Multimodal Treatment of a Peripheral Primitive Neuroectodermal Tumor Originating From the Thoracic Cavity in a Dog.

Peripheral primitive neuroectodermal tumor (pPNET) is a very rare, highly malignant tumor encountered in young dogs, with only four necropsy cases in the veterinary literature. A 1.5-year-old male intact French Bulldog presented for evaluation of progressive left forelimb lameness and ipsilateral Horner's syndrome. Whole body computed tomography identified a 12 cm mediastinal mass with extension into the vertebral canal and multifocal metastatic lesions. Histological and immunohistochemical results were consistent with a pPNET. Chemotherapy, radiation therapy, immunotherapy, and targeted therapy were administered. Precision-based tumor testing, including DNA sequencing and chemosensitivity assays, were performed to guide systemic treatment recommendations. A partial remission was observed 3.5 months from presentation based on imaging and improved clinical status. After 7 months of treatment, diffuse metastatic disease, including intracranial spread, was observed and the dog was euthanized.