| Objective: This study sought to explore the interobserver variability in the delineation of the tumour bed(based on clips, seroma or both clips and seroma) and to evaluate the dosimetric impact of tumour bed delineation variability and organ at risk(OAR) during external-beam partial breast irradiation(EB-PBI) planned utilizing four-dimensional computed tomography(4DCT) scans. Meanwhile, to investigate the potential dosimetric benefits of 4DCT compared with three-dimensional C T(3DC T) in the planning of radiotherapy for EB-PBI.Methods: Patients with a seroma clarity score(SCS) 3~5 and ?5 surgical clips in the lumpectomy cavity after breast-conserving surgery who were recruited for EB-PBI underwent 3DCT and 4DCT simulation. Based on the ten sets of 4DC T images acquired, the tumour bed formed using the clips, the seroma, and both the clips and seroma(defined as GTVC, GTVS and GTVC+S, respectively) were delineated by five radiation oncologists using specific guidelines. The following parameters were calculated to analyse interobserver variability: volume of the tumour bed(GTVC, GTVS and GTVC+S), coefficient of variation(COVC, COVS, COVC+S), and matching degree(MDC, MDS, MDC+S). The combined volume of the tumour bed(GTVC, GTVS and GTVC+S) delineated by one radiation oncologist on the 10 phases of 4DCT was defined as the internal gross target volume(termed IGTVC, IGTVS and IGTVC+S, respectively). Three treatment plans were established using the 4DCT end-inhalation(EI) images(termed EB-PBIC, EB-PBIS, EB-PBIC+S, respectively). The following parameters were calculated to analyse: volume of the target, homogeneity index(HI), conformal index(CI), and the dosimetric variance of the OAR. For each patient a conventional 3D conformal plan(3D-CRT) was generated based on GTVC+S on the EI phase of 4DCT. The treatment plan based on the 4DCT EI phase images was copied and applied to the end-exhalation phase(EE) and 3DCT images(defined as EB-PBIEI, EB-PBIEE, EB-PBI3 D, respectively).Results: Statistical significance was observed among the intraobserver on GTVC, GTVS and GTVC+S(p?0.05). The interobserver variability for GTVC and GTVC+S and for COVC and COVC+S were statistically significant(p?0.05). Significant differences in interobserver variability were observed for MDC vs MDS, MDC vs MDC+S, MDS vs MDC+S(p=0.000, 0.032, 0.008), the interobserver variability of MDS was smaller than that of MDC and MDC+S(MDS>MDC+S>MDC). The volume of IGTVC+S was significantly larger than that of IGTVC and IGTVS. Similarly, the volume of planning target volume(PTV) based on both clips and seroma was markedly larger than that of PTV based on clips and seroma. The EB-PBIS plan resulted in the lowest ipsilateral normal breast and ipsilateral lung doses compared with the EB-PBIC and EB-PBIC+S plans. There were no significant differences in the homogeneity index or conformity index between the three treatment plans(p=0.878, 0.086). The tumour bed volume based on 3DCT was significantly greater than that of EI and EE volumes(p= 0.002 each). The PTV coverage of EB-PBI3 D was significantly less than that of EB-PBIEI or EB-PBIEE(p=0.001 and p=0.025, respectively). However there were no significant differences in the HI or CI between the three treatment plans(p = 0.125 and p = 0.536, respectively). The EB-PBI3 D plan resulted in the largest organs at risk dose.Conclusions: When the SCS was 3~5 points and the number of clips?5 in the lumpectomy cavity based on 4DCT images, the interobserver variability was minimal for the delineation of the tumour bed based on seroma. The volume variability delineated based on clips, seroma or both clips and seroma did not show a marked influence on the dosimetric distribution, but resulted in dosimetric variability for organs at risk. There was a significant patient benefit when using 4DCT to plan 3D-CRT for EB-PBI with regard to reduced non-target organ exposure, and might result in poor dose coverage when the PTV is determined using 3DCT. |
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