PV QA 4 - Poster Viewing Q&A 4
Purpose/Objective(s): To explore the differences and correlation between the target volumes defined using preoperative prone diagnostic magnetic resonance (MR) image and postoperative prone computed tomography (CT) simulation image based on deformable image registration (DIR) for external-beam partial breast irradiation (EB-PBI) after breast conserving surgery (BCS).
Materials/Methods: Eighteen breast cancer patients suitable for external-beam partial breast irradiation (EB-PBI) were enrolled. The gross tumor volume (GTV) delineated on the preoperative diagnostic MR images was defined as GTVMRI, GTVMRI+1 and GTVMRI+2 were defined as 10 mm and 20 mm margins around the GTVMRI, respectively. Tumor bed (TB) delineated on the postoperative prone CT simulation images acquired during free breathing was defined as GTVTB, CTV and PTV were defined as 10 mm and 15 mm margins around the GTVTB, respectively. The target volume of the whole breast contoured on the MR and CT images were defined as BreastMRI and BreastCT, respectively. The MR and CT images were registered deformably in a commercially available deformable registration algorithm.
Results: The median volumes of GTVMRI, GTVMRI+1 and GTVMRI+2 were 4.64cm3, 34.33cm3, 105.27cm3 respectively, and 17.22cm3, 53.46cm3, and 118.00cm3 for GTVTB, CTV and PTV respectively. GTVTB and CTV were significantly greater than GTVMRI and GTVMRI+1, respectively (P = 0.000, 0.000). A statistically significant positive correlation was found between GTVMRI and GTVTB, GTVMRI+1 and CTV, GTVMRI+2 and PTV (P = 0.028, 0.047, 0.034). When based on DIR of the thorax, the conformal index (CI) and degree of inclusion (DI) between BreastMRI and BreastCT were 0.56, 0.82, respectively; the CI and DI of GTVTB-GTVMRI, GTVTB-GTVMRI+1, CTV-GTVMRI and CTV-GTVMRI+1 were 0.02, 0.07, 0.04, 0.17 and 0.08, 0.27, 0.43, 0.66, respectively; When based center-coincidence of the compared targets, the CI and DI of GTVTB-GTVMRI, GTVTB-GTVMRI+1, CTV-GTVMRI and CTV-GTVMRI+1 were 0.19, 0.31, 0.05, 0.38 and 0.85, 0.86, 0.89, 1.00, respectively.
Conclusion: For the patients enrolled for EB-PBI, when defining the target on the preoperative prone MR images, the target volumes were significantly smaller compared to that on postoperative prone CT images, but the a statistically significant positive correlation was found between the MR and CT target volumes. Although based on DIR, there were relatively poor spatial overlap whether the whole breast or the targets between the preoperative prone diagnostic MR images and the postoperative prone simulation CT images.
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