Perivascular pattern of tumor cells infiltration. (A) Several tumor cell foci are seen, all localized around vessels (hematoxylin and eosin). (B through D) The perivascular pattern is highlighted by staining for CD30 (B) or ALK (C) or by double labeling for vessels, with anti-CD31 (brown) and for ALK protein (red). (E) In the small-cell variant case, CD30 is strongly expressed by large cells surrounding the vessel, whereas the small cell population is only weakly positive.