Thymic Sarcomatoid Carcinoma with Metastasis in a Rabbit

Background Thymic tumours are frequent in rabbits, with almost exclusive reports of thymomas. Here, we report the first documented case of thymic sarcomatoid carcinoma in a rabbit, a tumour type that has been previously described in humans. Materials & Methods A 5-year-old female neutered rabbit was presented with bilateral recurrent prolapse of the third eyelid due to bilateral exophthalmos. Thoracic radiographs and a CT-scan revealed a cranial mediastinal mass along with numerous nodules in both lungs. Due to progressive respiratory distress, the rabbit was euthanised 119 days after initial presentation. Results Necropsy revealed a 2x4x6 cm large firm mass cranial to the heart, as well as multiple pulmonary nodules measuring 1-5 mm in diameter. Histological examination showed a moderately cellular neoplasm composed of spindle cells arranged in interwoven bundles or haphazard patterns. The tumour cells were embedded in an abundant collagenous stroma. Tumour cells exhibited moderate nuclear pleomorphism, but low mitotic activity. Scattered small aggregates of small lymphocytes were admixed between tumour cells, along with widespread necrosis, multifocal calcification, and numerous multinucleated giant cells. Immunohistochemistry showed diffuse immunopositivity of the spindle cell population for vimentin and multifocal labelling for cytokeratin AE1/3, cytokeratin 5/6, and cytokeratin 7. The pulmonary masses showed the same histological findings as the thymic tumour. Conclusion Histological and immunohistochemical findings of the thymic tumour are consistent with a sarcomatoid carcinoma. A carcinoma is diagnosed based on the positive labelling of some tumour cells for epithelial markers and the presence of pulmonary metastasis.

The Schwarzman Animal Medical Center
Michigan State University