Margin index is not a reliable tool for predicting residual disease after breast-conserving surgery for DCIS

We previously introduced the concept of margin index as a method for prediction of residual disease after attempted breast-conserving therapy (BCT). We sought to apply the margin index to patients with ductal carcinoma in situ (DCIS) to determine its reliability in predicting residual disease.

We identified all patients with DCIS who were treated with BCT from 2004 to 2010. Margin index was calculated as follows: margin index = closest margin (mm)/tumor size (mm) × 100. A receiver operating curve was created using the derived margin index and the presence or absence of residual disease in the re-excision specimen. Sensitivity and specificity were calculated at various margin indices to identify the optimum margin index.

Of 380 patients undergoing attempted BCT, 109 (29%) underwent re-excision. Of 109 patients undergoing re-excision, 46 (42%) had positive margins and were excluded from the study, 15 (14%) were excluded due to inability to determine the size of DCIS on pathology reports, and 48 (44%) met study criteria and were included in the analysis. Of 48 patients undergoing re-excision, 19 (40%) had residual disease. The receiver operating curve c index was 0.65. However, there was no optimum margin index that reliably predicted the presence or absence of residual disease.

Margin index is not a reliable method for prediction of residual disease after attempted BCT with close margins in patients with DCIS only. This may be a reflection of the complexities in accurately determining DCIS size and margin status in pathologic specimens.