (1987-1992). J Am Vet Med Assoc 204 (11), 1782-1785 PubMed.
  • Carpenter J L & Valentine B A (1992) Squamous cell carcinoma arising in two feline thymomasVet Pathol 29 (6), 541-543 PubMed.

Other sources of information

  • Withrow S J & MacEwen E G (2020) Small Animal Clinical Oncology. 6th edn. Saunders W B, Philadelphia pp 778-781.
  • Carpenter J L, Andrews K L and Holzworth  J (1987) Chapter 11. Tumors and tumor-like lesions. In: Holzworth J (Ed) Diseases of the cat  - Medicine and surgery. pp 439-441. WB Saunders.