(Sunday, 7/12/2020) [Eastern Time (GMT-4)]
Room: AAPM ePoster Library
Purpose: To compare the setup accuracy of a Surface Imaging System (AlignRT) and kV Images (standard of care) vs. CBCT in left breast DIBH radiotherapy.
Methods: Ten locally-advanced left breast patients treated with VMAT were evaluated in 25 fractions. Weekly AlignRT surface images, CBCT, and 2DkV AP/LAT images (n=5) were acquired in DIBH. AlignRT was registered to planning CT by matching a breast region of interest (ROI, covering the left breast and 1/3 of the contralateral breast) on patient’s surface, 2DkV images were registered to planning DRRs and CBCT to planning CT by matching bony anatomy (chest wall and sternum). The registration provided couch shifts for each imaging modality. Using CBCT as the reference, the differences in couch shifts using AlignRT and kV imaging were compared. A second AlignRT image was acquired during last beam delivery to evaluate intrafraction motion.
Results: Both AlignRT and kV images achieved clinical acceptable accuracy in most patients, with absolute mean difference in translations <5.0 mm and rotations <2.0° from CBCT. However, differences up to 9.0 mm (2 patients) and 5.0° (2 patients) were observed, reflecting poor correlation between surface and anterior bony anatomy position. The mean absolute translational differences in reference to CBCT were (VRT=3.2mm, LNG =3.9mm, LAT=3.3mm) for AlignRT and (VRT=3.5mm, LNG=4.7mm, LAT=2.3mm) for 2DkV imaging, with p-values of (0.59, 0.41, 0.02), respectively. The mean absolute rotational differences were (RTN=0.9°, ROL=1.4°, PIT=1.4°) for AlignRT and (RTN=0.9°, ROL=1.2°, PIT=1.1°) for 2DkV imaging, with p-values of (0.76, 0.07, 0.26), respectively. 2DkV images have systematic bias of VRT=-2.1mm and LNG=-2.4mm, likely because the acquisition was triggered at the lower DIBH gating threshold. The intrafraction motion in DIBH monitored by AlignRT was small with absolute mean translations <1.7mm and rotations <0.7°.
Conclusion: AlignRT provides similar setup accuracy as kV images for VMAT breast DIBH radiotherapy.
Funding Support, Disclosures, and Conflict of Interest: This work was supported in part through the NIH/NCI Cancer Center Support Grant P30 CA008748.
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