Genetic and Response-Based Risk Classification Identifies a Subgroup of NCI High Risk Childhood B-Lymphoblastic Leukemia (HR B-ALL) with Outstanding Outcomes: A Report from the Children's Oncology Group (COG)

Overall improvements in ALL outcomes have been attributed in part to refinements in risk classification that affect treatment intensity. The COGdeveloped a real-time disease classification protocol utilizing clinical, biologic and early disease response measures from local and central reference laboratories.

Patients between 1-30 years were enrolled on the COG AALL03B1 classification study at the time of B-ALL diagnosis and subsequently initiated a 3-drug (standard risk; SR) or 4-drug induction (HR) based on NCI risk group. All patients underwent standardized testing at approved or central laboratories to detect favorable (triple trisomies of chromosomes 4, 10 and 17 [TT] or ETV6-RUNX1 fusion) and unfavorable (hypodiploidy [DNA index <0.81 or chromosomes < 44], MLL rearrangements, BCR-ABL1 or iAMP21) cytogenetic abnormalities. At the end of induction therapy, patients > 1 year of age with B-ALL were classified into low, standard, high or very high risk groups for treatment allocation. Variables used for risk classification included age, initial WBC, extramedullary disease status, blast cytogenetics and early treatment response based on bone marrow morphology and day 29 marrow minimal residual disease (MRD). Rapid early response (RER) was defined as M1 (<5% blasts) bone marrow by day 15 plus flow cytometry-based MRD < 0.1% on day 29 of induction. Those with either M2/M3 (≥ 5% blasts) day 15 marrow or MRD ≥ 0.1% at day 29 were deemed slow early responders (SER).

From December 2003 - September 2011, 11,196 (11,144 eligible) patients enrolled on AALL03B1; 89% enrolled on a frontline ALL therapeutic trial, 96% of whom were evaluable for post-induction treatment assignment. Among these patients, 5104 and 2791 respectively, were treated on companion clinical trials for NCI SR (AALL0331, 65%) or HR (AALL0232, 35%) B-ALL. Patients with very high risk features (BCR-ABL1, hypodiploidy, induction failure, or poor response at day 43) did not continue on AALL0232/AALL0331 post induction, but did have outcome data captured for analysis. The distribution of induction response was 84% RER and 16% SER with 5-year event-free (EFS) and overall survival (OS) rates of 89.3% and 95.2%, respectively for RERs and 67.9% and 84.3%, respectively for SERs. Five-year EFS and OS rates for SR and HR patients combined according to cytogenetic subtype are summarized in the Table. The overall frequencies for the genetic subsets were: ETV6-RUNX1 26%; TT 21%; hypodiploidy 1.5%; MLL 2%; BCR-ABL1 2.6% and iAMP21 2%. Five-year EFS varied according to cytogenetic subset ranging from 70% (unfavorable) to 95% (favorable). Notably, 5-year OS was over 98% for the favorable cytogenetic subsets of combined SR and HR patients that accounted for almost half of all patients. In a subsequent analysis using current COG MRD response measures (RER=day 8 blood MRD <1% and day 29 marrow MRD <0.01%), HR patients with favorable cytogenetics who were CNS1 had 5-yr EFS and OS of 94.9% and 98.1% (n=243), respectively.

In conclusion, real-time classification was feasible in close to 10,000 patients enrolled on COG ALL trials and identified patients with varying outcomes for risk-based treatment allocation. While the COG has not previously utilized favorable cytogenetic features to risk classify NCI HR B-ALL patients, outcomes for this subgroup who also have rapid MRD responses during induction are excellent and suggest that these patients will not benefit from further chemotherapy intensification.