I found this article, which hopefully helps to answer your question. The article link is:

http://emedicine.medscape.com/article/207631-overview

eMedicine Specialties > Hematology > Stem Cells and Disorders
Acute Lymphoblastic Leukemia
Karen Seiter, MD, Professor, Department of Internal Medicine, Division of Oncology/Hematology, New York Medical College

Causes
•Less is known about the etiology of acute lymphoblastic leukemia (ALL) in adults compared with AML. Most adults with ALL have no identifiable risk factors.
•An increased prevalence of acute lymphoblastic leukemia (ALL) was noted in survivors of the Hiroshima atomic bomb but not in those who survived the Nagasaki atomic bomb. Most leukemias occurring after exposure to radiation are AML rather than ALL.
•Rare patients have an antecedent hematologic disorder (AHD) such as myelodysplastic syndrome (MDS) that evolves to acute lymphoblastic leukemia (ALL). However, most patients with MDS that evolves to acute leukemia develop AML rather than ALL.
•Increasingly, cases of acute lymphoblastic leukemia (ALL) with abnormalities of chromosome band 11q23 following treatment with topoisomerase II inhibitors for another malignancy have been described. However, most patients who develop secondary acute leukemia after chemotherapy for another cancer develop AML rather than ALL.
Updated: Mar 10, 2010