Preoperative Local Staging Of Rectal Cancer: Correlation Between Thin-section MRI Associated With DWI And Histopathologic Findings

Objective To choose the optimal b value of diffusion weighted imaging(DWI) for rectal carcinoma,and to evaluate the value of apparent diffusion coefficient(ADC) in the prognosis of rectal carcinoma.Both thin-section MR imaging(small FOV) associated with DWI(b= 1000 s/mm²) and conventional MR imaging were used to do preoperative local stage of rectal cancer(TN/T/N),to compare these two methods according to the gold standard of pathological stage;And mainly to approach the application value of the first method in T staging.Materials and Methods Fifty-two patients with rectal carcinoma were collected from Dec.2007 to Jan.2009.The standards of the enrollment of patients are as follows:the rectal carcinoma was diagnosed by biopsy,no radiotherapy or chemotherapy was performed before the examination of magnetic resonance imaging and the tumors were excised by operation and were confirmed by pathology at last. Forty of the fifty-two patients were met the standards and enrolled in this study. Among them,twenty-two were male and eighteen were female,aged from 29 to 83 years old,averaged 62.83 years old.MR scan was performed on GE 1.5 T Twin Speed Infinity with Exciteâ…¡scanner with Torsopa phased array coil.The patients were on liquid diet the day before the examination of MR without any special bowel preparation.SE-EPI-DWI sequence was performed with three different b values(500s/mm²,800s/mm²,1000s/mm²).DWI images with different b values were acquired,the ADC and eADC images were obtained by corresponding post processing.The patients also underwent conventional MR imaging applying feet-first way:axial T₁WI,T₂WI and fsT₂WI (FOV:360×360mm,matrix:224×320,slice thickness:6mm);sagital T₂WI; Thin-section MR imaging and DWI were also underwent:axial T₁WI,T₂WI,and fsT₂WI(FOV:180×180mm,matrix:224×256,slice thickness:4mm);sagital T₂WI and axial DWI with b value of 1000 s/mm².The signal intensities of DWI images and ADC values were measured respectively within the lesions.The data were analyzed:a.DWI and ADC images with different b values were reviewed,the contrast-to-noise ratio(CNR) and signal-to-noise ratio(SNR) were calculated to determine an optimal b value;b.The discrepancies in ADC values among the lesions with different b values were compared statistically;c.With the same b value,the discrepancies in ADC values among the lesions with different differentiations were compared statistically;d.An optimal ADC threshold values were chosen according to receiver operating characteristic(ROC) curve to differentiate the differentiations in rectal carcinoma in order to evaluate its malignancy before operation.To calculate the accuracy of both methods on TN/T/N staging according to gold standard of pathological staging and to analysis as follows:a.To compare the accuracy of both methods on TN staging statistically;b.To compare the accuracy of both methods on T staging statistically;c.To compare the accuracy of both methods on N staging statistically;in order to evaluate the value of thin-section MR imaging associated with DWI in preoperative local staging of rectal cancer.Results DWI images in rectal carcinoma revealed:a.On DWI images with different b values,the rectal carcinomas were all of high signal and the signal attenuated with b value increasing,the discrepancies of the signal intensity between different b values are statistically different;the CNR and SNR also attenuated with b value increasing,but the discrepancies of the CNR and SNR between different b values are not statistically different,and the lesions could be recognized clearly and the ADC values within lesions could still be measured with b value reaching 1000 s/mm²;b.The discrepancies of the ADC values between different b values are statistically different(p=0.001),and the ADC values attenuated with b value increasing;The discrepancies of the ADC values between different b values(b=500 s/mm² and b=800 s/mm²) are not statistically different(p=0.060),The discrepancies of the ADC values between different b values(b=800 s/mm² and b=1000 s/mm²) are also not statistically different(p=0.288),The discrepancies of the ADC values between different b values(b=500 s/mm² and b=1000 s/mm²) are statistically different(p=0.002);c.With each b value,The discrepancies of the ADC values between different differentiations of rectal carcinoma are all statistically different;d. By ROC curve analyzing,the threshold values of ADC obtained from each b value are contributory to differentiate the different differentiations in rectal carcinoma, especially with b value of 1000 s/mm²,the area under ROC curve is the largest;the optimal threshold values of ADC is 0.600×10³mm2/s.Thin-section MR imaging is better on showing the anatomic structures of bowel and the encroachment extent of tumor on surrounding structures than conventional MR imaging.The application of DWI is helpful for differentiating the mass with stool and mucus and is helpful for showing the encroachment extent of tumor,as a result,the staging is more accurate.The statistical analysis about the accuracy of both thin-section MR imaging associated with DWI and conventional MR imaging reveald:a.The accuracy of both methods on TN staging are 67.5%and 45%,the discrepancy is statistically significant(p=0.044);b.The accuracy of both methods on T staging are 85%and 65%,the discrepancy is statistically significant(p=0.040);c. The accuracy of both methods on N staging are 75%and 65%,the discrepancy is statistically nonsense(p=0.332).Conclusions The optimal b value in DWI for rectal carcinoma is 1000 s/mm² in the study.Features on DWI and ADC values are both helpful for differentiating the tumors with different differentiations on some extent,so they can be used to prognose the malignancy before operation.DWI is a helpful complement for conventional MR examination.Thin-section MR imaging(small FOV) associated with DWI(b=1000 s/mm²) is better than conventional MR imaging in preoperative local staging of rectal cancer(TN) and improves the accuracy to 67.5%.The former method is of more superiority especially in T staging and raise the accuracy to 85%; The two methods are statistically not different in N staging.