Project Summary Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase encoded by the ALK gene located on 5q35. Structural alterations including translocations, copy number gains and activating mutations targeting ALK occur in many types of human cancer, including lung cancer, non- Hodgkin lymphomas, Spitzoid melanocytic lesions, neuroblastoma and inflammatory myofibroblastic tumor. In over 80% of pediatric anaplastic large cell lymphoma (ALCL), the most common form of mature T cell lymphoma in this population, the chromosomal aberration t(2;5)(p23;q35) results in the expression of the constitutively active tyrosine kinase NPM-ALK. NPM-ALK positive lymphoma has served as a model for understanding ALK-mediated oncogenesis and development of targeted therapies, thus NPM-ALK related studies carry profound implications for the cancer field in general. Unfortunately, even with current intensive combined chemotherapy, approximately 30% of patients experiences disease progression or recurrence within two years of treatment. However, clinical or genetic factors that cause ALCL relapse are not known. Furthermore, prognostic biomarkers that can be easily obtained using non-invasive methods have not been clearly defined in patients receiving targeted therapies in ALCL. Our central hypothesis is that sensitive and specific quantitative assessment of circulating NPM-ALK transcript using digital droplet (dd)PCR in conjunction with plasma levels of ALK auto- antibody will serve as unique disease-specific biomarkers that will provide an opportunity for assessment of response to therapy and lead to prognostic biomarkers that may identify patients with high risk for relapse. Using plasma samples from uniformly treated patients enrolled in a Children's Oncology Group (COG) Phase II study of Brentuximab Vedotin and Crizotinib with newly diagnosed ALCL, we address our hypothesis through the following specific aims: 1) Determine the utility of ddPCR for minimal disease detection and disease monitoring in ALK+ ALCL, 2) Determine the prognostic utility of anti-ALK immune response in ALK+ ALCL 3) Investigate the prognostic significance of a multivariate model combining ddPCR and immune response to ALK in patients with ALK+ ALCL. The development of sensitive and precise assessment of minimal disseminated disease and/or minimal residual disease will play an important role in management of patients with ALK+ ALCL and facilitate decisions about discontinuation of treatment and identifying patients at risk of relapse/progression.