The clinical significance of blast cell morphology in childhood lymphoblastic leukaemia.

The morphological classification of ALL based on the FAB co-operative group's criteria is capable of identifying 10-15% of children with L2 disease who, given similar treatment, will fare less well than the 85-90% with the L1 variant. The significant features defining L2 morphology are a low cellular nuclear: cytoplasmic ratio and the presence of nucleoli. Children with L2 disease do not have higher leucocyte counts but are older, have "common" ALL less frequently, and more often have well-preserved marrow function at diagnosis. Their poor outlook is manifest not only by their higher relapse rate but also by a higher proportion failing to remit in the first instance. L2 morphology does not necessarily "breed true" and can arise in a small proportion of patients with previous L1 disease at the time of relapse. Other striking morphological features of lymphoblasts, including azurophil granules, vacuoles and "hand mirror" cells, have yet to have their significance defined, though the latter feature may be an unfavourable finding.