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Optimally Combined Proton-Photon Treatments

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J Unkelbach

J Unkelbach1*, M Bangert2 , K Bernstein3 , N Andratschke1 , M Guckenberger1 , (1) University Hospital Zurich, Switzerland, (2) German Cancer Research Center, Heidelberg, Germany, (3) Massachusetts General Hospital, Boston, MA


SU-I-GPD-T-171 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Proton treatment slots are a limited resource. Therefore, we investigate treatments in which most fractions are delivered with xrays and only a few with protons. We demonstrate how both modalities can be combined to optimally capitalize on the proton's ability to reduce normal tissue dose.

Methods: An optimal combined treatment must account for fractionation effects. We therefore perform simultaneous optimization of intensity-modulated proton (IMPT) and photon (IMRT) plans based on their cumulative biologically-effective dose (BED). Optimally combined proton-photon treatments are demonstrated for a sacral chordoma patient, in whom the gross tumor volume (GTV) abuts the bowel and rectum. We consider a standard-fractionated treatment with 30 fractions as the reference, and optimize a multi-modality treatment with 10 proton and 20 IMRT fractions.

Results: In the optimal combination of IMPT and IMRT plans, protons and photons deliver similar doses per fraction to the dose-limiting normal tissue (bowel and rectum overlaying the target volume). This optimally exploits the fractionation effect in these tissues and maximizes the BED to the target for a given normal tissue BED constraint. However, most of the GTV can be hypofractionated. Here, protons can deliver most of the dose. Consequently, less dose is delivered with IMRT, leading to an integral dose reduction in normal tissues. Thereby, 10 proton fractions could achieve more than 50% of the integral dose reduction in the gastrointestinal tract that is possible with 30 proton fractions (compared to 33% for a simple proton-photon combination in which both modalities deliver the same target dose).

Conclusion: A limited number of proton fractions can best be used if protons hypofractionate parts of the target volume while retaining near-uniform fractionation in dose-limiting normal tissues. This leads to non-trivial proton-photon combinations, in which IMRT and IMPT plans deliver inhomogeneous target dose distributions, which in combination deliver the prescribed BED.

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