Megavoltage irradiation effective for rare rabbit neoplasm
MedWire News: Megavoltage radiation therapy (RT) is an effective and well-tolerated treatment for thymomas in rabbits, a case series indicates.
Thymomas are unusual benign epithelial neoplasms that are typically slow-growing but potentially locally invasive. They rarely metastasize but as they grow can cause dyspnea, tachypnea, bilateral exophthalmos, and edema of the head, neck, and forelimbs.
Treatment options include surgery, RT, and adjuvant chemotherapy, which are associated with widely differing survival times.
In this study, Kathy Andres (San Francisco Veterinary Specialists, California, USA) and team examined the impact of megavoltage RT on outcomes in 19 rabbits with mediastinal masses due to thymoma.
The animals were aged 6.7 years on average at the time of diagnosis, 10 were female, their median weight was 1.45 kg, and breeds included Netherland dwarf, angora, silver martin, and rex.
Blood tests revealed abnormal complete blood counts in five rabbits and abnormal biochemistry results in 15. Radiographic findings included a cranial mediastinal mass effect in all animals and pleural effusion in six.
All rabbits were treated with megavoltage RT, with the protocol being definitively fractionated in six and coarsely fractionated in 13. The total radiation doses ranged from 24 to 48 Gy. Two rabbits also underwent adjuvant cytoreductive surgery.
In terms of outcomes, the time from first RT to resolution of clinical signs ranged from 4 to 42 days, Andres et al report. Serial thoracic X-rays were performed in six rabbits and showed that thymoma size declined by between 30.0% and 86.6%, although the magnitude of shrinkage was not correlated with symptom resolution.
The overall median survival time was 313 days. After excluding three rabbits that died within 14 days of radiotherapy, median survival time rose to 727 days.
Multivariate analysis identified just one factor - pretreatment bodyweight - that was significantly associated with survival. Rabbits weighing less than 1.57 kg had a median survival time of 312 days, compared with 727 days in rabbits weighing more than 1.57 kg.
Treatment was generally well tolerated, say the researchers, with just three rabbits developing complications related to RT. These were acute radiation field alopecia, delayed acute radiation-induced pneumonitis, and late-term radiation-induced myocardial fibrosis.
As noted, three rabbits died in the acute phase, and all died after extubation on the day of RT. A further three rabbits died during follow up due to thymoma recurrence and one rabbit died from each of renal fibrosis, radiation-induced cardiac failure, and liver failure.
Writing in Veterinary and Comparative Oncology, Andres et al conclude: "The results of this retrospective study demonstrate that megavoltage irradiation, either coarsely or definitively fractionated, is an effective treatment for rabbit thymomas, with long-term survival after therapy."
They add: "Prospective analysis using rabbits with confirmed thymomas randomly assigned to either a standardized definitively or coarsely fractionated RT protocol with standardized follow-up… would be helpful to establish an ideal RT prescription, identify additional prognostic indicators and potential side effects not found in this study."
By Joanna Lyford