Metastatic osteoblastic osteosarcoma of the jaw in a marble trout (Salmo marmoratus)

A female broodstock marble trout (Salmo marmoratus) approximately 5 years old, 45 cm in length and weighing 786 gr presented an evident mass protruding from the lower jaw associated with feeding impediment. Histopathology of the lower jaw showed expansion of connective tissue by an unencapsulated, poorly demarcated, infiltrative, moderately cellular neoplasm composed of bundles and streams of polygonal to spindle cells on a moderate amount of fibrovascular stroma. Abundant eosinophilic to amphophilic homogeneous matrix (consistent with osteoid) was interspersed among neoplastic cells. The cells were 10-15 µ min diameter, with indistinct cell borders, moderate to abundant eosinophilic fibrillar cytoplasm, often vacuolated, with central to paracentral oval nucleus and single to multiple distinct nucleoli. Mitoses averaged 2/HPF and atypia was severe. Multinucleated giant cells resembling osteoclasts were scattered among malignant osteoblasts. Trabeculae of immature bone lined by osteoblasts and osteoclasts (bone remodelling) were visible, together with islands of immature cartilage. Similar histopathological features were observed in the coelomic samples confirming these lesions as metastases of the primary mandibular neoplasm. The trichrome stain confirmed the deposits of homogeneous material as osteoid in nature (stained in red) both in the primary lesion and in the metastases. IHC with anti-osteocalcin antibody markedly stained neoplastic osteoblasts both in the primary lesion and in the visceral metastases. This is the first description of a metastatic mandibular osteosarcoma in a fish. In conclusion if the morphological diagnosis of bone tumours remains the gold standard in fish, IHC can help in refining the histological diagnosis also in this species when cross-reactive markers are available.