History: A 6-year-old female spayed Lionhead rabbit presented to Tufts Foster Hospital for Small Animals with a one-day history of inappetance. She also had a recent history of urine scald. On physical examination the rabbit was noted to be overweight with increased subcutaneous and abdominal fat stores. The results of a complete blood count revealed a leukocytosis (26.2 x 103cells/µl; reference range, 5 – 12 x 103cells/µl) with a lymphocytosis (23.4 x 103cells/µl; reference range, 1.25 - 6 x 103cells/µl). Radiographs were obtained (Figures 2 and 3 below).
Based on results of blood work and radiographs, what are your differentials? What do you recommend as the next step?
Radiographic findings and interpretation: There is a very large soft tissue mass in the cranial 2/3 of the thoracic cavity. The mass is primarily on midline but extends more into the right hemithorax compared to the left hemithorax. The mass is smooth and resulting in almost complete effacement of the cardiac silhouette. The most caudal aspect of the heart is seen on the VD projection and is displaced toward the left. The trachea is dorsally displaced by the mass. The visible pulmonary structures are within normal limits. The stomach contains a small amount of gas and fluid. The small intestines and cecum contain a combination of gas and fluid. The colon contains a combination of gas and fecal pellets. The caudoventral aspect of the liver is rounded and extends beyond the costal arch. There is adequate abdominal serosal detail. No abnormalities are seen in the visible bony structures. Differentials for the large cranial mediastinal mass include neoplasia such as lymphoma or thymoma. Ultrasound can be considered to guide FNA or biopsy of the mass.
Figure 4 is an ultrasound image obtained at the time of FNA
Treatment and outcome: An ultrasound guided aspirate of the mass was performed under general anesthesia. Figure 4 is an ultrasound image obtained at the time of FNA. Cytologic examination of the mass aspirate was consistent with thymoma. The owners were advised about options for the rabbit but supportive care was recommended (fluids, syringe feeding, prokinetic agents) to stabilize the patient prior to further treatment of the mass. The rabbit was initially treated in the hospital and then discharged for supportive care at home until the following week. Once the rabbit was eating and defecating normally, she was returned for thymoma treatment. Options were then discussed with the owner and a course of radiation therapy was chosen.
A CT scan was performed for radiation therapy planning (Fig. 5) and the rabbit was treated with external beam radiation using 6 MV photons in a 4 field box arrangement, 3 Gy/fraction for 10 biweekly fractions, for a total dose of 30 Gy. The size of the thymoma was noted to be 70% smaller by volume at a mid-therapy CT scan, indicating a positive response to treatment. The radiation field was adjusted to spare more of the lung parenchyma for the remaining radiation treatments. The rabbit will be returning for re-examination for thoracic radiographs after completing onemonth of radiation therapy.
KM Andres, M Kent, CT Siedlecki, J Mayer, J Brandao, MG Hawkins, JK Morrisey, K Quesenberry, VE Valli, RA Bennett. The use of megavoltage radiation therapy in the treatment of thymomas in rabbits: 19 cases. Veterinary and Comparative Oncology, 10(2), 82-94, 2012.
SM Huston, P Lee, K Quesenberry, A Pilny. Cardiovascular disease, lymphoproliferative disorders, and thymomas, in Quesenberry KE, Carpenter JW (eds): Ferrets, Rabbits, and Rodents: Clinical Medicine and Surgery (ed 3). Philadelphia, PA, WB Saunders, pp. 157–173, 2012