ESTRO 2021 Abstract Book

S1298

ESTRO 2021

exceed the rigid contour, without significantly affecting the target position. Optimisation times were long initially; in trying to achieve EMBRACE II style sparing whilst maintaining good coverage, hotspots were inadvertently created in the target. On two fractions, the TPS froze during segmentation, requiring termination of the session and restarting the process. Consequently, a new reference plan with more robust optimisation parameters was created (including increasing max number of segments), which reduced optimisation time and improved the dose distribution. Conclusion Lessons were learnt during the first radical cervix treatment on the MR-Linac to improve future treatments. Having a robust reference plan is imperative when undergoing online reoptimisation. Contour deformation was adequate but could be improved by changing slice thickness. To limit contouring times, edits should only be made if clinically significant. The bladder fills during the planning process, therefore rigid propagation of the bladder contour is recommended and drinking protocols can be designed to ensure the bladder is full when treatment starts. PO-1573 Margins based on inter-fractional surgical clip movement for breast tumor bed radiotherapy boost K.L. Gottlieb 1 , S.L. Krogh 1 , M.H. Nielsen 2 , E.L. Lorenzen 3 1 Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark; 2 Department of Oncology, Odense University Hospital, Odense, Denmark; 3 Laboratory of Radiation Physics, Department of Oncology; Odense University Hospital, Odense, Denmark Purpose or Objective Simultaneous integrated boost in breast cancer radiotherapy has the advantage, compared to sequential boost, of shortening the overall duration of the radiotherapy course. However, during the treatment course the boost target region, typically identified by surgical clips, may move relative to the other target regions. In this study, the inter-fraction movement of the surgical clips in the boost region relative to the remaining target regions was evaluated, and the corresponding required PTV margins are estimated. Materials and Methods Daily Cone Beam CT (CBCT) scans from all patients treated with simultaneous boost between October 2017 and November 2020 were included, yielding 1628 scans from 90 patients. The registrations done in the clinical workflow were used for this study. The workflow (illustrated in figure 1) was as follows: First, a boost specific match was performed, where the surgical clips in the boost region were registered to their position in the planning CT manually. Then, an automatic match on the chest wall and lymph-node regions (if present) was performed. We calculated the systematic (Σ) and random (σ) components. Corresponding required PTV margins were calculated based on a margin formula of 2.5 Σ+0.3 σ. The constant for σ (0.3) was based on a boost dose of 63 Gy and a whole breast dose of 51.52 Gy.

Figure 1. A clinical match of a patient. a) manual match using the clips and b) automatic match on the chest wall. Purple is the reference CT and Green is the CBCT.

Results The observed displacement of the surgical clips is shown in figure 2 with the corresponding systematic and random components. The displacement was similar in all directions with average components of Σ = 1.9mm and σ = 1.7mm. Based on this the corresponding PTV margin due to inter-fractional displacement of the clips is 5.2mm. Breathing motion could introduce additional random errors, but the impact is limited, e.g. an additional random error of 1.5mm would increase the PTV-margin to 5.4mm. Any systematic errors would increase the margin; however, the value of 1.9mm from clips displacement will be predominant with typical values, e.g. an additional systematic error of 1mm would increase the PTV margin to 5.8mm.