Objective To study the diagnostic value of spectral CT in atypical tuberculosis ball and lung cancer.Methods 45 cases of pulmonary nodule or mass received unenhanced and two phase enhanced scan in gemstone spectral imaging mode,and all pulmonary nodule or mass were confirmed by pathology.Selecting unenhanced,arterial phase and venous phases of spectrum image,atypical tuberculosis ball group and lung cancer group were analyzed.Normalized iodine concentration (NIC), the spectral curve slope (λHU),CT value of unenhanced and two phase enhanced spectrum image were measured and calculated.Two independent samples t-test was used for the statistic analysis of each spectrum parameters between the two groups.The receiver operating characteristic (ROC) curve was obtained for comparing the sensitivity and specificity of each spectral parameter.Results Among the 45 patients,11 were atypical tuberculosis ball, and 34 were lung cancer.Comparison NIC,two phase enhanced λHU and the net enhanced CT values of 70 keV monoenergetic image of the two groups, lung cancer group was higher than atypical tuberculosis ball group,and the difference was statistically significant(P<0.05);However, the unenhanced CT value of 70 keV monoenergetic image of the two groups was not statistically significant (P>0.05).When the NIC diagnostic threshold was 0.105, the maximum Youden index (0.733), the area under the ROC curve was 0.89, and sensitivity and specificity was 82.4% and 90.9%, respectively.Conclusion The quantitative parameters of spectral CT is helpful in the diagnosis of atypical tuberculosis ball and lung cancer,especially the NIC with high value.