International Consensus Contouring Recommendations for Stereotactic Body Radiotherapy of Non-Spine Bone Metastases

Abstract

Despite the increasing use of stereotactic body radiotherapy (SBRT) for non-spine bone metastases (NSBM), there are no established guidelines for clinical target volume (CTV) delineation. The objective of this study was to provide recommendations on CTV delineation based on international expert consensus contours. Eleven cases were contoured by nine international radiation oncologists with demonstrated expertise in NSBM SBRT via publications and/or clinical experience. Cases included metastases in long bones (femur, humerus), pelvic bones (ilium, ischium, acetabulum, pubic symphysis) and thoracic bones (rib, sternum, scapula, clavicle). For each case the gross tumor volume (GTV) was provided on the simulation CT scans, which were accompanied by corresponding MR imaging. Experts then contoured the CTV and completed a clinical survey. Agreement between expert CTV contours was analyzed with simultaneous truth and performance level estimation (STAPLE) using the kappa coefficient and with the Dice similarity coefficient (DSC), and summarized to establish contouring recommendations. All CTV contours were completed for each of the eleven cases. Six experts used a single dose level, while 3 used a two-dose level (high dose and low dose) simultaneous integrated boost (SIB) technique. For the SIB cases, only the high dose CTV was used for contour analysis. There was substantial agreement between expert contours across cases with a mean and median kappa measure of 0.72 and 0.71 (range, 0.61-0.86), respectively. The mean and median DSC values were 0.77 and 0.79 (range, 0.67-0.87), respectively. Consensus CTV contouring recommendations consist of: 1) A ≤5mm intraosseous ± ≤5mm extraosseous margin beyond the GTV; 2) An extraosseous CTV margin should be strongly considered in cases of associated soft tissue disease and/or significant cortical bone disruption; 3) All CTVs should be manually modified to spare uninvolved joint spaces and organs-at-risk while respecting natural anatomical barriers (e.g. peritoneal cavity, pleura). Consensus CTV contouring recommendations for NSBM SBRT were established based on analysis of international expert consensus contours with a high level of agreement. Detailed quantitative margin analysis in three-dimensions is ongoing to define the optimal extent of CTV margins. Until prospective clinical data are available to inform patterns of failure, these recommendations may serve as guiding principles for treating physicians.