The Impact of Different Simulation Modalities on Target Volume Delineation in Breast-Conserving Radiotherapy.

Purpose In the management of breast-conserving radiotherapy, computed tomography (CT) simulation is now commonly used to identify tumor bed while has difficulties defining precisely. We aimed to evaluate the impact of magnetic resonance (MR) and CT simulation on defining the postoperative tumor bed for breast-conserving radiotherapy in patients without the aid of surgical clips. Methods From August 2018 to March 2019, twenty patients with T1-2N0M0 breast cancer at our institution were enrolled. All the patients underwent breast-conserving surgery without implantation of surgical clips and were prepared to receive radiotherapy. CT and MR images were acquired on the same day for each patient. Three radiation oncologists independently assigned cavity visualization score (CVS) and delineated the tumor bed based on first the CT then the MR images. Interobserver variability was assessed by volumes, generalized conformity index (CIgen) and the distance between the centers of mass (dCOM). Differences in mean values for parameters were tested by paired t-test or one-way analysis of variance, as appropriate. Results First, the mean volumes of tumor bed derived from MR were 22%, 27% and 21% smaller than those based on CT images for each observer. In addition, the mean CIgen was significantly superior, and dCOM was smaller for MR than for CT images (CIgen: 0.59 vs 0.52, P= 0.008; dCOM: 1.30 cm vs 1.39 cm, P= 0.095). Moreover, the mean CVS was 3.23±1.34 and 2.43±0.92 for MR and CT images, respectively (P= 0.035). Last, a positive association was observed between the CVS and CIgen for both modalities (P< 0.01). Conclusion Compared to CT, MR can improve the visualization of changes in the postoperative tumor bed. In addition, MR can yield a more precise definition of the tumor bed and improve the consistency of tumor bed contouring in patients without surgical clips.