Thymoma in an 11-year-old dog: Radiation oncology perspective

Radiation therapy for canine thymomas is sparsely described in the literature. One study retrospectively evaluated 17 dogs receiving radiation therapy alone or in combination with various other therapies (corticosteroids, chemotherapy, surgery).1 Protocols varied from coarsely fractionated (palliative) to daily definitive protocols. The median survival for dogs was 248 days with an overall response rate, which included cats, of 75%. Complete responses were rare.1

Dr. Isabella PfeifferAnother retrospective study evaluated a palliative protocol for the sole treatment of thymoma in eight dogs.2 Dogs were treated once weekly with 7 to a Gray (Gy) per fraction for a total of six or seven fractions.2 The number of treatments was not determined by a set dose prescribed because of the tumor but by dose constraints chosen to avoid adverse effects to the lungs and heart. Radiation damage can cause severe, potentially life-threatening damage to the lungs and heart, such as radiation pneumonitis, myocarditis, pulmonary fibrosis, and myocardial failure. High doses of radiation per fraction increase the risk for those adverse effects.

In this study, two dogs experienced a complete and partial response determined by follow-up computed tomography (CT) scans. Two additional dogs had stable disease. The median survival time was not reached because five of the eight dogs were still alive at study completion. Two dogs died because of respiratory failure either due to myasthenia gravis or pleural effusion due to thymoma, and one dog died at 15 years old for unknown reasons. Acute and chronic radiation damage to the lungs was diagnosed by CT in four dogs and was self-limiting and asymptomatic in three dogs; one dog had a mild cough that resolved without intervention.2

Because thymomas can be associated with severe paraneoplastic syndromes, which predispose dogs to megaesophagus and increase the risk of aspiration pneumonia especially with repeated anesthesia induction, hypofractionated protocols are reasonable for at-risk patients. In patients without paraneoplastic syndromes and with a more favorable prognosis, definitive protocols to minimize late adverse effects to lungs and heart can be discussed with owners, as survival times in those few studies are encouraging.

To minimize radiation exposure to heart and lungs, radiation plans should be developed using CT scans and image-guidance.

References

1. Smith AN, Wright JC, Brawner WR Jr, et al. Radiation therapy in the treatment of canine and feline thymomas: a retrospective study (1985-1999). J Am Anim Hosp Assoc 2001;37(5):489-496.

2. Goto S, Murakami M, Kawabe M, et al. Hypofractionated radiation therapy in the treatment of canine thymoma: retrospective study of eight cases. Vet Radiol Ultrasound 2017;58(5):613-620.