Veterinary Society of
Surgical Oncology

General Considerations

  • thymoma is classified as invasive or non-invasive
  • thymoma is non-invasive in 50%-100% cats
  • non-invasive thymomas are well-encapsulated
  • invasive thymoma will invade adjacent structures such as cranial vena cava, thoracic wall, and pericardium
  • thymomas can also be cystic
  • thymoma arises from thymic epithelium and has variable mature lymphocyte involvement which can predominate, but the epithelium is the malignant component
  • lymphoid component exfoliates more readily than epithelial component and hence it can be difficult to differentiate thymoma from cranial mediastinal LSA
  • DDx: thymic LSA, thymic carcinoma, thymic branchial cyst, ectopic thyroid and parathyroid neoplasia, aortic body tumor, metastatic carcinoma, and rib and sternal sarcomas extending into mediastinal space

Clinical Features

  • thymoma is very rare in cats
  • male predisposition with a male-to-female ratio of 2.5:1
  • median age: 10.8 years
  • thymoma occurs in older cats compared to thymic LSA
  • histopathologic features: mixture of thymic epithelial cells and small lymphocytes in variable proportions
  • mast cells detected in 50% of feline thymoma
  • SCC has been reported to arise from feline thymoma
  • invasive thymoma may be more common in cats (55% in one report)
  • metastasis has not been reported

Clinical Signs

Non-Invasive Thymoma

  • asymptomatic or non-specific signs associated with large space-occupying thoracic mass
  • exercise intolerance, coughing, dyspnea, dysphagia, and weight loss
  • coughing and dyspnea due to pleural effusion or compression of trachea or segmental bronchi
  • dysphagia and drooling secondary to esophageal compression or megaesophagus
  • laryngeal paralysis with peripheral nerve entrapment
  • paraneoplastic syndromes associated with thymoma: myasthenia gravis, hypogammaglobulinemia, hypercalcemia, and aplastic anemia

Invasive Thymoma

  • clinical signs and paraneoplastic syndromes are the same as non-invasive thymoma
  • cranial vena cava syndrome: edema of submandibular area, neck, thoracic inlet, and thoracic limbs, and association with pleural effusion (particularly chylothorax)
  • pneumothorax and hemothorax have also been reported with invasive thymoma

Paraneoplastic Syndromes

  • paraneoplastic syndromes associated with thymoma include:
  • myasthenia gravis
  • hypogammaglobulinemia
  • hypercalcemia
  • aplastic anemia
  • myasthenia gravis is present in the Okas cat and 40% of dogs with thymoma
  • myasthenia gravis may be either focal or generalized with megaesophagus and generalized weakness
  • thymic monocytes may become immunogenic resulting in formation of antibodies directed against acetylcholine receptors and resulting in development of myasthenia gravis
  • thymoma is also associated with other immunogenic diseases with 20%-40% of dogs presenting with autoimmune disease such as immune-mediated anemia, polymyositis, and exfoliative dermatitis (cats)
  • cardiac myositis causes 3rd degree atrioventricular block

Non-Thymic Neoplasia

  • high incidence of 2nd non-thymic malignancy associated with thymoma due to possible association with deficient immunologic surveillance
  • 2nd tumors include both sarcomas and carcinomas


  • clinical signs
  • physical examination: caval syndrome and auscultation changes associated with pleural effusion
  • hematology and serum biochemistry are usually unremarkable
  • lymphocytosis (> 20,000 cells/µL) and pseudohyperparathyroidism are occasionally observed
  • thoracic radiographic findings include:
  • space occupying mass with dorsal elevation of trachea and esophagus
  • caudal displacement of cardiac silhouette
  • megaesophagus and aspiration pneumonia with paraneoplastic myasthenia gravis
  • minimal pleural effusion with non-invasive thymoma
  • pleural effusion which may obscure mass with invasive thymoma
  • pulmonary metastasis
  • ultrasonography: mixed echogenicity with cavitation compared to homogenous hypoechogenicity with LSA
  • advanced imaging (i.e., CT or MRI)
  • FNA or needle-core biopsy: predominance of lymphocytes rather than epithelial cells may confuse diagnosis
  • immunohistochemistry may be required for definitive diagnosis (cytokeratin)
  • other tests include FeLV and FIV in cats and acetylcholine receptor antibody titres
  • thymoma can be differentiated from cranial mediastinal lymphoma by:



  • exploratory thoracotomy required to differentiate non-invasive and invasive thymoma
  • median sternotomy usually required due to size of tumor, but lateral intercostal thoracotomy can be used for smaller lesions or in cats (although adjacent rib resection sometimes required)
  • non-invasive thymomas do not adhere to intrathoracic structures and removed using blunt-sharp dissection
  • cranial vena cava and phrenic nerves are located along the craniodorsal aspect of cranial mediastinal mass
  • invasive thymomas usually invade vital structures and are difficult surgical candidates
  • venous grafts are used in humans, and has been reported in the dog, for thymomas invading the cranial vena cava

Radiation Therapy

  • thymomas are radiation-sensitive tumors in cats, dogs, and humans
  • 75% response rate in cats and dogs with thymomas, including 20% CR
  • lymphoid component of thymoma may determine completeness of response
  • adverse effects: pneumonitis and pericarditis


  • chemotherapy is usually ineffective, but can be attempted in combination with corticosteroids for invasive thymoma
  • partial and complete responses are uncommon
  • corticosteroids may provide either prolonged stable disease or even partial or complete response
  • response to corticosteroids is due to cytotoxic effects on T lymphocytes which can represent a large non-neoplastic component of thymoma
  • cisplatin, ifosfamide, corticosteroids, doxorubicin, maytansine, cyclophosphamide, vincristine, and procarbazine are used in single or multiple agent protocols in humans with invasive and metastatic thymomas

Other Treatment

  • immunosuppressive therapy or anticholinesterase treatment for myasthenia gravis
  • motility drugs, H 2 antagonists, and antibiotics for prophylactic management of megaesophagus


  • prognosis is excellent
  • no local recurrence or metastasis in 10 cats surviving perioperative period
  • myasthenia gravis has been reported in 2 cats postoperatively
  • MST for cats with surgery: 21 months (range, 6-36 months)
  • MST for cats with radiation therapy: 720 days






FeLV Status



Lymphocytes, epithelial cells, and mast cells






Minimal response

Early and rapid response

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