Thymomas are tumors that arise from the thymus, an organ that is part of the immune system and is found in the cranial mediastinum (a portion of the chest cavity). Thymomas are relatively rare tumors but have been reported to occur in several rabbits. The overall incidence in pet rabbits is approximately 8%; such low occurrence of thymomas in rabbits may be related to the short life span of rabbits, as thymomas tend to arise in older animals (of other species) as well as adult humans. The most common differential diagnoses that should be considered in rabbits with a cranial mediastinal mass are thymic lymphoma and lung or mediastinal abscess. Thymomas tend to be slow growing but have the potential for local invasion into the lung and surrounding tissue and even metastasis to other organs.Clinical signs of thymoma in rabbits can be quite variable and usually don’t occur until the mass has reached an appreciable size. When present, the clinical signs are usually caused by the presence of a space-occupying mass in the thoracic (chest) cavity. Signs associated with a space-occupying mass include shortness of breath, exercise intolerance, difficulty breathing, and open-mouth breathing. The eyes may appear to bulge out of the skull, and neck, head and forelimb edema (fluid build-up) may also be seen.
Thoracic radiographs are useful in the initial identification of a cranial mediastinal mass but will not provide detailed information on the full extent of the disease, or the consistency of the mass. To get this information, advanced imaging techniques, such as computed tomography (CT scan) or magnetic resonance imaging (MRI) will need to be performed. Ultrasonography can be used to verify the presence of a mass, to characterize the consistency (solid and/or cystic), and is useful in performing guided aspirates.
Making the definitive diagnosis of thymoma requires getting a sample of the mass, using either aspiration or biopsy. An aspiration can be performed using a long, thin needle that is inserted into the mass while being guided by ultrasound, and then some cells are removed from the mass by suction. Some tumors do not release cells very well, so this may be non-diagnostic. However, this is less invasive than a biopsy, in which a small piece of the mass is removed either with surgery or using a special core needle biopsy. For both procedures, the rabbit must be anesthetized to avoid movement during the sample collection.The treatments most commonly used in rabbits with thymoma are surgical removal and/or radiation therapy. Because these treatment options can be expensive and carry considerable risk to the patient, it should be noted that survival time without treatment in a rabbit has been documented, but in only one rabbit. That rabbit lived for 4 months without treatment after identification of a thymoma before developing severe respiratory distress, and was euthanized at that time. Surgery is the treatment of choice in all species when there is a solitary tumor as it has the best chance of removing the entire tumor and therefore effecting a cure. The use of pre-operative imaging (CT scan) may allow appropriate selection of surgical candidates with non-invasive tumors. Radiation therapy has been used in the treatment of human, feline, canine, and rabbit thymomas. Thymomas are radiosensitive tumors but the proximity to critical normal tissues (heart, lung) may limit the radiation dose that can be delivered based on the extent and location of the tumor. Radiation pneumonitis (inflammation of the lung tissue) and fibrosis (scarring of the tissue) may develop with radiation treatment. Radiation therapy is recommended after incomplete surgical resection of thymomas or if the patient is not considered a good surgical candidate.
Both treatment options carry considerable risk to the rabbit and the decision of which treatment to use should be discussed thoroughly with your veterinarian or a veterinary oncologist in your area. There are pros and cons to each treatment option. Both are expensive, so cost may become an issue. Radiation therapy is less invasive but involves multiple (3-12) anesthetic episodes that could cause problems, especially if there is underlying heart or lung disease. Surgery is very invasive because the chest cavity has to be opened, but has a better chance of curing the disease. There are other factors as well that should be discussed with your veterinarian. Based on the information gathered from my own practice for this article surgery has approximately a 50% survival rate from the surgery itself, with a good chance of clinical cure of the disease. Radiation therapy has an 80% chance of the rabbit surviving the procedure, but a chance of recurrence and post-treatment complications that is greater than surgical removal of the tumor.
by James K. Morrisey, DVM, DABVP (Avian Practice)
Cornell University, College of Veterinary Medicine
*Reviewed and approved by HRS Health Committee 10/06