Impact of the Delineation Accuracy of Lumpectomy Cavity on Planning Target Volume in Partial-Breast Irradiation Using Brachytherapy and External-Beam Radiotherapy

Background: In partial-breast irradiation (PBI), the accurate target volume delineation based on the lumpectomy cavity (LC) remained difficult due to uncertain LC identification. However, the impact of accurate LC delineation on the planning target volume (PTV) has not been investigated. Material and Methods: Between September 2018 and April 2020, 159 patients receiving perioperative PBI with multicather-interstitial brachytherapy have been evaluated. While LC delineated using implanted catheters as fiducial markers was used as a reference, conventional LC was virtually delineated on computed tomography with clips. PTV1-cm margin and PTV2-cm margin, which means 1cm and 2cm expansion from LC, were developed and assumed for brachytherapy and external-beam PBI, respectively. The target accuracy and the impact of the delineation accuracy of LC on PTVs were evaluated. The geographic miss index (GMI) and normal tissue index (NTI) were used as accuracy indices and were defined as the percentage of under- and overestimating volume, respectively. Results: The PTV1-cm margin and PTV2-cm margin were significantly larger than the reference volume, 57.9cm3 vs. 37.9cm3 (P<0.001) and 113.2cm3 vs. 93.8cm3 (P<0.001), respectively. The GMI and NTI of LC were 27.3% and 41.2%, respectively. Although the GMI in the PTV1-cm margin and PTV2-cm margin was significantly reduced to 9.8% (P<0.0001) and 9.9% (P<0.0001), respectively, the NTI was not significantly improved in the PTV1-cm margin, which was 41.8% (P=0.60) but was improved in PTV2-cm margin, which was 23.1% (P<0.0001). Conclusion: The GMI in PTV1-cm margin was reduced to be as low as PTV2-cm margin. Although PTV2-cm margin was associated with lower NTI, the absolute volume was almost double with PTV1-cm margin. Although further research was required, brachytherapy-based PBI may be a reasonable option to achieve tumor control and cosmesis using conventional delineation method.