A method to deliver energy modulated planar proton arc therapy (EMPPAT)

Ulrich W. Langner, John G Eley, Marianna Guerrero, Katja Langen, Sina Mossahebi, Mingyao Zhu, Warren D'Souza, Byongyong Yi


Even though arc therapy is regularly used in photon therapy, it is not commercially available in proton therapy. We developed a concept to deliver proton arcs in an effective and practical way and discuss its application and advantages through a proof-of-principle dosimetric planning study. The concept of proton arc therapy which uses a tertiary energy modulator has been suggested so that only a single energy is requested from the cyclotron per arc (one gantry revolution). A dose plane with 1-2 cm thickness perpendicular to the beam is assigned for each gantry angle, where each angle will be used as a control point. This plane sweeps the target volume while the gantry moves. Comparison plans were created between a two field single field optimization (SFO) proton plan, a 6x photon volumetric arc plan, and 24 SFO plans from different gantry angles each day, using a cylindrical phantom with 25 cm diameter. Plan quality were evaluated based on volumetric integral dose, robustness of the technique, and distribution of relative biological effectiveness (RBE) dose. Plans with PTVs of varying size and position were evaluated to investigate the advantages of this method compared to current methods. Worst case GTV robustness D95% increased from 80.2%, to 90.8%, to 90.2% for the proton arc as the target diameter increased from 1cm, to 3 cm, to 11 cm. As the target was shifted from the surface to the center of the volume, the robustness did not change significantly. For the target closer to the surface the VMAT integral dose was similar to that of the proton plans, but the difference increased markedly as the depth of the target increased. With VMAT, almost the entire volume receives 10% of the dose, while the V10% is substantially less for all the proton techniques. The integral dose was about a factor of 2 less for proton arcs compared to VMAT and even less than for the standard two field technique, although the V5% was less for the two-field technique the V20% was less for proton arc method, thus decreasing the integral. The mean RBE’s of the 5 cm diameter PTV volume were 1.11 and 1.26 for the single field SFO plan and for the arc plan, respectively. The peak of the RBE distribution is at the center of the PTV for the arc plan, while it is at the distal end of the PTV for the single beam plan. This planning study shows that proton arc is a promising delivery method, even if it is perceived to give up the largest advantage of protons, i.e. no dose to a large volume. It gave less integral dose to a simulated volume than VMAT and even current proton treatment techniques. The method proposed here also provides significant advantages in RBE dose distribution.


Proton Arc, RBE, Energy Modulator

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DOI: http://dx.doi.org/10.14319/jpt.31.2

Copyright (c) 2017 Ulrich W. Langner, John G Eley, Marianna Guerrero, Katja Langen, Sina Mossahebi, Mingyao Zhu, Warren D'Souza, Byongyong Yi

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© Journal of Proton Therapy (ISSN 2469-5491)

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