ESTRO 37 Abstract book

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ESTRO 37

combined proton-photon treatments should be considered, in which most fractions are delivered with IMRT and only a few with IMPT. Here, we demonstrate how both modalities can be combined to optimally capitalize on the proton's ability to reduce integral normal tissue dose. Material and Methods We consider treatment sites where the GTV is eligible for hypofractionation, but where dose-limiting normal tissues overlay parts of the PTV. As an example, we consider the sacral chordoma patient in figure 1, in whom the GTV (green contour) abuts the bowel (red contour). 70 Gy and 54 Gy in 30 fractions are prescribed to the GTV and the PTV (blue contour). The maximum dose to the bowel is constrained to 54 Gy. The part of the bowel that overlaps with the PTV represents the dose-limiting normal tissue and can only be protected through fractionation. Hence, IMRT and IMPT fractions should both deliver the same dose per fraction of 1.8 Gy. However, protons avoid the dose bath to the gastrointestinal tract. Hence, it is desirable to deliver an overproportionate dose with protons. This is possible by hypofractionating the GTV with protons. To plan such nontrivial proton-photon combinations, we present a novel method to simultaneous optimize IMRT and IMPT plans based on their cumulative biologically effective dose (BED). Results Figure 1 illustrates a treatment with 10 IMPT and 20 IMRT fractions. Figures 1a and 1b show the dose per fraction delivered with IMRT and IMPT in an optimized combination of the two modalities. Figure 2a shows the corresponding DVHs for the GTV and the bowel. Protons and photons both deliver similar doses per fraction to the high dose region of the bowel and thereby optimally exploit the fractionation effect in the dose-limiting normal tissue. However, a proton fraction delivers, on average, twice the dose to the GTV. Figure 1c shows the cumulative equieffective dose (which is proportional to BED) and demonstrates that both modalities combined deliver the prescribed BED to the target volume. Evaluating the DVH of the cumulative BED (Figure 2b) shows that the optimized combination (solid line) improves on a simple combination (dashed line), which consists of separately optimized IMRT and IMPT plans that deliver the same dose per fraction to the GTV. In this example, the optimized combination with 10 IMPT fractions achieves 53% the integral dose reduction in the gastrointestinal tract that is possible with 30 IMPT fractions (compared to 33% for a simple combination). Conclusion A limited number of proton fractions can best be used if protons hypofractionate parts of the target volume while maintaining near-uniform fractionation in dose-limiting normal tissues.