Objective To investigate the diagnostic value of apparent diffusion coefficient (ADC)for discriminating orbital lymphomas from other orbital mass lesions.Methods In this prospective study,87 subjects (22 orbital lymphomas and 65 other orbital mass lesions) were enrolled.These patients underwent serial MR and echo-planar DW imaging examination of the orbits with b values of 0 and 700 s/mm2 at 1.5 T MR (GE Signa Excite).ADC maps were reconstructed,and the ADC values of the orbital masses were calculated.Mass ADC was also compared with that of normal-appearing white matter (ADC ratio,ADCR).The receiver operating characteristic curves (ROC) were constructed using optimal cut point of ADC and ADCR to differentiate between orbital lymphomas and other orbital mass lesions.The areas under the ROC curve for ADC and ADCR were also calculated.Results The mean ADC and ADCR of orbital lymphomas were (0.77 ± 0.17) × 10-3 mm2/s and 0.89 ± 0.21,respectively.The mean ADC and ADCR of other orbital mass lesions were (1.36 ± 0.38) × 10-3 mm2/s and 1.51 ± 0.43,respectively.Lymphomas had lower ADCs and ADCRs than other orbital mass lesions (t =-9.620,-9.003,P =0.000).The areas under the ROC curves of ADC and ADCR diagnosing lymphoma were 0.94±0.03 and 0.91 ±0.03,respectively.An ADC of less than 0.804 × 10-3 mm2/sec and ADCR of less than O.956 were optimal for predicting lymphoma (sensitivity,77.3％ for both; specificity,98.4％ and 92.3 ％,respectively ; and accuracy,93.1％ and 88.5 ％,respectively).Compared with pathological results,both ADC and ADCR had high correlations (Kappa values were 0.806 and 0.696,respectively).Conclusion Diffusion-weighted imaging can be applied as a complementary tool in the detection of orbital lymphomas.