Metastatic gingival haemangiosarcoma in a dog arising within a collision tumour

Abstract A 9‐year‐old mixed‐breed dog was presented with a bleeding oral mass on the gingiva. The gingival mass had been present for over a year before it started to bleed. Histopathology after surgical excision revealed a rare collision tumour composed of gingival haemangiosarcoma and peripheral odontogenic fibroma with incomplete margins. Follow‐up imaging identified multiple pulmonary nodules and subcutaneous masses, with biopsy confirming metastatic haemangiosarcoma. The patient underwent five cycles of doxorubicin chemotherapy, which was well tolerated but did not slow metastatic progression. Subsequent treatments included mass excision for palliation and oral piroxicam; however, acute kidney injury and neurological signs later developed. The patient ultimately deteriorated and died 208 days after diagnosis. This case highlights an uncommon oral presentation of haemangiosarcoma and the diagnostic challenges of mixed tumours.