TU-H-CAMPUS-JeP2-01: Inter-Observer Delineation Comparison of Visible Glandular Breast Tissue On Magnetic Resonance Imaging and Computed Tomography (prone and Supine)

Purpose: Breast cancers predominantly arise from Glandular Breast Tissue (GBT). If the GBT can be treated effectively post-operatively utilising radiotherapy this may be adequate volumetric coverage for adjuvant breast radiotherapy. Adequate imaging of the GBT is necessary and will be assessed between MRI and CT modalities. GBT visualisation is acknowledged to be qualitatively superior on Magnetic Resonance Image (MRI) compared to Computed Tomography (CT), the current radiotherapy imaging standard, however this has not been quantitatively assessed. For radiotherapy purposes it is important that any treatment volume can be consistently defined between observers. This study investigates the consistency of CT and MRI GBT contours for potential radiotherapy planning. Methods: Ten experts (9 breast radiation oncologists and 1 radiologist) contoured the extent of the visible GBT for 33 patients on MRI and CT (both without contrast), which was performed according to a contouring guideline in supine and prone patient positions. The GBT volume was not a conventional whole breast radiotherapy planning volume, but rather the extent of GBT that was indicated from the CT or MR imaging. Volumes were compared utilizing the dice similarity coefficient (DSC), kappa statistic, and Hausdorff Distances (HDs) to ascertain the modality that was most consistently volumed. Results: The inter-observer concordance was of substantial agreement (kappa above 0.6) for the CT supine, CT prone, MRI supine and MRI prone datasets. The MRI GBT volumes were larger than the CT GBT volumes (p<0.001). Inter-observer conformity was higher for CT than MRI, although the magnitude of this difference was small (VOI<0.04). Conformity between modalities (CT and MRI) was in agreement for both prone and supine, DSC=0.75. Prone GBT volumes were larger than supine for both MRI and CT. Conclusion: MRI improves the extent of GBT delineation. The role of MRI guided, GBT-targeted radiotherapy requires investigation in a clinical trial. This work was supported by a grant number APP1033237 from Cancer Australia and the National Breast Cancer Foundation.