Review of thymoma
Most dogs with a cranial mediastinal mass will present with signs of dyspnea (usually rapid, shallow breathing), coughing
and exercise intolerance. Other signs may include regurgitation, vomiting or gagging secondary to esophageal compression or
paraneoplastic myasthenia gravis. Generalized myasthenia gravis may also occur with a primary complaint by the owner of recurrent
weakness or collapse. Precaval syndrome (swelling of the head, neck and/or thoracic limbs) is possible if the mediastinal
mass causes compression of or invades the cranial vena cava. On physical examination, muffled lung sounds are often noted.
Image 4, 5, and 6.
Cranial mediastinal masses are usually thymoma or lymphosarcoma. Other causes may include ectopic thyroid tissue, branchial
cyst, chemodectoma or thoracic wall tumor. Lymphadenopathy due to infectious or inflammatory causes can also be found in the
cranial mediastinum. Fluid within the cranial mediastinum (transudate, exudate, hemorrhage) can occasionally mimic a mediastinal
Hypercalcemia may occur in both thymoma and lymphoma. Non-specific azotemia secondary to pre-renal and renal causes may be
found. Animals with lymphoma and liver involvement may have variable increases in serum ALP, ALT and total bilirubin. Hyperphosphatemia
can be seen with renal failure, and hypophosphatemia is usually associated with hypercalcemia of malignancy.
Two or three thoracic view radiographs (ventrodorsal or dorsoventral and one or two lateral views) are the preferred way to
diagnose an intrathoracic mass versus pulmonary, airway or pleural diseases causing respiratory signs.
Tracheal elevation is a consistent sign of a mediastinal mass on the lateral image. Differentiating a pulmonary or thoracic
wall mass from a mediastinal mass may be done with the ventrodorsal view. The mediastinum should be twice the width of the
spine in the dog. Fat in obese dogs can widen the mediastinum in the absence of a true mass. Pulmonary masses will usually
be positioned lateral to the mediastinum, and thoracic wall masses will be peripheral and often cause rib lysis or spreading.
Animals with thymoma and paraneoplastic myasthenia gravis can have signs of megaesophagus and possibly aspiration pneumonia.
Pleural effusion can also be present in mediastinal masses, usually containing either neoplastic cells or chyle.
Ultrasound of the cranial mediastinum can be useful in differentiating a cranial mediastinal mass from pleural fluid and may
be helpful in determining an aspiration or biopsy site.
Ultrasound of the abdomen is indicated in staging of lymphoma, to determine intra-abdominal organ involvement. Because thymomas
are rarely metastatic, abdominal ultrasound is not routinely performed in these dogs, except when attempting to differentiate
thymoma from lymphoma or in the case of potential intra-abdominal organ dysfunction, based on hematologic or serum chemistry
Image 7, 8, and 9.
Aspiration cytology can differentiate thymoma from lymphoma in many instances. Thymomas contain mature lymphocytes, neoplastic
epithelial cells and often mast cells. Lymphomas are usually lymphoblastic with large, immature lymphocytes. A lymphocytic
lymphoma or a cystic thymoma may cause cytology to be misleading, and tissue samples may be required. Cytology of pleural
effusion can also be diagnostic for lymphoma if there are exfoliated lymphoblasts present.
Thymomas are invasive or non-invasive. Staging should include thoracic radiography to rule out obvious metastatic disease
and further testing based on clinical signs, physical examination or hematologic and serologic findings.
Imaging of the thoracic cavity may not indicate the invasiveness of disease, and the presence of effusions should not rule
out exploratory surgery. Exploratory surgery is still the best staging procedure to determine resectability.
Surgery is the preferred treatment for thymomas, but some will not be surgically resectable. About 70 percent of these masses
are resectable. In the face of unresectable disease, radiation therapy may be useful, either as solitary therapy or as an
adjunct to incomplete removal. About 75 percent of thymomas appear to have a response to radiation, although complete remissions
are rarely achieved. Partial remission and relief of clinical signs for variable lengths of time is common. Megavoltage radiation
appears to be well-tolerated in most dogs, with hair loss and dermatitis being the most common complications.
Pneumonitis can be a potentially life-threatening complication. The optimal course of radiation has not been determined. Chemotherapy
may also play a role in the treatment of thymomas.
Prednisone alone has caused remission in some animals. Prednisone is also useful in the management of myasthenia gravis along
with anticholinesterase drugs.
Other chemotherapeutic drugs have not been reported as useful in thymomas. However, most treatment courses have been under
the presumption of lymphoma. Lymphoma protocols may have some utility in lymphocyte-rich tumors. Cisplatin is the drug used