International Journal of Surgical Pathology 1–6 © The Author(s) 2016 Reprints and permissions: sagepub.com/journalsPermissions.nav DOI: 10.1177/1066896916683447 journals.sagepub.com/home/ijs Case Reports Introduction Anaplastic large cell lymphoma kinase (ALK)–negative anaplastic large cell lymphoma (ALCL) is a rare CD30- positive T-cell lymphoma morphologically identical to ALK-positive ALCL but lacks ALK protein expression and was included as a provisional entity in the 2008 World Health Organization (WHO) classification. 1 However, fur- ther studies have shown that the epidemiology and clinical outcome of ALK-negative ALCL is considerably different from ALK-positive ALCL or other CD30-positive T-cell lymphomas and hence, in the 2016 revision of WHO clas- sification of lymphoid neoplasm, it is now considered as a distinct entity. 2 ALK-negative ALCL typically occurs in adults in contrast to ALK-positive variants which affects younger individuals and has a worse prognosis when com- pared with ALK-positive ALCL. Patients usually present in an advanced stage (stage III or IV) of disease and extra- nodal involvement is common. Histologically, there is effacement of lymph node architecture by diffuse and cohesive sheet-like infiltrates of pleomorphic tumor cells with multilobated and wreath-like nuclei. ALCL are also characterized by the presence of the “hallmark” cell, which is a large cell with horse-shoe shaped nuclei and a promi- nent central Golgi zone. ALK-negative ALCL express one or more T-cell lineage antigens or may show lack of expression of any T-cell antigens also known as the ”null- cell” phenotype. However, they are still considered to be of T-cell origin since they show clonal T-cell receptor gene rearrangement. 1,3 Morphologically and immunophenotyp- ically ALK-negative ALCL and CHL show overlapping features, posing diagnostic difficulties. 4 ALK-negative ALCL may show prominent mixed inflammatory back- ground resembling classical Hodgkin lymphoma (CHL) and conversely, syncitial variant of nodular sclerosis type of CHL may have confluent sheets of tumor cells with minimal inflammatory cell infiltrate mimicking ALCL. 4,5 683447IJS XX X 10.1177/1066896916683447International Journal of Surgical PathologyArun et al case-report 2016 1 Tata Medical Center, Kolkata, West Bengal, India Corresponding Author: Indu Arun, Department of Pathology, Tata Medical Center, 14 Main Arterial Road (E-W), Newtown, Rajarhat, Kolkata, West Bengal 700156, India. Email: drindups@gmail.com PAX-5 Positivity in Anaplastic Lymphoma Kinase–Negative Anaplastic Large Cell Lymphoma: A Case Report and Review of Literature Indu Arun, MD 1 , Paromita Roy, MD 1 , Neeraj Arora, MD 1 , Saurabh Jayant Bhave, MD 1 , Reena Nair, MD, DM 1 , and Mammen Chandy, MD, FRACP, FRCPA 1 Abstract Anaplastic lymphoma kinase (ALK)–negative anaplastic large cell lymphoma (ALCL) is a subtype of T-cell lymphomas that may mimic several other malignancies morphologically. With the help of immunohistochemistry, most cases of ALCL can be diagnosed on the basis of expression of T-cell lineage associated antigens. However, aberrations in the expression of immunohistochemical markers pose diagnostic challenges. The morphological and immunophenotypic features of ALCL show considerable overlap with classical Hodgkin lymphoma (CHL), which is a B-cell lymphoma. The 2008 World Health Organization Classification of Tumours of Haematopoietic and Lymphoid Tissues suggests that staining for the B-cell transcription factor, paired box 5 (PAX-5), is helpful in differentiating between them, as it is weakly positive in most CHL and should be negative in ALCL. We report a rare case of ALK-negative ALCL, which was positive for PAX-5 and CD15, mimicking CHL by immunohistochemistry, resulting in a diagnostic dilemma. Keywords ALK negative, anaplastic large cell lymphoma, immunohistochemistry, PAX-5 positivity, clonal T-cell receptor gene rearrangement