ESTRO 38 Abstract book

S1172 ESTRO 38

uncertainty was computed as the mean DICE over all raters for each organ and patient. Statistical analysis included paired t-testing to evaluate differences between visualization of organs on the planning CT and corresponding kV CBCT images. Results Eighteen organs-at-risk were evaluated across seven patients; a total of 52 kV CBCTs were acquired and contoured by each rater for comparison. The median DICE were 0.85 +/- 0.21 (planning CT), 0.82 +/- 0.11 (breath hold CBCT), and 0.84 +/- 0.11 (free breathing CBCT). Neither breath hold (p=0.14) nor free breathing kV CBCT (p=0.88) were statistically significantly different from planning CT for the inter-rater delineation variability across all organs-at-risk and patients. Limiting to a selection of abdominal organs only, the median DICE values were 0.93 +/- 0.07 (planning CT), 0.93 +/- 0.10 (breath hold CBCT, p=0.53), and 0.92 +/- 0.16 (free breath CBCT, p=0.15). Conclusion Inter-rater ability to delineate organs-at-risk was not statistically different between images acquired on a novel ring gantry kV CBCT unit versus diagnostic quality simulation CT images. Prospective evaluation of the utility of kV CBCT imaging to enable online adaptation applications is. EP-2118 Effects of interfraction uncertainty with Strut Adjusted Volume Implant applicator K. Miyaura 1 , T. Fujii 2 , T. Kubo 2 , H. Shinjoh 1 , M. Kato 1 , K. Toyofuku 1 , A. Niiya 1 , R. Kobayashi 1 , Y. Ozawa 1 , K. Murakami 1 , M. Morota 1 , Y. Ito 1 , A. Imai 1 , Y. Kagami 1 1 Showa University, Radiation oncology, Tokyo, Japan; 2 Showa University, Deportment of Radiological technology, Tokyo, Japan Purpose or Objective We started accelerated partial breast irradiation (APBI) after breast conservation surgery using Strut Adjusted Volume Implant (SAVI) from March 2014. As the SAVI applicator does not displace after implantation, planning only at treatment start. However, slight displacement of the SAVI applicator and its influence on the DVH parameter may be considered. We examined changes in the interfraction uncertainty and DVH parameters of the SAVI applicator using daily CT. Material and Methods In this study, we examined 50 patients. We use checking CT for each treatment to confirm that the SAVI applicator is not displaced. Therefore, after contouring the objects to CT, imprinting of the initial plan was performed, and the change of the DVH parameter due to the interfraction uncertainty of the applicator was calculated. The region of interest to be evaluated is a PTV = "PTV_EVAL" for dose assessment excluding the SAVI catheter, adding 2 mm from the skin side and 0 mm from the chest wall, adding a 3-dimensional margin of 10 mm to the SAVI catheter, Skin = "SKIN", chest wall = "CHESTWALL". Results The average coefficient of variation was "PTV_EVAL" V 90%: 1.0%, V 150%: 7.2%, V 200%: 8.1%, "SKIN" D 1 cm 3 : 5.6%, "CHEST WALL" D 1 cm 3 : 4.1%. In "PTV_EVAL", the variation of V90% is small, and the influence by interfraction uncertainty is considered to be small. However, the variation was increased at a higher dose region. "SKIN" was more variant than "CHESTWALL". Conclusion E-posters Brachytherapy Electronic Poster: Brachytherapy: Breast

Conclusion The ACDS is developing a comprehensive audit for SABR treatments. Large discrepancies between calculated and measured dose in different medium such as bone are to be further explored. References/ Acknowledgements 1. Guidelines for safe practice of stereotactic body (ablative) radiation therapy. Faculty of Radiation Oncology, The Royal Australian and New Zealand College of Radiologists. (2015) 2. Solberg TD, Balter JM, Benedict SH et al. Quality and Safety Considerations in Stereotactic Radiosurgery and Stereotactic Body Radiation Therapy. PRO (2011) 2(1), p2-9. 3. Stereotactic Ablative Body Radiation Therapy (SABR): A resource. UK SABR Consortium. Version 4.0 (2013). EP-2117 Novel kV CBCT Imager on Ring Gantry Radiotherapy Unit Permits High Inter-rater Contour Uniformity L. Henke 1 , B. Cai 1 , S. Rudra 1 , B. Fischer-Valuck 1 , P. Gabani 1 , P. Samson 1 , A. Srivastava 1 , M. Roach 1 , E. Laugeman 1 , J. Luo 1 , S. Mutic 1 , G. Hugo 1 , H. Kim 1 1 Washington University/Barnes Jewish Hospital, Radiation Oncology, Saint Louis- MO, USA Purpose or Objective A novel kV imaging system coupled with a ring gantry radiation treatment system is now commercially available. Improved on-board kV CBCT acquisition time (17-40 seconds) and image quality using this device may allow online adaptive radiotherapy. The purpose of this work was to evaluate the image quality of the on-board kV CBCT as defined by inter-rater contour variability across a spectrum of anatomical structures. Material and Methods Seven patients undergoing routine external beam radiation therapy were enrolled on an IRB-approved, prospective imaging study. Patients were imaged with the ring gantry kV CBCT system at breath hold and/or free breathing. Imaged sites included the head & neck, thorax, abdomen, and pelvis. Patient anatomy was independently contoured by 7 radiation oncology physicians on both the original simulation CT (diagnostic quality) and kV CBCT images. Sequence of contouring was randomized across raters to minimize bias and all kV CBCT images were contoured prior to diagnostic scans to mitigate physician learning of patient anatomy. Inter-rater contour variability was assessed by computing a consensus contour with STAPLE and computing DICE between each individual rater contour and the consensus contour. Inter-rater