SU‐G‐BRA‐04: Simulation of Errors in Maximal Intensity Projection (MIP)‐Based Lung Tumor Internal Target Volumes (ITV) Using Real‐Time 2D MRI and Deformable Image Registration Based Lung Tumor Tracking

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Purpose:To evaluate the effect of inter‐ and intra‐fractional tumor motion on the error in four‐dimensional computed tomography (4DCT) maximal intensity projection (MIP)–based lung tumor internal target volumes (ITV), using deformable image registration of real‐time 2D‐sagital cine‐mode MRI acquired during lung SBRT treatments.Methods:Five lung tumor patients underwent free breathing SBRT treatment on the ViewRay, with dose prescribed to PTV (4DCT MIP‐based ITV+3–6mm margin). Sagittal slice cine‐MR images (3.5×3.5mm pixels) were acquired through the center of the tumor at 4 frames per second throughout the treatments (3–4 fractions of 21–32 minutes duration). Tumor GTVs were contoured on the first frame of the cine and tracked throughout the treatment using off‐line optical‐flow based deformable registration implemented on a GPU cluster. Pseudo‐4DCT MIP‐based ITVs were generated from MIPs of the deformed GTV contours limited to short segments of image data. All possible pseudo‐4DCT MIP‐based ITV volumes were generated with 1s resolution and compared to the ITV volume of the entire treatment course. Varying pseudo‐4DCT durations from 10‐50s were analyzed.Results:Tumors were covered in their entirety by PTV in the patients analysed here. However, pseudo‐4DCT based ITV volumes were observed that were as small as 29% of the entire treatment‐ITV, depending on breathing irregularity and the duration of pseudo‐4DCT. With an increase in duration of pseudo‐4DCT from 10–50s the minimum volume acquired from 95% of all pseudo‐4DCTs increased from 62%–81% of the treatment ITV.Conclusion:A 4DCT MIP‐based ITV offers a ‘snap‐shot’ of breathing motion for the brief period of time the tumor is imaged on a specific day. Real time MRI over prolonged periods of time and over multiple treatment fractions shows that the accuracy of this snap‐shot varies according to inter‐ and intra‐fractional tumor motion. Further work is required to investigate the dosimetric effect of these results.