A study of the dosimetric impact of daily setup variations measured with cone‐beam CT on three‐dimensional conformal radiotherapy for early‐stage breast cancer delivered in the prone position

• Type of Material: Article
• Authors: Annie Xiao, The University of Chicago; Jessica Jutzy, The University of Chicago; Greg Hubert, The University of Minnesota; Meghan Edens, The University of Chicago; Maxine Washington, Emory University; Yasmin Hasan, The University of Chicago; Steven J. Chmura, The University of Chicago; Hania A. Al-Hallaq, The University of Chicago
• Language: English
• Date: 2020-12-01
• Publisher: The American College of Medical Physics and American Institute of Physics
• Publication Version: Final Published Version
• Copyright Statement: © 2020 The Authors.
• License: Creative Commons Attribution 4.0 International
• Final Published Version (URL): http://dx.doi.org/10.1002/acm2.13080
• Title of Journal or Parent Work: Journal of Applied Clinical Medical Physics
• Volume: 21
• Issue: 12
• Start Page: 146
• End Page: 154
• Grant/Funding Information: This work was presented in part at the American Association of Physicists in Medicine (AAPM) annual meeting in Nashville, TN in 2018. Support for institutional review board approval was provided by the University of Chicago Comprehensive Cancer Center support grant P30 CA014599.
• Keywords: setup margins; breast cancer; cone‐beam CT; prone positioning; whole‐breast radiotherapy
• Research Categories: Health Sciences, Medicine and Surgery; Health Sciences, Oncology; Health Sciences, Radiology

Abstract

Purpose To evaluate the dosimetric impact of daily positioning variations measured with cone‐beam computed tomography (CBCT) on whole‐breast radiotherapy patients treated in the prone position. Methods Daily CBCT was prospectively acquired for 30 consecutive patients positioned prone. Treatment for early‐stage (≤II) breast cancer was prescribed with standard dose (50 Gy/25 fractions) or hypofractionation (42.56 Gy/16 fractions) for 13 and 17 patients, respectively. Systematic and random errors were calculated from the translational CBCT shifts and used to determine population‐based setup margins. Mean translations (±one standard deviation) for each patient were used to simulate the dosimetric impact on targets (PTV_eval and lumpectomy cavity), heart, and lung. Paired Student’s t tests at α = 0.01 were used to compare dose metrics after correction for multiple testing (P < 0.002). Significant correlation coefficients were used to identify associations (P < 0.01). Results Of 597 total fractions, 20 ± 13% required patient rotation. Mean translations were 0.29 ± 0.27 cm, 0.41 ± 0.34 cm, and 0.48 ± 0.33 cm in the anterior–posterior, superior–inferior, and lateral directions leading to calculated setup margins of 0.63, 0.88, and 1.10 cm, respectively. Average three‐dimensional (3D) shifts correlated with the maximum distance of breast tissue from the sternum (r = 0.62) but not with body‐mass index. Simulated shifts showed significant, but minor, changes in dose metrics for PTV_eval, lung, and heart. For left‐sided treatments (n = 18), mean heart dose increased from 109 ± 75 cGy to 148 ± 115 cGy. Shifts from the original plan caused PTV_eval hotspots (V105%) to increase by 5.2% ± 3.8%, which correlated with the total MU of wedged fields (r = 0.59). No significant change in V95% to the cavity was found. Conclusions Large translational variations that occur when positioning prone breast patients had small but significant dosimetric effects on 3DCRT plans. Daily CBCT may still be necessary to correct for rotational variations that occur in 20% of treatments. To maintain planned dose metrics, unintended beam shifts toward the heart and the contribution of wedged fields should be minimized.