Impact of inter-observer variations in target volume delineation on dose volume indices for accelerated partial breast irradiation with multicatheter interstitial brachytherapy

Purpose: To investigate dosimetric impact of inter-observer variation in clinical target volume(CTV) delineation for patients undergoing interstitial partial breast brachytherapy. Methods: Five radiation oncologists delineated CTV in twenty patients who underwent multi-catheter partial breast brachytherapy. Five treatment plans for each patient were graphically optimized for CTV of all observers and evaluated using coverage index(CI), external volume index(EI), overdose volume index(OI) and conformal index(COIN). In addition, volume enclosed by prescription isodose(V100), its spatial concordance(CIcommon), mean coverage of all CTVs with common volume of prescription dose (V-100_common) and mean CTV coverage for all pairs of observer with common prescription volume of respective pairs(V100_ pair) were also computed. Results: The mean +/- standard deviation(SD) of CI and COIN ranged from 0.756 +/- 0.076 to 0.840 +/- 0.070 and 0.591 +/- 0.090 to 0.673 +/- 0.06 respectively. When a plan made for CTV of individual observer was evaluated on CTV of all observers, the maximum variations(q < 0.05) in the mean CI, COIN, OI and EI were 10.6%, 11.4%, 10.6% and 72.7% respectively. The observed mean +/- SD of V100, CIcommon of V100, CTV coverage with V-100_common and V-100_pair was 160.7 +/- 52.1, 0.70 +/- 0.09, 73.1 +/- 8.1% and 77.9 +/- 7.3% respectively. Conclusion: Inter-observer variation in delineation of CTV showed significant dosimetric impact with mean CTV coverage of 73.1% and 77.9% by common and paired prescription dose volume respectively among all observers. (C) 2018 Elsevier B. V. All rights reserved. Radiotherapy and Oncology