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Preliminary Results of the Dosimetric Impact of Set-Up Accuracy for An Electron Breast Boost Technique

S Davidson*, B Mason, K Kisling, R Barrett, A Bonetati, M Ballo, S Kirsner, MD Anderson Regional Care Centers, Houston, TX

SU-E-T-315 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To analyze the dosimetric impact of set-up accuracy of an electron breast boost technique using a lateral decubitus position with and without a breast compression device.

Methods: Reproducibility of the breast boost set-up was verified for 19 patients and 99 fractions using Cone Beam CT (CBCT). 3D-3D matching was performed between the CBCT and the initial planning CT for each boost fraction by matching the tumor bed and clips. Shifts in all three dimensions were recorded for each fraction. The dosimetric impact of the daily positioning error was achieved by re-running the initial treatment plans incorporating the shifts recorded for each fraction. Comparison of the tumor bed and scar coverage was analyzed for both plans.

Results: The range of set-up errors based on CBCT was: 1.5 cm anterior to 8 mm posterior, 1.3 cm superior to 2.3 cm inferior, and -2.4 cm to 2.1 cm laterally. Three patients had set-up errors that were greater than or equal to 2 cm which is the normal margin placed on the tumor bed and scar. Two of these three patients had unacceptable coverage as defined by the V90 when compared to the original plan. The remaining 17 patients had no discernible difference in coverage (V90). Whether patients had breast compression seemed to have little impact on reproducibility of the set-up.

Conclusion: 14 patients with a breast compression device and 5 patients without a breast compression device were studied using CBCT to quantify set-up error from 99 fractions. A 2 cm margin around the tumor bed plus scar volume seems to be adequate to account for set-up error. No appreciable difference was seen in set-up error between patients with breast compression or without. The two patients with unacceptable coverage were large breasted with deep seated tumors.

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