Localization of the surgical bed using supine magnetic resonance and computed tomography scan fusion for planification of breast interstitial brachytherapy.

Purpose: To evaluate the feasibility of supine breast magnetic resonance imaging (MR) for definition and localization of the surgical bed (SB) after breast conservative surgery. To assess the inter-observer variability of surgical bed delineation on computed tomography (CT) and supine MR.

Materials And Methods: Patients candidate for breast brachytherapy and no contra-indications for MR were eligible for this study. Patients were placed in supine position, with the ipsilateral arm above the head in an immobilization device. All patients underwent CT and MR in the same implant/treatment position. Four points were predefined for CT-MRI image fusion. The surgical cavity was drawn on CT then MRI, by three independent observers. Fusion and analysis of CT and MR images were performed using the ECLIPSE treatment planning software.

Results: From September 2005 to November 2008, 70 patients were included in this prospective study. For each patient, we were able to acquire axial T1 and T2 images of good quality. Using the predefined fusion points, the median error following the fusion was 2.7 mm. For each observer, volumes obtained on MR were, respectively, 30%, 38% and 40% smaller than those derived from CT images. A highly significant inter-observer variability in the delineation of the SB on CT was demonstrated (p<0.0001). On the contrary, all three observers agreed on the volume of the SB drawn on MR.

Conclusion: Supine breast MRI yields a more precise definition of the SB with a smaller inter-observer variability than CT and may obviate the need for surgical clips. The volume of the SB is smaller with MRI. In our opinion, CT-MRI fusion should be used for SB delineation, in view of partial breast irradiation.