Rare Case of ALK-Positive Large B-Cell Lymphoma in a Young Adult: Diagnostic and Therapeutic Challenges.

We report a rare case of ALK-positive large B-cell lymphoma (ALK + LBCL) in a previously healthy 20-year-old male, who presented with two weeks of worsening diffuse abdominal pain and weight loss, and was subsequently found to have extensive mesenteric and retroperitoneal lymphadenopathy. Biopsies confirmed the diagnosis of Stage II bulky ALK + LBCL, supported by characteristic immunohistochemical findings including granular cytoplasmic ALK staining and the absence of CD20 expression. Due to the rarity of this aggressive disease, treatment options remain uncertain, and a multidisciplinary consensus for management was required. The patient was treated with dose-adjusted EPOCH chemotherapy, with rituximab added in subsequent cycles despite predominant CD20 negativity. Interim PET/CT scans demonstrated a favorable metabolic response, spotlighting the diagnostic and therapeutic challenges in managing this rare and poorly understood lymphoma subtype.