Clinical importance of speed of response to therapy in childhood lymphoblastic leukaemia.

Speed of response to therapy predicts outcome in childhood lymphoblastic leukaemia. This observation has been made studying both blood and bone marrow in children on widely differing treatment regimens from the 1970s to the present day. It appears to be independent of other classical prognostic factors such as age and diagnostic white cell count. Currently some major collaborative groups are using the rate of initial disease clearance to risk-stratify subsequent therapy and this practice may increase. The best way to measure the rate of disease clearance remains to be defined. Watching disappearance of peripheral blood blasts is the least invasive method but possibly the least sensitive. Molecular quantitation of minimal residual disease (MRD) after achievement of conventional remission is much more sensitive but less specific. It cannot be applied to all patients and is costly and time consuming. The degree of marrow infiltration remaining after 7 or 14 days may fall between the two but is often difficult to estimate reliably and reproducibly due to technical limitations. The three techniques may reflect response to therapy in a way slightly different from each other and may not be direct correlates. The best compromise may be to use all three but to reserve MRD study only for those who clear their blood and bone marrow after 7 days.