An Efficiency and Feasibility Trial Comparing Novel Contouring and Dosimetric Techniques in 4DCT Respiratory Gating Planning for SBRT in NSCLC

Abstract

The gold standard for tumor delineation in free-breathing SBRT for NSCLC at our facility is a 10-phase binning method to produce each patient’s internal tumor volumes (ITV). We attempted to assess the reproducibility of tumor volume and location in three dimensions when contouring lung tumors with a single ITV (ITVc) on a 4DCT image set showing the extent of tumor displacement with active, cinematic respiratory motion. This novel technique would, in theory, provide a ten-fold decrease in the required total number of tumor volumes to be contoured. Therefore, we also evaluated for any variances in efficiency in regards to contouring time and workflow between these two techniques. Our endpoints were reproducibility of tumor volumes and locations, as well as noninferiority in workflow efficiency. 25 patients (5 patients with tumors in each of the 5 lobes) were selected from patients treated at RPCI. Each patient underwent a 4DCT simulation that provided image sets of ten sequential phases of a complete respiratory cycle. The phases each represented approximately 10% of a complete inhalation and exhalation. The cinematic image set is a compilation of the ten phase-based scans on which contours can be drawn while actively viewing or scrolling through the respiratory cycle. The contrast was set to 240 HU and -160 HU, which allowed for the clearest image on all the phases. Each tumor was contoured as ten separate respiratory-phase specific volumes that were then binned together as an ITV (ITV10) and was then contoured as a single ITV (ITVc) using the cinematic image set. Both techniques were then compared with respect to their volumes as well as to their center of mass and lobe/location. Time spent to complete each method was also recorded and contrasted. Only 8 of the 25 evaluated cases ended up with an acceptable (<5%) difference from the standard 10-phases ITV [range 1.3%-24.4%; median 9.9%]. However, these are very small volumes making even a small change result in large change in percentage. No correlation, either on the geographical location of the tumor or on the tumor volume, could be identified that explains why excessive deviation occurred in certain patients and not in others. The mean difference between the centers of mass was relatively small for tumors in all lobes except the left lower lobe (LLL). The LLL tumor experienced more deviation between the centers of mass, but the difference was still small (0.8 mm). While the ITVc method of tumor volume delineation offers substantial gain in workflow efficiency, variance in volumes compared to the current standard are too significant to report noninferiority. A larger sample size is needed to assess the accuracy of this method. This study is being repeated with additional researchers and clinicians providing contours, as results can rely heavily on user-specific techniques and skill.