ALK-positive anaplastic large cell lymphoma (ALCL) is a T-cell lymphoma that is characterized by a pleomorphic cell population in which a variable proportion of cells have distinctive eccentric, horseshoeor kidney-shaped nuclei, often with an eosinophilic region near the nucleus (hallmark cells).^1,2,3,4,5,6 In addition, the tumor cells express CD30 and exhibit genetic alterations involving the ALK gene on 2p23.^{7,8,9,10,11,12,13,14}

ALK-negative ALCL is a CD30-positive T-cell non-Hodgkin lymphoma that morphologically resembles ALK-positive ALCL but lacks chromosomal rearrangements of the ALK gene.^{3,15,16,17,18,19} Criteria to distinguish ALK-negative ALCL from other CD30-positive peripheral T-cell lymphomas remain imprecise, making this entity a diagnostic challenge; this difficulty is further compounded by a lack of specific genetic biomarkers for ALK-negative ALCL.¹⁷

ALCL is a well-recognized clinicopathologic entity accounting for 2% of all adult non-Hodgkin’s lymphomas (NHL) and about 13% of pediatric NHL.¹ ALCL frequently involves both lymph nodes and extranodal sites.¹¹ Mediastinal disease is less frequent than in classic Hodgkin lymphoma, occurring in 35% to 59% of patients.^4,20 The mediastinum is uncommonly the sole site of disease.^21,22

ALK-positive ALCL occurs most commonly in children and young adults, although cases can occur at any age; there is also a male predominance (male-to-female ratio, 3.0).^4,8,21