Every Breath You Take – Evaluating Respiratory Motion of the Bony Thorax in the Context of Bone SBRT

Abstract

Stereotactic Body Radiation Therapy (SBRT) delivers highly conformal treatment in a single or few fractions of high-dose radiation yielding excellent local control rates with minimal toxicity. There is growing interest in using SBRT to treat oligometastases, which potentially includes any anatomical site where metastases can occur. Bone is one of the most common sites of metastatic disease and although methodology to treat SBRT spine is well established, very little is known about the technical considerations of bone SBRT, particularly regarding respiratory motion in the bony thorax. The purpose of this study was to evaluate respiratory motion in thoracic bone metastases and to determine if the motion differs based on anatomic location. The first 70 patients treated with thoracic bone SBRT planned with four-dimensional computed tomography (4DCT) were identified from a prospective institutional database. To quantify motion, a region of interest representative of the treatment target was contoured by a single user in Pinnacle on the 0% (inhale) and 50% (exhale) datasets using fixed autocontour threshold values to eliminate subjectivity. A point of interest was automatically placed in the centroid of each volume and the relative positional difference of these points was calculated to measure the lateral (LR), anterior/posterior (AP) and superior/inferior (SI) motion. A total linear distance (LD) vector was calculated from these values. Rib lesions were further categorized as anterior, lateral or posterior based on axial position as well as superior (1-4), middle (5-7) and inferior (8-12) depending on rib location. Rib motion was analyzed using analysis of variance (ANOVA), if it was significant (p<0.05) a post hoc analysis was done to evaluate pairwise comparisons amongst the rib location groups. There were 15/16 sternal lesions assessed as 1 patient had significant bone destruction and could not be reliably contoured. Mean (range) LR, AP and SI sternum motion was 0.2 mm (0-0.7), 1.2 mm (0-2.6) and 0.1 mm (0-0.3) respectively and the LD was 1.2 mm (0-2.6). Forty-seven rib lesions were assessed for motion; 7 were excluded due to bone destruction preventing use of auto contour tool. The mean (range) LR, AP and SI motion for all rib lesions was 1.1 mm (0-4.0), 2.4 mm (0-8.4) and 0.1 mm (0-0.8) respectively and LD was 2.8 mm (0.1-8.5). When categorized by rib location, there were 9 superiorly, 22 middle and 16 inferior located rib lesions. The mean LD was 3.2 mm, 3.5 mm and 1.6 mm for superior, middle and inferior lesions respectively. There was a significant difference between the groups, with the inferiorly located moving less in terms of AP motion (p=0.02) and LD (p=0.04). In terms of axial position, there were 6 anterior lesions, 27 lateral lesions and 14 posterior lesions. The mean LD was 3.6 mm, 3.3 mm and 1.3 mm for anterior, lateral and posterior lesions respectively. There was a significantly less motion for posteriorly located lesions in terms of AP motion (p=0.005) and LD (p=0.02). This data suggests that the respiratory motion of the bony thorax is variable but not trivial in the context of high precision radiation therapy techniques with small margins. It appears that lesions located in the sternum and posterior and/or inferior aspect of the ribs move less; however, contouring guidelines, margins and image guidance technique need to be strongly considered before eliminating motion assessment when planning thoracic bone SBRT.