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

## TL;DR

A dog with a chest tumor developed internal bleeding after a targeted radiation treatment, a rare complication not previously seen in veterinary patients.

## Contribution

First documented case of acute tumor-related bleeding in a dog after stereotactic body radiotherapy.

## Key findings

- The dog developed breathing difficulty and anemia one day after completing SBRT.
- Imaging confirmed intra-tumoral hemorrhage and hematoma formation.
- The patient recovered with supportive care and showed reduced tumor volume on follow-up.

## Abstract

Stereotactic body radiotherapy is a highly targeted radiation treatment that has been used in dogs to manage mediastinal tumors such as thymomas and heart base tumors. Most side effects are mild and temporary, such as tiredness, radiation-induced inflammation of the lungs or esophagus, or abnormal heart rhythms. Although rare, bleeding within a tumor has been reported in people shortly after stereotactic body radiation therapy. Until now, such an acute hemorrhagic complication had not been documented in veterinary patients. This report describes a Pomeranian dog with a tumor in the cranial mediastinum that received stereotactic body radiation therapy. The day after the final treatment, the dog developed breathing difficulty and anemia. imaging and blood test results supported a diagnosis of tumor-associated bleeding. The patient recovered with supportive care and remained clinically stable during follow-up. In addition to presenting this rare complication, this case also discusses possible radiobiological mechanisms. Although rare, this type of hemorrhage can be life-threatening and may cause different symptoms depending on its location. Clinicians should be aware of this risk when planning treatment and managing patients.

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.