Objective: ALK-positive large B-cell lymphoma is an aggressive form of lymphoma characterized by the expression of the ALK protein alongside the loss of pan-B-cell and pan-T-cell markers. Unfortunately, there is no established standard treatment for this subtype, resulting in a poor prognosis. Understanding the expression of aberrant markers is essential for accurate diagnosis and improved patient management.

Case Report: A 13-year-old male presented with intestinal obstruction to pediatric surgery and subsequently underwent exploratory laparotomy. Histopathological examination of the specimen revealed the presence of monomorphic tumor cells with plasmablastic morphology, which expressed CD45, CD79a, and CD3. Considering the patient's age and the tumor`s morphology, further analysis showed the loss of other B-cell and T-cell markers, along with the expression of ALK, CD138, CD38 and lambda light chain restriction. Thus, the current case emphasizes the necessity of a comprehensive immunohistochemical analysis to accurately diagnose ALK-positive large B-cell lymphoma with aberrant CD3 expression, thus preventing misdiagnosis as T-cell lymphoma.

Conclusion: It is crucial to recognize the uncommon ALK-positive large B-cell lymphoma that exhibits aberrantly expressed CD3 to prevent misdiagnosis. Identifying this condition may allow for the incorporation of ALK inhibitors, potentially improving patient outcomes.