Thymomas in Canines

Introduction

The thymus is derived from the third pharyngeal pounch and starts to involute at 4–5 months of age. Histologically, the thymus consists of small lymphocytes and spherical corpuscles called Hassall bodies. Its function revolves around cell-mediated immunity as well as maturation and selection of T cells. Thymomas account for 33–64% of thymic masses in dogs. While we can also see thymomas in cats, this is more rare. We typically see thymomas in our older dog population with a prevalence in medium to large breeds. Dogs typically present with a range clinical signs, including weakness, anorexia, dyspnea, cough, weight loss, vomiting/regurgitation, polyuria/polydipsia, and/or cranial vena cava syndrome. Dogs can also present with paraneoplastic syndromes, including myasthenia gravis (17–46% of dogs with thymomas) and hypercalcemia.

Diagnosis

Mediastinal masses in general are typically first identified with thoracic radiographs. Other differentials for mediastinal masses include lymphoma, thymic carcinoma, cyst, hemangiosarcoma, chemodectoma, ectopic thyroid tumors, lipoma, and others. For further identification and establishment of treatment options, a thoracic CT is needed. It is important to note that nonthymic tumors have been reported in 27% of dogs with thymomas, therefore, abdominal ultrasound or CT may also be warranted. There have been some studies looking at specific CT characteristics to determine whether we can differentiate cranial mediastinal lymphoma from thymic epithelial neoplasia. In one study, diagnosis of thymic epithelial neoplasia was associated with heterogeneous attenuation in pre- and post-contrast images. Conversely, envelopment of the cranial vena cava by the mass was less likely with thymic epithelial neoplasia than lymphoma. However, in addition to a CT scan (and often under the same anesthetic event), a fine needle aspirate of the mass can be performed and submitted for cytology for a more definitive diagnosis. Both lymphoma and thymomas may be composed primarily of small lymphocytes, therefore, differentiation can be challenging. In three studies, a presumptive diagnosis of thymoma was achieved in approximately 20%, 40%, and 77% of mediastinal masses after FNA and cytology. In cases where diagnosis is challenging, flow cytometry can be pursued, however, an adequate cellular sample must be obtained.

Treatment

Stabilization of the patient, if indicated, is first and foremost. If the tumor is amenable to surgery, this is the gold standard. To remove thymomas, typically a median sternotomy is performed. Most patients do very well following surgery and, barring any complications, are able to be discharged after 1–2 days of hospitalization. Reported complications include hemorrhage, aspiration pneumonia, infection, cardiac arrect, and seroma. Depending upon the size and invasiveness of the tumor, video-assisted extirpation as well as thoracoscopic removal of mediastinal masses have been reported with good success rates (8–11% conversion rates). Surigcal excision of thymomas has been associated with a long survival time (635–1825 days MST). One- and three-year survival rates were 73% and 49% in one study. The overall prognosis for a patient diagnosed with myasthenia gravis is more guarded, and while surgery may reverse this condition, it is not a guarantee. In one recent paper, 6/9 dogs with myasthenia survived the postoperative period. Of these dogs, 3/6 had complete resolution and 2/6 dogs had a partial resoluction. Other negative prognostic factors include diagnosis of a second, non-thymic tumor, megaesophagus, preoperative aspiration pneumonia, vascular invasion, larger sized mass (also increased risk for recurrence), metastasis, incomplete resection, and presence of postoperative complications.

If surgery is not an option due to tumor invasiveness or owner preference, radiation is another effective treatment modality. In one study of eight dogs treated with hypofractionated RT (total administered dose was 48–49 Gy with one fraction per week for a total of six to seven fractions), MST was not reached (8–1128 days) and the one-year survival rate was 75%. Possible reported complications include tracheitis, esophagitis, pneumonitis, lung fibrosis, pericarditis, heart failure, and cardiac arrhythmias.

Chemotherapy is not well evaluated in veterinary patients with thymomas, and there is limited evidence regarding benefit. Stable disease after treatment of thymomas with high-dose prednisone has been reported in both human and veterinary medicine. However, this would likely only be offered if surgery and radiation therapy were not feasible or declined.

References

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