Description: Publication date: Available online 6 July 2018 Source: Radiotherapy and Oncology Author(s): Ritu Raj Upreti, Ashwini Budrukkar, Udita Upreti, Tabassum Wadasadawala, Shagun Misra, Lavanya Gurram, Rima Pathak, Deepak D. Deshpande 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(V 100 ), its spatial concordance(CI common ), 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(V 100_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( ρ  〈 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 V 100 , CI common of V 100 , 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.