Purpose: To investigate the effects of breathing variation on gating window internal target volume (ITVGW) in respiratory gated radiation therapy. Method and Materials: Two-dimensional dynamic MRI (dMRI) of lung motion was acquired in ten volunteers and eight lung cancer patients. Resorted dMRI using 4DCT acquisition method (RedCAM) was generated for selected subjects by simulating the image rebinning process. A dynamic software generated phantom (dSGP) was created by moving a solid circle (to mimic the "tumor") with dMRI-determined motion trajectories. The gating window internal target area (ITAGW, 2D counterpart of ITVGW) was determined from both RedCAM and dSGP/dMRI. Its area (A), major axis (L1), minor axis (L2), and similarity (S) were calculated and compared. Results: In the phantom study of 3 cm tumor, measurements of the ITAGW from dSGP (A=10.0±1.3 cm2, L 1=3.8±0.4 cm, and L2=3.3±0.1 cm) are significantly (p<0.001) greater than those from RedCAM (A=8.5±0.7 cm2, L 1=3.5±0.2 cm, and L 2=3.1±0.1 cm). Similarly, the differences are significantly greater (p<0.001) for the 1 cm tumor (A=1.9±0.5 cm2, L 1=1.9±0.4 cm, and L 2=1.3±0.1 cm in dSGP; A=1.3±0.1 cm2, L 1=1.5±0.2 cm, and L 2=1.1±0.1 cm in RedCAM). In patient studies, measurements of the ITA GW from dMRI (A=15.5±8.2 cm2, L 1=5.0±1.1 cm, and L 2=3.8±1.2 cm) are also significantly greater (p<0.05) than those from RedCAM (A=13.2±8.5 cm2, L 1=4.3±1.4 cm, and L 2=3.7±1.2 cm). Similarities were 0.9±0.1, 0.8±0.1, and 0.8±0.1 in the 3 cm tumor phantom, 1 cm tumor phantom, and patient studies, respectively. Conclusion: ITVGW can be underestimated by 4DCT due to breathing variations. An additional margin may be needed to account for this potential error in generating a PTVGW. Cautions need to be taken when generating ITVGW from 4DCT in respiratory gated radiation therapy, especially for small tumors (<3 cm) with a large motion range (>1 cm).
- dynamic MRI
- internal target volume
- residual tumor motion
- respiratory gated radiation therapy
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging