Malignant lymphomas of the skin: their differentiation from lymphoid and nonlymphoid cutaneous infiltrates that simulate lymphoma.

Malignant lymphomas of the skin, excluding mycosis fungoides, are pathologically, immunologically, and clinically heterogeneous. Varying patterns and degrees of cutaneous infiltration are encountered in all histologic subtypes of non-Hodgkin's lymphomas. Immunologic studies have shown relatively equal numbers of cases with B and T cell phenotypes, but true histiocytic lymphomas of the skin also occur. Patients may be of any age, and they may have lymphoma in any clinical stage. A low clinical stage and a low-grade histologic subtype are significant factors for long survival. The differential diagnosis includes a variety of lymphoid infiltrates that are referred to as a group as cutaneous lymphoid hyperplasia; it also includes lymphomatoid papulosis as well as nonlymphoid cutaneous infiltrates such as myeloid leukemias, histiocytosis X, malignant histiocytosis, regressing atypical histiocytosis, and neuroendocrine (Merkel) cell carcinoma. Distinction of cutaneous lymphomas from these entities requires comprehension of multiple variables, including immunologic as well as morphologic and clinical factors.