Cytotoxic lymphomas are tumours derived from T or natural killer (NK) lymphocytes with a cytotoxic phenotype. Neo-plastic cells typically express at least one cytotoxic protein such as T-cell intracellular antigen-1 (TIA-1), granzyme B or perforin, and show a variable expression of CD56 [1-8]. Although cytotoxic NK/T-cell lymphomas are commonly described as aggressive neoplasms, the expression of cytotoxic proteins themselves is not restricted to a specific group of lymphomas, as they can be observed in cases of mycosis fungoides (rarely in early lesions, more commonly in late stages of the disease; see Chapter 2), and commonly in the so-called CD30+ cutaneous lymphoproliferative disorders (see Chapter 4). Expression of cytotoxic proteins is also the rule in cases of subcutaneous T-cell lymphoma (see Chapter 5). In short, cytotoxic proteins do not have any diagnostic or prognostic value per se, and their expression should be evaluated in the context of the clinicopathological and molecular features of the lesions. It has been shown recently that the expression of cytotoxic proteins and inhibitory receptors varies in different types of cytotoxic lymphomas, suggesting that they may differ with regard to their functional profiles [9].

We would also like to stress that these lymphomas show many overlapping clinicopathological features and that classification may be subjective in some cases. In particular, distinction from mycosis fungoides and from subcutaneous T-cell lymphoma can be difficult (see Chapters 2 and 5). Moreover, in spite of extensive phenotypical and genotypical studies, a few cases defy precise classification [8,10].

It is important to emphasize that for most of these lymphomas, cytomorphological features are variable and are not associated with prognostic features. In addition, cytomor-phology is similar in all of these entities, and may be characterized by predominance of small-, medium- or large-sized cells (usually with pleomorphic nuclei). Thus, cytomorpho-logical aspects are neither useful for a specific diagnosis and classification of the lymphoma, nor are a feature associated with the biological behaviour, and should always be analysed together with all other clinical, histopathological, phenotyp-ical and molecular genetic features.

Finally, it should be emphasized that distinguishing between primary and secondary cutaneous involvement is less important for this group of tumours than for most other skin lymphomas [8,11]. In fact, cases with a primary cutaneous presentation usually develop extracutaneous dissemination within a short period of time, and the prognosis is usually very poor, regardless of the results of staging investigations at presentation.

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