ESTRO 36 Abstract Book

S393 ESTRO 36 _______________________________________________________________________________________________

Purpose or Objective The use of stereotactic body radiation therapy (SBRT) is increasing rapidly in patients with bone metastases. This technique involves high precision dose delivery, for which accurate gross tumor volume (GTV) contouring is crucial. This study compares inter-observer agreement in bone metastases delineated on CT, MR and CT with co- Twenty consecutive patients with bone metastases treated with SBRT were selected. All patients received CT and MR imaging in treatment position prior to radiotherapy. CT images were obtained with a Philips large bore CT scanner (1 mm slice thickness). A Philips 1.5 Tesla MRI scanner was used to acquire T1- and T2- weighted images in transversal direction. Coronal and/or sagittal images were acquired, including 3D T1FFE mDIXON scan (slice thickness 1.1mm) and diffusion weighted imaging (slice thickness 4 mm). Five observers from three institutions independently delineated GTV after a training set of two patients and a consensus meeting. First, GTV was delineated on CT images only. A second delineation was based on CT images with co-registered MR images. At least four weeks after the delineation on CT-MR combined, the GTV was contoured on MR imaging only. Average volumes of the contours per patient and imaging modality were calculated. The generalized conformity index (CI) was used to quantify inter-observer agreement. Significant differences between the average volumes and CI were analyzed by Wilcoxon signed rank test. Observer count maps were generated for visual comparison of agreement for each case and imaging modality. Results Mean GTV volume delineated on MR (43.4 ± 49.7 cm 3 ) was larger compared to CT-MR (40.2 ± 49.4 cm 3 ) and CT (34.8 ± 34.8 cm 3 ). Compared to CT, the mean volume of GTV was 11% larger on CT-MR and 35% on MR (Figure 1B). A large variation in CI was found in all imaging modalities: CT (range: 0.15-0.75), CT-MR (range: 0.17- 0.71) and MR (0.14-0.80). Mean CI were significantly higher on CT-MR compared to CT (Table 1). An example of a count map is shown (case 18, Figure 1B). For this case, mean volume of the GTV was almost doubled on MR compared to CT, which might be explained by better visibility of the extra-osseous disease on MR imaging. registered MR imaging. Material and Methods

Conclusion This multicenter contouring study demonstrated large inter-observer variation in GTV delineation for all investigated imaging modalities. Delineation of GTV on MR imaging resulted in larger volumes and marginal better inter-observer agreement compared to CT only delineations. These results suggest that future research should focus on guidelines to improve agreement on GTV delineation in these spine and non-spine bony metastases. PO-0747 Setting defaults in palliative radiation: a value-driven approach to improving care L. Puckett 1 , L. Lee 1 , I. Zhang 1 , P. Zuvic 1 , P. Gilbo 1 , L. Potters 1 , B. Bloom 1 1 Northwell Health / Hofstra Northwell School of Medicine, Radiation Medicine, Lake Success, USA Purpose or Objective Single fraction (fx) and hypofractionated (1 or 5 fx) radiation treatment (RT) provide superior value and reduced treatment length in the palliation of bone metastases. Despite data and recent guidelines recommending reduced treatment duration, there has been a slow adoption of this practice in the USA and worldwide. Previous examination of our academic and community multi-center practice from 2004 - 2016 revealed that single fx RT utilization has remained at 16% and hypofractionationed courses have remained at 72% since 2012. We hypothesized that enacting evidence- based, treatment-guiding defaults would further increase hypofractionation utilization. Material and Methods Institutionally, palliative bone metastasis treatments are monitored by our Quality Assurance (QA) committee. On 2/29/2016, two distinct consensus-driven and evidence- based clinical directives were created within our electronic health system for use with either simple or complicated bone metastasis, irrespective of primary histology. The simple and complex treatment directives had default prescriptions of 8 Gy/1fx or 20 Gy/5fx,