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Session Item

Tuesday
August 31
11:00 - 12:15
N105-N106
Recent developments in the treatment of rectal cancer
Sebastian Meltzer, Norway;
Vincenzo Valentini, Italy
Preoperative chemoradiation therapy is an integral part of the treatment of advanced rectal cancer. Intensification strategies in the neoadjuvant setting are debated, with improved local control at the cost of increasing toxicity. This session will provide an overview of dose-intensification trials, and what is needed to obtain a complete response. Further, the hype and pitfalls of radiosensitation and concomitant chemotherapy, as well as the total neoadjuvant treatment strategy will be discussed. Updated results from the RAPIDO trial will be presented. Lastly, patient selection to organ preservation strategies will be debated, with prediction models of complete response in the interface between research and clinic.
Symposium
Clinical
Margins based on inter-fractional surgical clip movement for breast tumor bed radiotherapy boost
Karina Lindberg Gottlieb, Denmark
PO-1573

Abstract

Margins based on inter-fractional surgical clip movement for breast tumor bed radiotherapy boost
Authors:

Karina Lindberg Gottlieb1, Simon Long Krogh1, Mette Holck Nielsen2, Ebbe Laugaard Lorenzen3

1Laboratory of Radiation Physics, Department of Oncology, Odense University Hospital, Odense, Denmark; 2Department of Oncology, Odense University Hospital, Odense, Denmark; 3Laboratory of Radiation Physics, Department of Oncology; Odense University Hospital, Odense, Denmark

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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. 

Material 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.


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.

Conclusion

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.