When a Lump is More Than Just a Lump: Anaplastic Large Cell Lymphoma Presenting as a Pediatric Breast Mass.

Anaplastic large cell lymphoma (ALCL) is a rare non-Hodgkin T-cell lymphoma characterized by a cluster of differentiation-30 positivity. Subtypes are characterized by positive or negative anaplastic lymphoma kinase (ALK) expression. ALCLs account for about 10% to 15% of all pediatric non-Hodgkin lymphomas and more than 90% of the cases are ALK-positive. We report a rare case of pediatric systemic ALK-negative ALCL with an atypical presentation as a painful breast mass. Despite the general benign features of most pediatric breast masses, it is important to consider malignant systemic diagnoses like the one reported here.