Terms Used for Neoplasms

The term 'tumour' is usually a synonym for neoplasm, but it has also been used in a broader context to indicate any tissue swelling or mass, including those that are not neoplastic. Hyperplastic lesions and non-neoplastic diseases such as lymphocystis and Mycobacterium infection have sometimes been referred to as tumours (Weissenberg, 1965; Post, 1987; Berthiaume et al., 1993; K. Anders and Yoshimizu, 1994). Campana (1983) stated that he used tumour 'in a loose sense' because of uncertainty about whether skin lesions of starry flounders (Platichthys stellatus) were neoplastic. Because the term 'tumour' can be ambiguous, the terms neoplasia (for the disease) and neoplasm (for the lesion) are preferred when the objective is to clearly state the diagnosis (Sirica, 1989).

The names used for fish neoplasms are similar to those used for mammalian neoplasms. Typically the name includes an indication of the tissue or cell type of origin and whether the disease is benign or malignant. Malignant neoplasia, commonly known as cancer, is usually indicated by the terms carcinoma or sarcoma. However, some types of neoplasms have names that vary from this pattern. Papillomas, for example, are named for the papillary appearance of the mass rather than for the cell type. Other exceptions are names used for neoplasms that are invariably malignant, such as lymphoma, melanoma and various 'blastomas' (such as nephroblastoma). There also have been changes over time in the names used for some types of neoplasms; e.g. hepatocellular carcinoma was usually termed 'hepatoma' in older literature.

Indications that a fish neoplasm is malignant include the cellular appearance and behaviour of the lesion. These criteria are similar to those used for mammalian neoplasms, but there is considerably less documentation (and for many lesion types, no documentation) about recurrence after surgery or the clinicopathological outcome. For most fish neoplasms, invasiveness is perhaps the most important criterion used to determine malignancy.

The categories of benign and malignant for neoplasms of fish have been questioned because of the prognostication implied with the term 'malignant' (i.e. potentially life threatening) and because fish neoplasms are less aggressive than their mammalian counterparts (Hayes and Ferguson, 1989). As previously mentioned, clinical experience with most types of neoplasms in fish is limited, so that the eventual outcome is unknown. A conclusion that a fish neoplasm is malignant implies that some of the morphological features associated with malignant neoplasms of mammals are present, but the terms malignant and benign should be used cautiously.

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