I’ve never heard of rapid leukaemia. Could you mean acute leukaemia?
Leukaemia is cancer of the blood or bone marrow, and acute leukaemia involves very rapid increase in the number of immature blood cells in the bone marrow. This makes the bone marrow too crowded, so it can’t produce healthy blood cells. Because this happens so fast, acute leukaemia needs to be treated quickly.
I haven’t come across the term ‘rapid leukemia’ before – but leukemia is cancer of the blood or bone marrow, which kills around 200,000 people die from worldwide each year. I suppose that ‘rapid leukemia’ is a relatively quick progression of the disease.