Suspected Tumor-Related Hemorrhage as a Rare Complication of Stereotactic Body Radiotherapy in a Dog with Cranial Mediastinal Mass: A Case Report.

Stereotactic body radiotherapy (SBRT) has been increasingly used in dogs for mediastinal tumors and is generally considered a precise and relatively safe treatment, with clinically significant complications reported only rarely. A cranial mediastinal mass was incidentally identified in a 10-year-old Pomeranian dog and cytologically diagnosed as a carcinoma. SBRT was performed using volumetric-modulated arc therapy, with a total dose of 27 Gy delivered in three fractions on alternate days. One day after completing treatment, the dog developed acute dyspnea and anemia. Thoracic radiography revealed mediastinal widening and pleural effusion. Subsequent imaging and hematological assessments suggested intra-tumoral hemorrhage and hematoma formation. The patient was managed conservatively with supportive therapy, resulting in gradual clinical improvement. Follow-up computed tomography (CT) demonstrated a 25% reduction in contrast-enhancing tumor volume, accompanied by a large non-enhancing region presumed to represent hematoma. Despite these changes, the patient remained clinically stable during follow-up. This case represents the first documented report of an acute hemorrhagic complication following SBRT in a veterinary patient, emphasizing the importance of awareness of this rare adverse event during treatment planning and client communication.