Usefulness Of Volume Perfusion Computed Tomography In Differentiating Histologic Subtypes Of Thymic Epithelial Tumours

Background and PurposeThymic epithelial tumors（TETs）are the most frequent primary neoplasm of the anterior mediastinum.There are different optimal therapeutic strategies and prognoses for different thymic tumors.Currently,imaging plays a vital role in the identification and evaluation of thymic tumors as well as in the follow-up monitoring for its recurrence.As a major modality to assess the mediastinal tumors,computed tomography（CT）can provide detailed information regarding tumor size,shape,homogeneity,contrast enhancement,and so forth,but it cannot accurately differentiate different subtypes of WHO classification.Tumor angiogenesis was significantly correlated with invasiveness in TET patients,and it should increase with higher risk.However,our previous study is out of expectation in suggesting that the maximal contrastenhanced range in LRT（types A and AB）was significantly higher than in other TETs.So,further study for resolving this question is warranted to clarify the hemodynamic features of different subtypes of TETs.Volume perfusion CT（VPCT）is a relatively new technique enabling the acquisition of functional perfusion-based data complementary to conventional morphologic CT,which has been increasingly used for oncologic imaging over the past few years.The CT perfusion parameters correlate well with tumor angiogenesis reflected by histological markers such as microvessel density and vascular endothelial growth factor expression,and it can predict tumoral grading.Histopathology research demonstrated that thymomas and thymic squamous cell carcinomas differ significantly in their vascular architecture and expression of key angiogenic growth factors.Therefore,we hypothesize that VPCT parameters are useful in differentiating the TET subtypes.We aim to evaluate the performance of volume perfusion computed tomography（VPCT）parameters in differentiating the World Health Organization（WHO）subtypes of thymic epithelial tumours（TETs）.Materials and Methods This study was approved by the local Ethics Committee,and informed written consent were obtained.Fifty-one TET patients confirmed by histopathological analysis and underwent conventional CT and a 48-s volume perfusion CT scan of the tumor bulk before any treatment were included in this study.CT features,such as tumor size,shape,density and invasion of mediastinal structures,etc,were evaluated in relationship to simplified subtypes（low-risk thymoma（LRT,typs A,AB and B1）,high-risk thymoma（HRT,types B2 and B3）and thymic carcinoma（TC））of TETs using chi-square Fisher’s exact test.Time to peak and the VPCT parameters（blood volume（BV）,blood flow（BF）,mean transit time（MTT）and permeability（PMB））based on volume of interest（VOI）or region of interest（ROI）were compared for differences among LRT,HRT and TC by one-way ANOVA.Receiver operating characteristic curve（ROC）were performed to determine the potential efficiency for differentiating the LRT from HRT and TC by VPCT parameters.Results（1）Fifty-one TETs included 23 LRT,17 HRT and 11 TC.Round,oval and plaque proportion in LRT,HRT and TC were 56.5%,17.6%and 63.6%;34.8%,35.3%and 18.2%;8.7%,47.1%and 18.2%,respectively.Smooth,irregular and lobulated edges in LRT,HRT and TC accounted for 39.1%,43.5%and 17.4%;5.9%,23.5%and 70.6%;18.2%,18.2%and 63.6%,respectively.The proportion of necrotic or cystic changes in LRT,HRT and TC were 17.4%,41.2%and 54.5%,respectively.Lymph node enlargement or blood metastasis were not found in all patients with LRT and HRT.In 11 cases of TC,4 cases（36.4%）developed mediastinal lymph node enlargement,and 2 cases（18.2%）appeared pericardium or pleural nodules.The proportion of pericardial effusion or pleural effusion in LRT,HRT and TC were 21.7%;11.8%and 63.6%.Shape,edge,tumor necrosis,pericardium or pleural effusion,and lymph node enlargement were statistically significant in three groups（P<0.05）.Mean diameter,maximum diameter and calcification of the tumor had no statistical difference in three groups（P value were 0.597,0.465 and 0.382,respectively）.（2）The BV_VOI,PMB_VOI,BV_ROI,and PMB_ROI values in LRT were significant higher than the value from HRT and TC(BV_VOI:13.75,6.17 and 5.48 ml/100 ml;PMB_VOI:22.47,9.56 and 13.37 ml/100 ml/min;BV_ROI:14.75,6.87 and 6.06 ml/100 ml;PMB_ROI:24.05,9.79 and 15.63 ml/100 ml/min,respectively;all P<0.05/3).However,the BF_VOI,MTT_VOI,BF_ROI,MTT_ROI values and time to peak did not differ between LRT and HRT or TC groups（P>0.05/3）.The AUC,sensitivity,specificity,accuracy and cutoff value,respectively,for differentiating LRT from HRT and TC for BV_VOI,BV_ROI,PMB_VOI,and PMB_ROI values were as follows:BV_VOI,0.928,73.9%,96.4%,86.3%,and 9.29 ml/100 ml;BV_ROI,0.911,78.3%,92.9%,86.3%,and 9.88 ml/100 ml;PMB_VOI,0.890,73.9%,92.9%,84.3%,and 17.33 ml/100 ml/min;and PMB_ROI,0.842,60.9%,100.0%,82.4%,and 21.99ml/100 ml/min;respectively.The mean time to peak of TETs was 25.5sec.Conclusion（1）Conventional CT features of TETs might reflect their pathology subtypes,thus aiding the initial evaluation of TETs before treatment.（2）These results suggest that VPCT could be useful in differentiating LRTs from HRTs and TCs preoperatively.