Possible involvement of cerebrospinal fluid drop metastasis in the survival of French bulldogs after radiotherapy for presumed glial brain tumors.

Background: The French bulldog (FB) is a popular breed, and the number of FBs raised has increased. In recent years, there has been an increasing number of reports of FB brain tumors. Radiotherapy is widely used and believed to be the most effective therapeutic modality against brain tumors in small animal clinical practice; therefore, it is necessary to examine the outcomes of radiation therapy for brain tumors of FB. Aim: This study aimed to investigate the therapeutic effectiveness of radiotherapy on presumed glial brain tumors in FBs and to investigate the possible causes of poor prognosis of patients by reviewing magnetic resonance imaging (MRI) findings at clinical worsening after radiotherapy. Methods: Medical records and MRI images of 15 FBs with presumed glial tumor based on MRI that were treated by radiotherapy were retrospectively analyzed. Results: Among the 15 FB cases, 11 completed their originally scheduled radiotherapy. Radiotherapy reduced the size of primary tumors on MRI in 11 cases; however, the median survival time (MST) of these 11 cases was 509 (range: 74-900), which contrasts with previous reports of radiotherapy for canine brain tumors. The major cause of death was the recurrence of neurological symptoms. MRI features of recurrent cases and cases that died in the early clinical course were characterized by clear enhancement in the leptomeningeal region around the brainstem to the cervical spinal cord, although primary lesions remained. These MRI findings were compatible with those of CSF drop metastasis. Conclusion: Our findings reveal that the primary lesions in FB glial brain tumors are radiosensitive. However, CSF drop metastasis might account for the recurrence of clinical symptoms and shorten the survival time in the affected FBs, and CSF drop metastasis should be considered, even after tumor regression.