"Triton tumor" in a Haflinger - case report, histology, therapy

A 20-year-old Haflinger gelding was submitted with a history of neoplasms as big as a child's head located craniomedial at both thighs. Due to the morphology and predilection sites, initially there was the clinical suspicion of equine sarcoids. Because of advanced tumor growth the treatment consisted in surgical excision under general anesthesia. In the present case representative sites of neoplasia were embedded in paraplast and then asessed by light microscope at the H.-E.-stained cut. In the course of histological examination after routine processing multiple neoplasms composing the clinical visible proliferation medial on the right tight could be identified. In this patient, among other things, a tissue formation was diagnosed, whose histomorphology appeared similar to malignant Triton tumors in humans. These kind of neoplasia are very fast-growing, malignant nerve sheath tumors of peripheral nerves with rhabdomyoblastic differentiation. An immunohistological characterization was done to exclude other mesenchymal tumors (e.g. rhabdomyosarcoma or ulcerated equine sarcoid). The neoplastic cells were checked on a possible expression of S-100-protein, glial fibrillary acidic protein, (GFAP) and desmin. Both, the spindle and the rhabdomyoblastic tumor cells, showed an moderate to high-grade intracytoplasmic expression of S-100-protein. However, an expression of Desmin and GFAP could not be evidenced. The immunohistological result confirmed the suspected diagnosis of a malignant peripheral nerve sheath tumor with rhabdomyoblastic differentiation. To the authors knowledge this is the first description of such a Triton tumor in the horse. So far, there is no data regarding the specific therapy and prognosis in this species. The treatment included surgical excision and subsequent adjuvant therapy with autogenous vaccines. In a period of two years no recurrence in the case has been observed.