Alk-positive anaplastic large cell Lymphoma: First documented case of primary mandibular involvement in a 12-Year-Old male.

Anaplastic large cell lymphoma (ALCL), ALK-positive, is a rare subtype of peripheral T-cell lymphoma, most commonly affecting lymph nodes and soft tissues in children and young adults. Primary bone involvement is uncommon, and mandibular localization is exceptionally rare. We present the first reported case of ALK-positive ALCL presenting as a primary bone lymphoma of the mandible in a 12-year-old male. The patient reported with a diffuse, progressively enlarging swelling on the right side of the face. Imaging revealed an ill-defined, FDG-avid (SUV max 13.5) destructive lesion involving the right mandibular body, with no evidence of disease elsewhere. Histopathological examination showed a high-grade malignant neoplasm composed of cohesive sheets of large atypical cells with marked cytoplasmic clearing, nuclear pleomorphism, prominent nucleoli, and atypical mitoses, accompanied by bone destruction and a mixed inflammatory infiltrate. Due to the undifferentiated morphology, an extensive immunohistochemical panel was applied. The tumor was negative for epithelial, B-cell, plasma cell, melanocytic, and myogenic markers, but positive for CD45, CD3, CD30, ALK, and EMA, confirming a diagnosis of ALK-positive ALCL of T-cell lineage. Bone marrow aspiration and biopsy showed no evidence of infiltration, establishing the mandible as the primary site. The patient was treated with Brentuximab Vedotin in combination with the R-CHOP regimen and showed an excellent clinical response with gradual resolution of swelling. This case highlights the diagnostic complexity of undifferentiated mandibular tumors in pediatric patients and emphasizes the importance of integrating morphology, immunophenotyping, and systemic evaluation to identify rare presentations of peripheral T-cell lymphomas.