Malignant thymus tumour progressing into acute leukaemia with non-specific esterase activity.

We describe an 8-year-old girl who presented a massive mediastinal tumour of thymic origin. Surgical excision was unsuccessful. Irradiation treatment (4,500 rad) together with intensive combination chemotherapy resulted in regression of the tumour. After three months, an increase of blasts was observed in the peripheral blood and bone marrow. The terminal phase was characterized by acute leukaemia, which did not respond to chemotherapy. Despite the thymic origin of this malignancy, the blast cells contained non-specific esterase activity as determined by naphthol-AS-acetate as substrate. The NaF-sensitive form of this enzyme is a characteristic of monocytes-macrophages. Other monocyte-macrophage markers were, however, negative. We conclude that the NaF-sensitive naphthol-AS-acetate esterase is not limited to tumours of the monocyte-macrophage system.