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Assessment of Rotational Lung Tumor Motion and Its Influence On Treatment Margins for Stereotactic Body Radiosurgery (SBRT)

Q Xu

Q Xu1*, J Fan2, T LaCouture1, Y Chen1, (1) Cooper Medical Center, Mt Laurel, NJ, (2) Virtua Fox Chase Cancer Center, Philadelphia, PA

SU-E-J-214 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To quantify rotational motion of lung tumors under radiotherapy based on fiducial imaging. Tumor rotational motion in lung has not been well understood due to difficulties of imaging and target delineation. In this study the rotational motion of fiducial clusters were measured for assessing the treatment margins necessary for adequate dose coverage to CTV.

Methods:25 patients who underwent CyberKnife based SBRT were recruited. Three to five fiducials were implanted in or near the tumor. The reference fiducial locations were determined using a breath-hold CT. Orthogonal X-ray image pairs were acquired for modeling and tumor tracking during treatment. These images were used to reconstruct the fiducial locations in 3D. A rigid-body registration was derived between the measured and reference fiducial locations. The mean distance between the corresponding fiducial pairs was used to evaluate the registration. 2796 pairs of images in 106 fractions of treatment were analyzed.

The rotational motion of fiducial clusters was used to simulate the tumor rotational motion for assessing the adequacy of the PTV margins. A margin of 3mm was used to expand the upper lobe (UL) target and 5 mm for lower lobe (LL) target. Additional tracking errors were included in the analysis for tumor coverage under alignment of the rotated CTV with the PTV.

Results:The tumor rotational angles in LL and UL were 0.25°±5.7° vs 0.40°±2.1° in roll, -0.21°±7.3° vs 0.05°±1.8° in pitch, and 0.23°±5.3° vs. 0.1°±2.1° in yaw, respectively. In 94.4% (LL) and 97.1% (UL) of the total imaged tumor locations, the CTV was 100% covered by the corresponding PTV. The mean missing coverage of the CTV for the rest locations were of 4.4% and 1%, respectively.

Conclusion:The reported angles were highly correlated to the distance to the diaphragm. Appropriate margins need to be applied for different lobes to ensure CTV coverage.

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