Morphologic variants and ALK staining illustrating 2 representative ALCL belonging to clusters C1 and C2, revealed by unsupervised analysis. (A) Common type ALCL (cluster C1) consists of large cells, including hallmark cells with a horseshoe shaped nucleus (arrow). Hematoxylin and eosin staining, magnified 1000 ×. (B) All malignant cells were strongly positive for ALK. Immunoperoxidase staining, magnified 1000 ×. (C) Small cell variant (cluster C2) with a predominant population of small cells with irregular nuclei, and scattered hallmark cells (arrow). Hematoxylin and eosin staining, magnified 400 ×. (D) Most of these cases are associated with a perivascular disposition of neoplastic cells highlighted by ALK staining. Immunoperoxidase staining, magnified 400 ×. Images were captured using a Leica DMR microscope equipped with an HC PL Fluotar 40×/0.85 numerical aperture (NA) or 100×/1.30 NA oil objective lens (Leica, Solms, Germany) and a Leica DFC 300 Fx camera. Images were processed using Leica IM50 image acquisition software.