MRI reduces variation of contouring for boost clinical target volume in breast cancer patients without surgical clips in the tumour bed

Abstract

Introduction: Omitting the placement of clips inside tumour bed during breast cancer surgery poses a challenge during delineation of lumpectomy cavity clinical target volume (CTV_LC). We aimed to quantify inter-observer variation and accuracy for CT- and MRI-based segmentation of CTV_LC in patients without clips.

Materials and methods: CT- and MRI-simulator images of 12 patients, treated by breast conserving surgery and radiotherapy, were included in this study. Five radiation oncologists recorded the Cavity Visualization Score (CVS) and delineated CTV_LC on both modalities. Expert-consensus (EC) contours were delineated by a senior radiation oncologist, respecting opinions of all observers. Inter-observer volumetric variation and generalized conformity index (CI_gen) were calculated. Deviations from EC contour were quantified by the accuracy index (AI) and inter-delineation distances (IDD).

Results: Mean CVS was 3.88 +/- 0.99 and 3.05 +/- 1.07 for MRI and CT, respectively (p=0.001). Mean volumes of CTV_LC were similar: 154 +/- 26 cm³ on CT and 152 +/- 19 cm³ on MRI. Mean CI_gen and AI were superior for MRI when compared with CT (CI_gen: 0.74 +/- 0.07 vs. 0.67 +/- 0.12, p=0.007; AI: 0.81 +/- 0.04 vs. 0.76 +/- 0.07; p=0.004). CI_gen and AI increased with increasing CVS.  Mean IDD was 3 mm +/- 1.5 mm and 3.6 mm +/- 2.3 mm for MRI and CT, respectively (p=0.017).

Conclusions: When compared with CT, MRI improved visualization of post-lumpectomy changes, reduced the inter-observer variation and improved the accuracy of CTV_LC contouring in patients without clips in the tumour bed.