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Calculation of PTV Margins for Whole Breast DIBH Radiotherapy Using Real-Time Surface Imaging Data

J Crosby

J Crosby1*, H Kang2 , M Malin1 , Y Hasan1 , S Chmura1 , H Al-Hallaq1 , (1) The University of Chicago, Chicago, IL, (2) Loyola Univ Medical Center, Chicago, IL


SU-K-605-11 (Sunday, July 30, 2017) 4:00 PM - 6:00 PM Room: 605

Purpose: PTV margins for whole-breast radiotherapy treatments delivered during voluntary deep inspiration breath-hold (vDIBH) were calculated using real-time surface imaging (SI) data.

Methods: Patients (n=33) treated with mono-isocentric technique to the left (n=16) and right (n=17) breast and regional nodes via field-in-field tangents and supraclavicular fields were included. Treatment beams were gated using AlignRT by registering the whole breast region-of-interest to the surface generated from the simulation CT scan. AlignRT recorded real-time displacements every 0.3s in each dimension and the beam-on-state. Using an in-house MATLAB script, the means and standard deviations of the displacements during vDIBH for each treatment fraction were used to calculate PTV margins per the Van Herk equation. The intra-DIBH stability and the intra-fraction reproducibility were estimated as the 5th-to-95th percentile range of the corresponding data points in each breath-hold and fraction, respectively.

Results: A total of 4022 breath-holds were detected over 740 fractions in which a median dose of 200cGy was delivered. Each patient was monitored for an average of 5.72±2.46 minutes/fraction. Calculated PTV margins were 5.5mm (VRT), 5.20mm (LNG), and 7.05mm (LAT). The intra-DIBH stability and the intra-fraction reproducibility were estimated to be 0.92 mm and 1.96 mm, on average, respectively. The isotropic PTV margin calculated using SI data was smaller than others’ x-ray imaging based calculations (6.2mm versus 9.3-10.9mm) despite similar database sizes. When compared to breast surface displacement using spirometry-based positioning, the intra-DIBH variability and intra-fraction reproducibility of SI agreed to within 0.82mm and 0.32mm, respectively.

Conclusion: While the intra-DIBH variability and intra-fraction reproducibility are similar to those obtained with spirometry-based positioning, the overall breast surface reproducibility is superior to x-ray-based positioning as evidenced by smaller PTV margins calculated using SI. Although a PTV margin of 6mm is required, patients can reproduce their breath-hold position to within 2mm on any given treatment day.

Funding Support, Disclosures, and Conflict of Interest: Funded by the University of Chicago Biological Sciences Division Summer Research Fund

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