# A Novel Markerless Lung Tumor-Tracking Method Using Treatment MV Beam Imaging
## 2 Materials and Method  
Continuous MV projection images were acquired in beam's eye view (BEV) by an electronic portal imaging device (EPID) during radiation therapy treatment. Figure 1 illustrates the geometry of the imaging system. A 2-dimensional (2D) axial CT slice represents a patient. Typically, a patient is positioned head first and facing up while lying on a treatment couch to locate the treatment target at the isocenter of a linear accelerator. Both an X-ray source and an EPID imager are mounted on a gantry which rotates around a horizontal axis corresponding to the patient's superior-inferior (SI) or head-foot direction. At any gantry angle (GA), a central radiation beam passes through the isocenter. The imager has two dimensions. As shown in Figure 1, at any gantry angle, one axis of the imager always coincides with the gantry rotation axis, the SI direction. The other orthogonal (Orth) component changes in space depending on the gantry angle. The Orth direction coincides with the patient's anterior-posterior (AP) direction at GA = 270\({}^{\circ}\) or lateral direction at GA = 0\({}^{\circ}\). Normally, the most significant component of lung tumor motions occurs in the SI direction. However, depending on the location of the tumor within the thorax, the AP or lateral motion contributions may be more pronounced than the SI direction. An overview of the markerless tiling technique flow chart is illustrated in Figure 2. Planning kilovoltage (kV) CT images was converted to MV CT image by introducing a kV-MVCT corresponding curve in order to match the digitally reconstructed radiographs (DRR) with MV treatment EPID images. The fitting corresponding curve was obtained by scanning the same CT electron density phantom with a kV CT scanner and an MV CT scanner (Tomotherapy, Accuray Inc., Sunnyvale, CA, USA). During a conventional treatment planning process, tumors and surrounding critical anatomical structures were contoured by physicians on the basis of the maximum intensity projection (MIP) images from the 4D CT scan data. MIPs provide an overall moving envelope of tumors and other regions of interest (ROIs) but do not give an exact snapshot at a particular timestamp. Lung tumors were removed from the anatomy, and the cavities were filled with tissues with averaged lung tissue CT numbers. The complete anatomy was divided into two separate parts, i.e., the tumor and the other structures. Two sets of DRRs were generated from the converted MV CT images using a ray-tracing  
Figure 1: Schematic diagram of the imaging geometry. The X-ray source and imager are mounted on a gantry rotating around a horizontal axis, patient’s superior–inferior (SI) direction. X-ray source and imager at gantry angle (GA) of 0\({}^{\circ}\) and 270\({}^{\circ}\) are superimposed. The lung tumor is expanded to a planning target volume (PTV) as demonstrated. The projection image of both PTV and tumor is illustrated on the imager. Imager has two axes: the superior–inferior (SI) direction always coincides with the patient’s SI axis, while the Orth axis changes in a 3D space with gantry rotation. Figure 2: Flow chart of the markerless lung tumor detecting algorithm. algorithm, which simply integrates the CT numbers along the X-ray trace from the X-ray source point to every imager pixel. For every MV projection image acquired, a pair of DRRs (background DRR consisting of the surrounding ROI with no tumor and tumor DRR consisting of only the tumor, as illustrated in Figure 3a,b) were generated. The same geometric configurations as those for acquiring MV projections after a pre-process were used to detect and remove the boundary edges (as illustrated in Figure 3c). A composite DRR was formed by a superimposition of the tumor DRR on a similar sized window of the background DRR with individual positions, as illustrated in Figure 3d.