Transformation of recurrent ALK-positive anaplastic large-cell lymphoma from common pattern to composite pattern (lymphohistiocytic and small-cell pattern) with a change in CD30 expression.

ALK-positive anaplastic large-cell lymphoma (ALK + ALCL) is characterized by expression of ALK and CD30. The current World Health Organization (WHO) classification recognizes five morphologic patterns: common, small-cell, lymphohistiocytic, Hodgkin-like, and composite. There are few reports about the morphologic transformation of ALK + ALCL. Today, we report an intriguing case of presumed transformation from common-pattern to composite-pattern (lymphohistiocytic and small-cell pattern) ALK + ALCL, with CD30 expression changing from positive to negative. This case expands understanding of morphologic transformation between different subtypes of ALK + ALCL and highlights common diagnostic pitfalls, including atypical morphology and negative CD30. Morphologic transformation of ALK + ALCL should be assigned importance, and comprehensive clinical history, histologic and immunophenotypic evaluation are vital to rendering the correct diagnosis of ALK + ALCL.