Target Volumes Comparison between Postoperative Simulation MRI and Preoperative Diagnostic MRI for Prone Breast Radiotherapy after Breast-Conserving Surgery

Abstract

To investigate the correlation and difference between target volumes determined by postoperative prone positioning magnetic resonance imaging (MRI) and preoperative prone diagnostic MRI for breast radiotherapy after breast-conserving surgery (BCS) based on deformable image registration (DIR). A total of 17 patients undergoing prone whole breast-irradiation after BCS were enrolled, all these patients were undergoing preoperative and postoperative prone MRI scans. The gross tumor volume (GTV) was delineated on the preoperative MRI images, then expanding 10 mm to simulate the preoperative lumpectomy cavity (LC). The LC were expended to clinical target volume (CTV), and planning target volume (PTV) on preoperative and postoperative MRI sequences, respectively. Preoperative and postoperative MRI was registered based on DIR by a commercially available software system. The target volume, the distance between the centers of mass (dCOM), conformity index (CI), degree of inclusion (DI) were compared to explore the target volume variation for pre- and post-surgery delineation. And evaluated the relationship between CILC/CIPTV and clinical factors. The volume of GTVpre was 3.05cm3(range, 1-23.63cm3). The median volume of LCpre were 26.84 cm3 (range, 11.75-102.28 cm3). 0.85cm3 smaller than the LCpre, the LCpost were 25.99 cm3 (range, 5.98-41.95) (P = 0.04). The dCOM, CI, and DI between LCpre and LCpost were 1.371cm (range, 0.533-5.447), 0.221 (range, 0.041-0.516), and 0.472 (range, 0.108-0.804). There was no significant statistically difference between PTVpre and PTVpost (120.73cm3 vs 131.67cm3, p = 0.723). Compared with LC, the CI and DI between PTVpre and PTVpost were increased to 0.446 (range, 0.233-0.700), 0.636 (range, 0.362-0.834), respectively. Meanwhile, the dCOM was reduced to 1.239cm (range, 0.413-3.642). There was no obvious linear correlation between the CI with the GTV volume, the ratio of the primary tumor volume to the breast volume, the distance from the primary tumor to the nipple and chest wall, and the Body Mass Index (BMI), respectively. For prone breast irradiation, even based on DIR, the spatial correspondence of the target volumes between preoperative and postoperative MRI images were still poor. Therefore, it is not reasonable to delineate postoperative LC refer to preoperative diagnostic MRI based on DIR. Preoperative MRI also cannot provide individualized guidance for LC delineation according to tailored patient.