고되었다 [3][4][5][6].국내에서는 방사성 요오드 치료와 관련하여 발생한 이차성 혈액종양으로 French-American-British (FAB) 분류에 의한 AML M1, M3 및 급성림프모구백혈병(ALL) L1이 각각 1예씩 보고되었으며 M3 증례는 t(15;17)(q22;q21), L1 증례는 t(11;19) (q23;p13) 핵형 이상을 동반하였다 [7][8][9] Ionizing radiation including I 131 might produce chromosomal translocation, causing hematologic malignancy. The incidence of leukemia following radioactive iodine treatment for thyroid cancer has been reported to be approximately 0.1 to 2.0% in Western countries, whereas fewer cases have been reported in Korea. We hereby report four cases of secondary hematologic malignancy, who received iodine therapy for thyroid cancer after thyroidectomy: two cases of acute lymphoblastic leukemia with t(9;22)(q34;q11.2), a case of MDS with 5q deletion, and a case of MDS with normal karyotype. Three cases of hematologic malignancy have developed after cumulative dosage of less than 800 mCi. The treatment intervals in two cases were less than 12 months, and the other two cases had I 131 therapy only once. Assessment of causality using the Naranjo probability scale for adverse drug reactions showed that a 'possible' relationship existed between the use of I 131 and secondary hematologic malignancy in all of the four cases in this report. (Korean J Lab Med 2008; 28:425-9)