ALK⁺ lymphoma, common type (lymph node, paraffin section). (A) Scattered ALK⁺ anaplastic tumor cells (labeled in red) are present in the paracortical area. Residual lymphoid tissue is ALK⁻. GC indicates a germinal center (×150). (Inset) Higher magnification (×800) of the same case showing cytoplasmic and nucleolar positivity of tumor cells for the ALK protein (arrowhead). (B) ALK⁺ lymphoma, common type (lymph node, paraffin section). Large anaplastic tumor cells show cytoplasm-restricted positivity for the ALK protein (arrow; ×800). (C) ALK⁺ lymphoma, common type (lymph node, paraffin section). Tumor cells consists of a mixture of large anaplastic elements expressing the ALK protein both in the nucleus and cytoplasm (arrow) and small tumor cells showing nucleus-restricted ALK positivity (arrowhead; ×800). (D) ALK⁺ lymphoma, small cell variant (lymph node, paraffin section). The tumor cell population mostly consists of small neoplastic elements showing nucleus-restricted ALK-positivity. A small percentage of large neoplastic cells that express the ALK protein both in the nucleus and cytoplasm (arrowheads) is present around blood vessels (*; ×500). (E and F) ALK⁺ lymphomas (paraffin sections). Rare small-size tumor cells showing nuclear-restricted ALK-positivity are detectable in the cortical area of the lymph node (E, arrow) and in the bone marrow (F, arrow). The arrowhead in (F) indicates an ALK⁻megakaryocyte (both ×800). (A through F) Immunostaining with the ALKc MoAb; APAAP technique.