# A Novel Markerless Lung Tumor-Tracking Method Using Treatment MV Beam Imaging
## 2 Materials and Method
The global NCC between the MV projection and the composite DRR is the cumulative NCC computed between the individual tile pairs:  
\[S=\ \frac{\sum_{u=1}^{U}\sum_{v=1}^{V}s(u,v)}{U\cdot V} \tag{2}\]  
The global NCC was maximized by adjusting the relative position of the tumor DRR with the background DRR window. The coordinates of the tumor DRR, fused onto a background that yields the highest similarity match between the global projection and the DRR, represents the tumor location on the projection. Different tiling matrix have been tested, and it was found that the global NCC could be increased when more tiles are used. As an example, the global NCC could be increased from 0.66 to 0.79 when a 2 \(\times\) 2 tiling is applied for identical geometric matching. However, when the tile size is too small, a mismatch of the tumor might occur. The smallest tile size should be larger than the tumor size. This study used a 2 \(\times\) 2 tiling technique. The proposed lung tumor-tracking algorithm was evaluated on a sophisticated respiratory torso phantom to determine its accuracy. A chest cavity phantom constructed with materials equivalent to natural bones and soft tissues was used. Tumors were simulated by two cylindrical bulks consisting of pliable bolus. Their volumes were 10.6 cm\({}^{3}\) and 6.9 cm\({}^{3}\), simulating a large and a small lung tumor, respectively. Two phantoms were configured with the large tumor or the small tumor at different locations. A tungsten sphere with a diameter of 2 mm was attached to the tumors. The tumors were fixed at one end of a Styrofoam rod and positioned on a computer-controlled moveable platform, such that the tumor and Styrofoam rod were independently movable inside the chest cavity of the phantom. Five tumor motion patterns were programmed to control a three-dimensional (3D) motion platform. They were all 3D sinusoidal respiratory motions with maximum motion ranges between 10 mm and 20 mm. All phantom studies were performed on a Truebeam linear accelerator (Varian Medical Systems, Palo Alto, CA, USA), equipped with a MV imager (EPID) with pixel sizes of 0.4 mm. MV treatment images were acquired continuously by an EPID in cine mode. Varian iTool (Varian Medical Systems, Palo Alto, CA, USA) was used to receive streaming MV BEV images instantly. Two 3D conformal radiation therapy plans were generated for corresponding phantoms/tumors. Each plan had nine treatment fields at different gantry angles. Each plan was delivered five times, while the tumor was driven by five different motion patterns. On average, there were about 170 images acquired for every treatment beam. Extra pre-processing was performed for the phantom study. This marker was detected by a pattern-matching algorithm in both CT images and MV images as ground truth of motions. The marker was removed from both images, and the holes were filled by expanding gradually from the edges of the holes. The markerless tumor tracking was then started on the modified images. BEV images of five lung cancer patients were acquired when patients were treated on a SynergyS (Elekta, Stockholm, Sweden) by stereotactic body radiation therapy (SBRT). The tumor average position and the cumulated probability within various ranges were studied to quantify tumor motion. The cumulated probability within a range, in a statistical manner, is the percentage of time when the tumor motion is equal to, or smaller than, a given range [20]. For an institutional protocol, an internal target volume (ITV) was contoured to include the motion of the tumor based on the 4D CT image set. The ITV is defined by the gross tumor volume (GTV) plus the tumor motion range, which was 5 mm according to the 4D CT results. An additional margin of 5 mm was added in all directions over the ITV resulting in the PTV. Hence, the contour of the PTV was similar to that of the GTV with an additional 10 mm margin. The cumulated probability within a range of 10 mm represents the ratio of time when the tumor is within the planning margin.