I don't think it's a medical term. It just means the disease had a "fulminant" (aggressive) course. Acute leukemia complicated by pneunomia.
Thanks. I know what it means, I just didn't know that this could happen in the case of cancer. It is a medical term like fulminant hepatitis for example (maybe you meant to say that it's not a different version of the disease). I just looked it up; it seems it's very rare.