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Setup Uncertainties in Locally Advanced Breast Cancer Patients Undergoing Regional Nodal Irradiation

Y Zhang

Y Zhang*, D Brinkmann , R Mutter , S Park , E Yan , D Pafundi , N Remmes , Mayo Clinic, Rochester, MN


PO-BPC-Exhibit Hall-24 (Saturday, March 5, 2016)  Room: Exhibit Hall

Purpose: To evaluate the setup uncertainties of the regional nodal stations using on-board kV imaging (OBI) when compared with the gold standard cone beam CT (CBCT)
Methods: 20 consecutive patients undergoing postmastectomy (n=14) or postlumpectomy (n=6) radiotherapy who required regional nodal irradiation were enrolled. The clinical target volume (CTV) included the breast/chestwall and regional lymph nodes (axillary, infraclavicular, supraclavicular and internal mammary nodal regions) and was treated using either free breathing (n= 9) or breath hold (n=11) technique. Daily patient setup was based on OBI matching to the ipsilateral anterior ribs and sternum. In order to encompass the entire CTV (20-28cm in the S/I direction), a multi-scan CBCT, consisting of a superior and an inferior scans, were acquired at the treatment position and stitched offline for assessment of setup reproducibility. Offline matching to the breast/chest wall CTV or the regional node CTV was performed on the stitched volume and setup uncertainties were represented by the shift differences between the CTV matches and the treated bony anatomy OBI match.
Results: A total of 168 CBCT sets were available for the analysis. Setup uncertainties (mean +/- SD) on the regional node CTV were -0.2mm+/-2.2mm, -0.3mm+/-4.0mm, and 0.3mm+/-2.3mm in the L/R, A/P and S/I directions, respectively. Setup uncertainties on the breast/chestwall CTV were 0.4mm+/-2.6mm, 0.5mm+/-3.0mm, and -0.7mm+/-3.6mm in each direction, respectively. Setup uncertainties were not significantly different for patients treated with breath hold compared to free breathing (p>0.1). Similarly, there was no significant difference in setup uncertainties between patients with intact/reconstructed breasts compared to nonreconstructed chest walls (p>0.1).
Conclusion: OBI matching to chest wall bony landmarks provides an acceptable localization surrogate for both the regional node CTV and the breast/chestwall CTV in lieu of daily CBCT. Daily OBI instead of CBCT also reduces radiation dose to the patient.

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