| Summary: | <p><strong>Purpose:</strong> The purpose of this study is to evaluate the dosimetric effect of rotational setup errors on the synchronous bi-lateral lung cancer plans generated by the intensity modulated proton therapy (IMPT) technique.</p><p><strong>Methods:</strong> The original IMPT plans were generated in for the left planning target volume (PTV) and right PTV of the left lung and right lung, respectively. Each plan was generated using two beams (lateral and posterior-anterior) with an isocenter placed at the center of the corresponding PTV. The IMPT plans were optimized for a total dose of 74 Gy[RBE] prescribed to each PTV with 2 Gy(RBE) per fraction. Original plans were recalculated by introducing simulated rotational errors. For each PTV, 18 rotational plans (±1⁰, ±2⁰, and ±3⁰) for each of the yaw, roll, and pitch rotations were generated. <strong></strong></p><p><strong>Results:</strong> Rotational errors caused the reduction in the clinical target volume (CTV) and PTV coverage in new rotational IMPT plans when compared to the original IMPT lung plans. The CTV D99 was reduced by up to 13.3%, 9.1%, and 5.9% for the yaw (+3⁰), roll (-3⁰), and pitch (+3⁰), respectively. The PTV D95 was reduced by up to 8.7%, 7.3%, and 4.6% for the yaw (+3⁰), roll (-3⁰), and pitch (+3⁰), respectively. The PTV V100 showed the highest deviation with a reduction of dose coverage by up to 40.1%, 31.8%, and 33.9% for the yaw (-3⁰), roll (-3⁰), and pitch (+3⁰) respectively. <strong></strong></p><p><strong>Conclusion:</strong> The rotational setup errors with magnitude of ≥2⁰ can produce a significant loss of dose coverage to the target volume in the IMPT of a synchronous bi-lateral lung cancer. The yaw had the most severe impact on the dosimetric results when compared to other two rotational errors (roll and pitch).</p>
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