Critical weight loss, as defined by ≥5% decrease in body weight, has been associated with increased morbidity and mortality in human patients with cancers of the head and neck. Weight loss has anecdotally been reported to occur frequently in veterinary patients undergoing radiation therapy and is hypothesized to be more severe in patients with cancers of the head and neck, along with those hospitalized during radiation therapy. The primary objective of this retrospective study was to evaluate the occurrence of critical weight loss in canine cancer bearing patients undergoing either definitive or palliative radiation protocols and to determine if weight changes were associated with radiation toxicity, tumour location or patient hospitalization status.Data from 47 dogs who underwent definitive and 43 dogs who underwent palliative radiation protocols at the University of Tennessee were included for analysis. Dogs were categorized based on tumour location (head/neck or other), hospitalization status (boarded or non-boarded) and radiation toxicity score. Weight recorded at the start of treatment, midway through treatment and at the final treatment was used for analysis. No significant differences were found in regard to weight change over time, location or hospitalization status when evaluated for both protocols. Overall, 5/90 dogs (5.5%) lost 5% or more of their body weight during therapy, and 7/90 dogs (7.7%) gained 5% or more of their body weight. The results of the current study suggest that critical weight loss occurs in a small percentage of canine patients undergoing radiation therapy, contrary to what is often anecdotally reported. K E Y W O R D Scancer, canine, radiation therapy, weight loss

Case summary A case of nasal adenocarcinoma as a suspected secondary malignant neoplasm following definitive radiation therapy and multiagent chemotherapy for nasal lymphoma is described. An 11-year-old spayed female domestic shorthair cat was presented for a 3-week history of progressive facial swelling located over the nasal planum and extending to the medial canthus of the right eye. The cat was previously diagnosed with nasal lymphoma and treated with chemotherapy and definitive radiation 2.5 years prior. Although a definitive diagnosis could not be obtained via cytology, recurrent lymphoma was suspected based on the cat’s history and recurrent clinical signs. A lymphoma-directed chemotherapy protocol was attempted, but no clinical response was achieved. The cat was euthanased owing to progressive clinical signs and a diagnosis of nasal adenocarcinoma was made on necropsy examination. Both the original diagnosis of nasal lymphoma and the secondary diagnosis of nasal adenocarcinoma were confirmed with immunohistochemistry. Relevance and novel information Secondary malignant neoplasm following radiation therapy is infrequently reported in the veterinary literature. In the few reports that exist, most have described sarcoma development in the dog following radiation therapy. In the present report, we describe a cat with a suspected radiation-induced nasal adenocarcinoma that developed 2.5 years after definitive radiation treatment for nasal lymphoma.