Quantification of Minimal Disease by Digital PCR in ALK-Positive Anaplastic Large Cell Lymphoma: A Step towards Risk Stratification in International Trials?

Minimal disseminated and residual disease (MDD/MRD) analyzed by qualitative PCR for NPM-ALK fusion transcripts are validated prognostic factors in pediatric ALK-positive anaplastic large cell lymphoma (ALCL). Although potentially promising, MDD quantification by quantitative real-time PCR in international trials is technically challenging. Quantification of early MRD might further improve risk stratification. We aimed to assess droplet digital PCR for quantification of minimal disease in an inter-laboratory setting in a large cohort of 208 uniformly treated ALCL patients. Inter-laboratory quality control showed high concordance. Using a previously described cut-off of 30 copies NPM-ALK/104 copies ABL1 (NCN) in bone marrow and peripheral blood, MDD quantification allowed identification of very high-risk patients (5-year PFS% 34 ± 5 for patients with ≥30 NCN compared to 74 ± 6 and 76 ± 5 for patients with negative or <30 NCN, respectively, p < 0.0001). While MRD positivity was confirmed as a prognostic marker for the detection of very high-risk patients in this large study, quantification of MRD fusion transcripts did not improve stratification. PFS% was 80 ± 5 and 73 ± 6 for MDD- and MRD-negative patients, respectively, versus 35 ± 10 and 16 ± 8 for MRD-positive patients with <30 and ≥30 NCN, p < 0.0001. Our results suggest that MDD quantification by dPCR enables improved patient stratification in international clinical studies and patient selection for early clinical trials already at diagnosis.