Change of target volume and its dosimetric impact during the course of accelerated partial breast irradiation using intraoperative multicatheter interstitial brachytherapy after open cavity surgery

Purpose To investigate the change of clinical target volume (CTV) and its dosimetric impact during the course of accelerated partial breast irradiation (APBI) using intraoperative multicatheter interstitial brachytherapy after open cavity surgery. Methods and Materials Twenty-two patients of APBI with intraoperative placement of catheters underwent computed tomography scans for the treatment planning before the first (CT1) and the last (CT2) treatment fraction. Delineation of lumpectomy cavity and CTV was done consistently on both CT data sets by one of the coauthors. Optimum plan (P CT1 ) was made on CT1. P CT1 was manually reproduced in CT2 which yielded plan P CT2 . Plans were compared using coverage index (CI), dose homogeneity index (DHI), external volume index (EI), overdose volume index (OI) and conformal index (COIN). Results The mean ± SD volume of lumpectomy cavity and CTV was 78.5 ± 40.7 cm 3 , 156.4 ± 69.0 cm 3 for P CT1 , and 84.7 ± 50.1 cm 3 ( p = 0.11), 165.7 ± 82.8 cm 3 ( p = 0.15) for P CT2 , respectively. CTV volume increase by ≥ 10% was observed in 9 cases however decrease of ≥10% was observed in 5 cases. Mean (SD) of absolute pairwise difference in CTV volume was found to be 13.2 (6.7) %. For cases with increase in CTV volume, significant ( p p = 0.004) increase of 45% in EI was observed, whereas COIN reduced significantly ( p = 0.001) by 13.5%. Overall 22 cases showed significant decrease of 5.8% and 8.1% in mean CI and COIN, respectively. Conclusions The change of CTV during the course of APBI using intraoperative multicatheter interstitial brachytherapy after open cavity surgery was found patient specific and showed a significant impact on coverage and conformity.