Applied Research Of IVIM-DWI In Differential Diagnosis Of Thymic Epithelial Tumors And Thymic Lymphomas

Background and PurposeThymus is one of human central immune organ,which is located in the anterior portion of the mediastinum.Various tumors,cysts,and other abnormalities related to the thymus can develop.Thymic epithelial tumors（TETs）accounts for 50%of all tumors in anterior mediastinum,and followed by lymphoma.Optimal therapeutic strategies and prognoses for thymic tumors differ depending on pathological type,especially because surgery is not always the first step in treatment.It is therefore critical to accurately identify histological type before treatment.In this study,we evaluated the performance of MRI routine sequence,IVIM-DWI parameters in differential diagnosis of thymoma,thymic carcinoma and thymic lymphoma.Materials and MethodsSixty-six patients（46 men,20 women,mean age 44±16 yrs,age range,5-72 yrs）including thymoma（24 cases）,thymic carcinoma（16 cases）and thymus lymphoma（26cases）confirmed by pathological analysis.3.0 T MRI routine sequence examination,DWI（b=0,1000 sec/mm²）,multi-b DWI with 9 b-values（0-1200 sec/mm²）was performed before surgery,to investigate the conventional MRI features of thymic tumors,including tumor size,shape,signal,internal septum,mediastinal lymphadenopathy,and adjacent major vascular relationship,peripheral pleura and pericardial invasion.Slow diffusion coefficient（D）,fast perfusion coefficient（D*）and perfusion fraction（f）were calculated with bi-exponential model（b-value:0-1200 sec/mm²）;Multi-b-value ADC(ADC_mb)and conventional ADC values were calculated by mono-exponential model（b-value:0-1200sec/mm² and 0,1000 sec/mm²,respectively）.The differences of MRI features among thymomas（Ts）,thymic carcinomas（TCs）and thymus lymphomas（TLs）were analyzed by one-way ANOVA or chi-square test.The DWI parameters were compared for differences among groups based on one-way ANOVA,respectively.Receiver operating characteristic curve（ROC）analyses were performed to determine optimum thresholds for differentiating the defined groups based on various parameters and to calculate sensitivity,specificity,and area under the curve（AUC）values.Results（1）According to the WHO pathological classification,66 cases of thymic tumor including,thymoma:1 case of type A（1.5%）,5 cases of type AB（7.6%）,2 cases of type B1（3.0%）,13 cases of type B2（19.7%）and 3 cases of type B3 thymoma（4.5%）,16thymic carcinomas（24.2%）,and 26 thymic lymphomas（39.4%）.Tumor maximum diameter,the uniformity of the signal,combination of pericardium and（or）pleural effusion,internal septum,mediastinal lymphadenopathy,presence of mediastinal large blood vessels invasion among three groups are statistically significant（P<0.01）.However,there was no significant statistical difference on the tumor necrosis,cystic changes,shape and contour among groups（P=0.083,0.223 and 0.34,respectively）.（2）The ADC、ADC_mb、D、D*values in TLs were significantly lower than the values from Ts and TCs（all P<0.05）,the parameter values were:ADC:1.28,0.94 and 0.62×10^-3 mm²/sec;ADC_mb,1.34,0.97 and 0.74×10^-3 mm²/sec;D,0.91,0.57 and 0.50×10^-3mm²/sec;D*,9.99,4.92 and 3.86×10^-3 mm²/sec.As for the f value,no significant differences were found among defined groups（P>0.05）.The AUC and cutoff value,respectively,for differentiating thymoma from thymic carcinoma:D value has better diagnostic efficacy(AUC,0.805;sensitivity,54.2%;specificity,93.8%;cutoff value,0.851×10^-3 mm²/sec);while ADC and D*value of AUC,sensitivity,specificity and cutoff value,respectively:ADC:0.789,95.8%,56.3%and 0.852×10^-3 mm²/sec;D*:0.807,79.2%,75.0%and 5.497×10^-3 mm²/sec.For differentiating thymoma from thymic lymphoma:ADC value has better diagnostic efficacy(AUC,0.995;sensitivity,95.8%;specificity,100%;cutoff value,0.854×10^-3 mm²/sec);ADC_mb,D and D*,the AUC,sensitivity,specificity and cutoff value were as follows:ADC_mb:0.925,95.8%,73.1%and0.856×10^-3 mm²/sec;D:0.885,70.8%,96.2%and 0.727×10^-3 mm²/sec;D*:0.886,83.3%,84.6%and 4.807×10^-3 mm²/sec,respectively.Conclusion（1）The MRI features of thymoma,thymic carcinoma and thymic lymphoma have certain characteristics,and MRI routine sequences are helpful for the differential diagnosis of thymic tumors.（2）Significant differences of IVIM-DWI parameters exsist among thymoma,thymic carcinoma and thymic lymphoma,IVIM-DWI is helpful for the differential diagnosis of thymic tumors.