Objective To evaluate the ability of the ratio of peripheral blood white blood cell(WBC)counts to platelet counts to predict the onset of radiation-induced thymic lymphoma(TL)in a mouse model.Methods Mice were subjected to fractionated total-body irradiation(TBI)to established a TL model before the changes of the WBC-to-platelet ratio during the development and progression of TL were investigated.Four-week-old male C57BL/6J mice were randomized into the normal(non-irradiation)group and radiation exposure group that was subjected to 1.8 Gy TBI once weekly for four consecutive weeks.The survival and TL-incidence of those two groups were compared within 370 days of TBI.Histomorphology and hematoxylin & eosin(H&E)staining of the thymus were used for definite diagnosis of TL while flow cytometry was adopted to detect the frequency changes of T cells in the thymus,bone marrow and spleen.Peripheral blood(PB)cell counts were measured to analyze the changes of peripheral hemogram during TL pathogenesis.Results No mice in the normal group were diagnosed with TL while 83％of the irradiated mic suffered from TL within 370 days of fractionated TBI(P＜0.0001).Using histopathologic technology,medium-sized tumor cells were observed in the thymus of irradiated mice diagnosed with TL.Cytometric analysis showed decreased frequencies of CD4 mono-positive cells and increased frequencies of CD8 mono-positive cells in the thymus,bone marrow and spleen of mice diagnosed with TL.PB analysis displayed a significant increase in the WBC-to-platelet ratio one week prior to the TL-caused death in the irradiated mice(P＜0.01).Conclusion Elevation of the peripheral blood WBC-to-platelet ratio can help predict the onset of IR-induced TL of mice.