SU‐FF‐I‐17: Reduction of Set‐Up Error Using Cone Beam‐CT in Patients Undergoing Partial Breast Irradiation
Purpose: Partial breast irradiation with multiple external beams requires accurate alignment of the target volume and treatment isocenter. On‐board plannar kV/MV imaging can verify set‐up based on boney landmarks. On‐board cone beam CT (CBCT) may provide additional soft‐tissue‐based information to further improve set‐up accuracy. In this study, we assess the utility of CBCT in patients receiving partial breast radiation therapy, who have been aligned with on‐board plannar kV/MV imaging. Method and Materials: Patients undergoing partial breast irradiation were imaged using an on‐board‐imager attached to the gantry of a Varian 21EX machine. Each patient was aligned to skin marks in the treatment position. Orthogonal kV images were acquired and registered to digitally reconstructed radiographs from the planning CT. Subsequently, a CBCT image data set was acquired and compared to the planning CT using both boney anatomy and soft tissue information which yielded estimations of residual setup error. Results: 10 patients were studied under an IRB protocol. Each patient had an average acquisition of 9 pre‐RT CBCT images. After 2D kV image registration based on boney anatomy, the average soft tissue residual error, based on the CBCT vs. planning CT, was 4, 2, and 3 mm in the Ant/Post, Sup/Inf, and Rt/Lt directions respectively. The ranges were 0–15, 0–9, and 0–7 mm, respectively. In one case, 30% of the planning tumor volume (PTV) would have been outside of the therapeutic isodose volume based on the pre‐RT CBCT images. Conclusion: CBCT is a practical tool for image registration while visualizing the soft tissue of the tumor bed. It provides additional anatomic information and soft‐tissue detail beyond that provided by planar radiographs. Such increased accuracy can significantly improve dosimetric coverage of the planning target volume. Conflict of Interest: Partially supported by a Varian research grant. © 2006, American Association of Physicists in Medicine. All rights reserved.
Fatunase, T; Wang, Z; Yoo, S; Yin, F; Marks, L
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