Cytologic, Histologic, and Ultrastructural Findings of Spontaneous, Cutaneous Fibrosarcoma in a Koi

OBJECTIVE: In teleosts, it is often difficult to differentiate between cutaneous neoplasms originating from peripheral nerves and fibrous tissues, and chromatophoromas, and ultrastructural examination is frequently required in addition to light microscopy for definitive diagnosis. METHOD: A 10-year-old koi (Cyprinus rubrofuscus) had a 3-week history of a 3-4 cm, cutaneous, ulcerated, multinodular mass on the dorsum. RESULTS: Fine needle aspiration revealed spindle cells with abundant, cytoplasmic, variably granular magenta pigment and moderate cellular and nuclear atypia. Biopsy was performed several weeks later and revealed neoplastic spindle cells forming intersecting streams separated by collagenous stroma. Ultrastructural examination showed neoplastic cells had cytoplasmic extensions that were continuous with collagen fibrils, consistent with fibrosarcoma; neoplastic cells lacked pigment and refracting organelles, ruling out chromatophoroma. The nature of the intracytoplasmic pigmented material observed on cytology is uncertain. Neither ultrastructural nor histologic correlates for this cytologically evident material were identified, and differentials for the material are discussed. DISCUSSION: In domestic animals, the presence or absence of concurrent inflammation can often be used to differentiate spindle cell neoplasia from fibroplasia on cytology; this feature is less useful in teleosts, however, as sarcomas are often heavily inflamed and chronic granulomas can be composed of a nearly uniform population of spindle cells. Additionally, there is morphologic overlap between poorly pigmented chromatophoromas and variants of soft tissue sarcoma (STS). For these reasons, definitive diagnosis of STS, chromatophoromas, and many other neoplasms in teleosts often requires a combination of light microscopy and transmission electron microscopy.