Abstract
Purpose: The ability to produce precisely targeted beams as small as 1 mm necessitates the understanding and management of intra‐fraction motion. This study evaluated lung motion in free‐breathing mice and compared free‐breathing imaging (3D and 4D reconstruction) to breath‐hold imaging for use in treatment planning. Methods: Five mice were imaged weekly for six weeks using the X‐RAD 225Cx system. Each week, CBCT projections were acquired during free‐breathing imaging under anesthesia and reconstructed into 3D and 4D images (6 phases). Superior‐inferior, anterior‐posterior, and right‐left motion were evaluated using deformable registration of the 4D images, and confirmed by manual measurements on fluoroscopic images. Next, the mice were intubated and their breath was held at full‐inhale for 20 seconds during image acquisition. Breath‐hold scans from the same session were compared to assess reproducibility. Results: The average voxel motion in the lungs of free‐breathing mice was 1.3 mm (stdev = 0.2mm) while the average maximum motion was 3.4 mm (stdev = 0.3mm). To ensure tumor coverage in free‐breathing mice we can apply the ITV concept to 3D‐CBCTs, using added margins defined by the 4D‐CBCT image sets. However, in an area of maximum motion, the expanded target volume could cover 30% the length of the lung. Adding this margin could Result in substantial normal tissue toxicity. Breath‐hold imaging was reproducible to within 0.6 mm except in areas close to the heart due to cardiac motion. A visual comparison of image quality found that the breath‐hold images were substantially sharper than both the 3D and 4D free‐breathing images due to blurring and reconstruction artifacts. Conclusion: Given the large respiratory motion relative to lung size in mice, breath‐hold imaging offers clear advantages in motion management and image quality. Breath‐hold treatments using IGRT are feasible and are recommended in cases of large tumor motion.
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