Radiation and Cancer Physics
SS 15 - Physics 3 - Treatment Planning
112 - Robustness and Organ Sparing Potential of Intensity Modulated Proton Therapy for Lung Cancer
Monday, October 22
5:05 PM - 5:15 PM
Location: Room 006
Robustness and Organ Sparing Potential of Intensity Modulated Proton Therapy for Lung Cancer
H. P. van der Laan1, M. Anakotta1, E. W. Korevaar2, M. Dieters1, J. F. Ubbels1, J. A. Langendijk1, S. Both1, C. Muijs1, and A. Knopf2; 1University of Groningen, University Medical Center Groningen, Department of Radiation Oncology, Groningen, Netherlands, 2Department of Radiation Oncology, University Medical Center Groningen, University of Groningen, Groningen, Netherlands
Purpose/Objective(s): Proton therapy is more sensitive (less robust) to geometrical and density uncertainties than photon therapy. In lung cancer, breathing motion increases these uncertainties, and therefore it is essential to thoroughly verify the robustness of proton therapy delivery procedures. Pencil-beam scanned intensity modulated proton therapy (IMPT) has the potential to reduce the dose to heart and lungs. Therefore, the aim of this study was to test the robustness of IMPT on weekly 4D repeat CT scans and to compare the robustness and organ sparing capabilities of IMPT to that obtained with volumetric arc photon therapy (VMAT).
Materials/Methods: Seventeen lung cancer patients, scheduled for curative chemoradiation, underwent a 4D planning-CT (pCT0) and 5 weekly 4D repeat-CT (rCT) scans. 4D-average scans were used for this study. CTVs including motion were delineated on pCT0 and on rCT scans. For VMAT only, a PTV was created. VMAT plans on pCT0 were 3D optimised for adequate PTV coverage (D98 ≥ 57 Gy), spinal cord dose (Dmax < 50Gy), and minimal heart and lung dose. IMPT plans were 3D optimised with similar objectives using CTV-based robust planning and reviewed by a radiation oncologist. Finally, VMAT and IMPT plans were reconstructed on each weekly rCT including setup (2 mm) and range (3%) error scenarios. Accumulated dose distributions were obtained by deforming and summing the weekly reconstructed dose distribution back to the reference pCT0 simulating 5 fractions per rCT. In case of inadequate summed CTV coverage on pCT0, the effect of treatment plan adaptation on one or more rCTs was simulated.
Results: The summed doses from the weekly rCTs on pCT0 resulted in adequate CTV coverage for all 17 VMAT plans and for 15 out of 17 IMPT plans. In 2 patients, IMPT treatment plan adaptation was required. Adapted plans in the first week (one patient) and in the first and second week (one patient) then also resulted in adequate CTV coverage in the accumulated plan. The summed CTV D98 on pCT0 was > 57 Gy in all plans and patients. On average the D98 was 58.2 Gy with VMAT and 57.8 Gy with IMPT. The spinal cord tolerance dose was exceeded in 2 patients by the VMAT plans only (with 0.5 Gy and 1.0 Gy, respectively). The average mean heart dose with VMAT was 5.3 Gy (SD: 6.6 Gy; range: 0.2 – 24.8 Gy) and with IMPT 1.0 Gy (SD: 1.2; range: 0.0 – 4.3 Gy; p < 0.007). The average mean lung dose with VMAT was 10.5 Gy (SD: 2.9 Gy; range: 6.0 – 17.7 Gy) and with IMPT 7.2 Gy (SD: 2.6; range: 2.8 – 14.5 Gy; p < 0.001).
Conclusion: Robust planned IMPT for lung cancer, with optional weekly plan adaptation, resulted in adequate target coverage similar to that of PTV-planned VMAT. Inter-fractional variation for breathing and anatomy were considered in this analysis under various error scenarios. With IMPT, no spinal cord dose thresholds were violated and significant and clinically relevant dose reductions were obtained for the heart and lungs compared to VMAT.
Author Disclosure: H. van der Laan: None. M. Anakotta: None. E.W. Korevaar: None. M. Dieters: None. J.A. Langendijk: Honoraria; IBA. Consultant; IBA. Research Collaboration With IBA; IBA. Research Collaboration With RaySearch; RaySearch. Research Collaboration With Mirada; Mirada. S. Both: None. C. Muijs: None.