Actual target coverage after setup verification using surgical clips compared to external skin markers in postoperative breast cancer radiotherapy

Publication date: Available online 19 April 2017
Source:Practical Radiation Oncology
Author(s): A. van der Salm, L. Murrer, I. Steenbakkers, R. Houben, L.J. Boersma
PurposeAfter changing from offline setup verification to online setup verification using external skin markers in breast cancer patients, we noticed an increase in localized acute skin toxicity beneath the markers. Also, in-vivo 3D–dose-measurements showed deviations between the delivered and the planned dose distribution. Therefore, we investigated the accuracy of setup verification using surgical clips in the tumor bed, with a focus on target coverage of whole breast and tumor bed.MethodsOrthogonal kilovoltage (kV)-images were acquired before every fraction in 35 breast cancer patients, deriving an online 3D–setup error by matching on external skin markers. In retrospect, a re-match was performed using surgical clips. For 155 fractions, i.e. 5–6 fractions/patient, a conebeam-computed tomography scan (CBCT) was available. Analysis concerned:1) visibility of the clips,2) migration of the clips,3) comparison of setup-errors according to both match methods4) comparison of target coverage by recalculating the dose on the online setup-corrected CBCT with the patient setup according to both match methods.External validation of the surgical clip based online setup verification was performed in 23 patients, by analyzing kV-images of 100 fractions, obtained after treatment.ResultsAll types of surgical clips could be visualized. The clip to center-ofmass (COM) distance decreased on average by 2 mm (SD 1) over the course of treatment. Setup differences between match methods were on average <0.5 mm in all directions. The reconstructed dose distributions showed that V95%, V107% and Dmean of the breast- and boost-PTV were similar for the planning-CT and the CBCTs, for both match procedures. An external validation in 23 patients showed reassuring setup errors < 2 mm.ConclusionsOnline setup verification using surgical clips results in comparable setup corrections and target volume coverage as verification using skin markers. By omitting skin markers acute skin toxicity beneath the markers is prevented.



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