Relapsing acute non-lymphatic leukemia with changing phenotypes.

Morphological and cytochemical findings in a case of acute myeloid leukemia (AML) with 3 relapses are described. The disease began as a subtype M4 with atypical eosinophils according to the French-American-British (FAB)-classification. At the time of the first relapse, the phenotype had converted into an M2-subtype, while the second relapse presented the picture of an M1-type of acute myeloid leukemia with non-granulated peroxidase-positive blasts. When the third relapse occurred, the diagnosis was again an M4-type of acute myeloid leukemia. The observed changes of the phenotype can be interpreted as an expression of tumor cell heterogeneity, even though the observed emergence of new leukemic clones would be attributed entirely or in part to effects of the applied cytostatic drugs, because tumor cell heterogeneity concerns also the response to therapy, which may result in a growth advantage for certain leukemic subpopulations. In so far, AML behaves like all other human malignant neoplasias. In addition, morphological changes in leukemias such as are reported here, have been observed long before the introduction of cytostatic drugs. Also, the validity of the FAB-classification is discussed in the light of the presented observation. It is concluded that the FAB-classification is not a classification in a strict sense, but merely a crude and simplistic list of subtypes of acute myeloproliferative diseases that is difficult to reproduce and that disregards major aspects of this group of diseases, for instance tumor cell heterogeneity.