Session Item

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. 

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.

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.

Figure 2. Histogram of the displacement of surgical clips relative to breast/chest and lymph node target regions as measured on 1682 daily CBCT scans. Corresponding systematic (Σ) and random components (σ ) is shown in each subfigure.

Displacement of the surgical clips in the boost region is a major uncertainty in simultaneous integrated boost for breast cancer radiotherapy. However, with the use of daily CBCT performing a match on breast/chest and lymph node regions, the additional uncertainty from clips displacement alone can be accounted for by adding a PTV margin to the boost region of 5mm.