Accumulated dose comparison of stereotactic MRgRT and proton therapy for central lung tumors
PD-0398
Abstract
Accumulated dose comparison of stereotactic MRgRT and proton therapy for central lung tumors
Authors: Moritz Rabe1, Miguel A. Palacios2, John van Sörnsen de Koste2, Chukwuka Eze1, Martin Hillbrand3, Claus Belka1,4, Guillaume Landry1, Suresh Senan2, Christopher Kurz1
1University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany; 2Amsterdam University Medical Centers, location VUmc, Department of Radiation Oncology, Amsterdam, The Netherlands; 3Kantonsspital Graubünden, Institut für Radio-Onkologie, Chur, Switzerland; 4German Cancer Consortium (DKTK), Partner Site Munich, Munich, Germany
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Purpose or Objective
Stereotactic
MR-guided adaptive photon radiotherapy
(MRgRT) and hypofractionated intensity-modulated proton therapy (IMPT) are techniques
to reduce the potential toxicity of radiotherapy for central lung tumors. This study
compared MRgRT with state-of-the-art robustly optimized IMPT for central lung
tumors. We used MRgRT fraction images to calculate accumulated doses and assess
the dosimetric effects of interfractional anatomical changes.
Material and Methods
Datasets
of ten central lung tumor patients, treated with MRgRT to a dose of 8×7.5 Gy prescribed to the PTV (GTV + 5 mm) at a
ViewRay MRIdian 0.35 T MR-Linac, were included in this study. The setup 3D-MRIs
acquired at the beginning of each treatment fraction were deformably registered
to the baseline 3D-MRI (pMRI) acquired at the treatment planning stage. The delivered
daily adapted fraction doses were deformed to the pMRI to accumulate the dose. For
IMPT, treatment plans with 8×7.5
Gy(RBE) were created on the baseline virtual CT (planning CT deformably
registered to the pMRI) used for initial MRgRT treatment planning. Assuming
gated proton beam delivery, an ITV with muscle tissue density override was
created by isotropically expanding the GTV by 3 mm to account for residual
motion within the gating window. The IMPT plans were robustly optimized with 3%
range and isotropic 6 mm position uncertainty to account for interfractional
changes and residual uncertainties as per published protocols. The IMPT plans
were recalculated on all eight fraction virtual CTs using a routine approach,
i.e., without online plan adaptation, which is not yet standard practice in
proton therapy. The resulting dose distributions were mapped to the pMRIs to
accumulate the dose. A DVH parameter analysis was conducted for the GTV, the
ipsilateral lung, and organs at risk (OARs) located within 2 cm of the PTV that
was clinically used for treatment planning. Only OARs with at least five patient
observations fulfilling this proximity criterion were included in the analysis.
Results
Exemplary
MRgRT and IMPT accumulated dose distributions are depicted in Figure 1. The
mean DVH parameters and their differences averaged over all patients are
summarized in Table 1. The accumulated GTV D98% was above the
prescribed dose for the MRgRT and IMPT plans for all patients, with a mean
difference of 1.2 Gy. The mean dose to the ipsilateral lung was 1.7 Gy higher on
average for the accumulated IMPT compared to the MRgRT doses. The average IMPT D1cc
was higher by 5.8 Gy for the ipsilateral bronchial tree, 8.3 Gy for the heart,
and 11.3 Gy for the chest wall when compared to the MRgRT D1cc.
Conclusion
For
the chosen combination of margin concept and robustness settings, and without
online plan adaptation, the accumulated IMPT D1cc were higher
for all OARs included in the analysis with a higher Dmean to the ipsilateral lung while achieving similar
target coverage. Inclusion of more patient datasets which can allow analysis of
additional OARs is required to validate our findings.