Morphology and immunophenotype of the small cell variant and the lymphohistiocytic variant of ALCL.

(A) Small cell variant of ALCL. Most of the cells have a small irregular nucleus, the cytoplasm is light, and larger cells are located in close vicinity to small vessels (Giemsa). (B) Immunoreactivity of a small cell variant of ALCL for CD30. The larger tumor cells are strongly positive, whereas the small tumor cells are only weakly positive (alkaline phosphatase–antialkaline phosphatase [APAAP]). (C) Immunoreactivity of a small cell variant of ALCL for ALK. The larger cells (often located in the vicinity of small vessels) are positive in the cytoplasm and the nucleus, whereas in the smaller cells, the labeling is restricted mainly to the nucleus (APAAP). (D) Lymphohistiocytic variant of ALCL. Note the broad cytoplasm and the frequently slightly eccentrically located nuclei, which are revealed immunohistologically as belonging to reactive histiocytes (see panel E) (hematoxylin and eosin). (E) Immunoreactivity of the same case as in (D) with the CD68 antibody clone PG-M1. Nearly all of the larger cells are positive. These PG-M1–positive cells, representing nonneoplastic bystander macrophages, obscure the few CD30⁺ tumor cells (see Figure 2F) (APAAP). (F) Immunoreactivity of the same case as in (D) for CD30. Only a minority of the large cells are positive and represent the tumor cell population (APAAP). (G) Immunoreactivity of the same case as in (D) and (E) for ALK. The labeled cells vary greatly in size, with a predominance of smaller cells. The smaller the cells, the more the labeling is restricted to the nucleus. The negative cells are the bystander macrophages (antibody clone ALK1; APAAP). (H) Immunoreactivity of the same case as in (D), (E), and (G) for perforin. The tumor cells are strongly positive, whereas the bystander macrophages are negative (antibody clone P1-8;APAAP).