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

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.