Inducible haemorrhage (epistaxis) secondary to lomustine administration without thrombocytopenia in a dog

Abstract A 13‐year‐old dog with no history of spontaneous bleeding was presented for evaluation of multiple diffusely distributed cutaneous mast cell tumours. No obvious metastasis was found, and chemotherapy with vinblastine/lomustine was initiated. After the second dose of lomustine was administered, the dog experienced epistaxis for 2–3 minutes per day along with heavy sneezing due to the discharge. Throughout additional chemotherapy cycles, intermittent nasal bleeding continued, and investigation into the causes of epistaxis found no obvious nasal causes or coagulopathies, but epistaxis resolved after lomustine administration was delayed. As the mast cell tumours began to grow again, another dose of lomustine was given without vinblastine, and epistaxis returned. Throughout the dog's treatment, it was never thrombocytopenic but often had thrombocytosis. There are reports of lomustine and other nitrosoureas affecting platelet function in people, and this could be considered a differential in dogs as well.