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PO-0877: Proton therapy of oesophageal cancer is more robust against anatomical changes than photons
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protons, IMPTPR plans were the most robust and only 4/26 (15%) decreased in coverage to below 99%. The CTV coverage for all patients and plans are shown in Fig 1. The most common anatomical changes are lateral target deformations, enlargement of mediastinum and changes in diaphragm position. The posterior proton plans are sensitive to target deformations, while the multiple photon fields are sensitive to all three types of changes (see Fig. 2).
PO-0877 Proton therapy of oesophageal cancer is more robust against anatomical changes than photons D.S. Møller1, M. Alber2, T.B. Nyeng1, M. Nordsmark3, L. Hoffmann1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark 2 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 3 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective Anatomical changes such as changes in the mediastinum and the diaphragm position are seen in oesophageal cancer patients during the course of radiotherapy. Field entrance through areas with a high risk of changes is often unavoidable with intensity modulated photon radiotherapy (IMRT) if target conformity and reduction of dose to especially lungs and heart is pursued. Delivery of proton therapy is highly sensitive to anatomical changes, but using only one posterior field may avoid high risk entrances. We investigate the sparing of normal tissue and the potential gain in robustness towards anatomical changes using intensity modulated proton therapy (IMPT) instead of IMRT. Material and Methods Twenty-six consecutive patients with medial or lower oesophageal or gastroesophageal junction(GEJ) cancer treated with IMRT (5-8 fields) were retrospectively planned with IMPT using one posterior beam. The fractionation schedules were either 41.4 Gy/23fx (preoperative regime, 22 patients) or 50Gy/27fx (definitive regime, 4 patients). To ensure dose coverage of the CTV for photon plans, a PTV (5 mm AP, 5mm LR, 8 mm CC) was used to account for uncertainties in planning and delivery. For protons, three different strategies were pursued. Robust optimization of the CTV (IMPTCR), robust optimization of the CTV and full coverage of the PTV (IMPTPR) and no robust optimization, but full coverage of the PTV (IMPTP). Robust optimization was performed accounting for 3mm isocenter shifts and 3% density uncertainty. IMRT and IMPT plans were compared in terms of dose to lungs and heart. For all patients, an additional surveillance CT-scan was obtained at fraction 10 and used for recalculation of both IMRT and IMPT plans, analysing the percentage of CTV receiving 95% of the prescribed dose. Results Using IMPT instead of IMRT reduced the lung and heart dose significantly regardless of the IMPT strategy (p<0.001 using a Wilcoxon signed rank test). The mean lung and heart doses decreased from sample median = 8.7Gy [1.6;16.3] and 17.1Gy [1.1;24.1] using IMRT to 2.2 Gy [0.5;8.5] and 9.1 Gy [0;15.5], using IMPTPR. Recalculation on the surveillance scans demonstrated that 7/26 (27%) IMRT plans showed CTV coverage < 99%. For
Conclusion Treating oesophageal cancer with protons has the advantage of decreasing dose to organs at risk and at the same time it improves the robustness towards common anatomical changes. Frequent imaging is still needed to identify patients with target deformations requiring adaptive treatment planning.
PO-0878 Plan adaptation on the MR-Linac: first dosimetric validation of a simple dose shift R. Koopman1, A.J.A.J. Van de Schoot1, J. Kaas1, T. Perik1, T.M. Janssen1, U.A. Van der Heide1, J.J. Sonke1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Patient positioning on the MR-Linac (MRL; Elekta AB, Stockholm) requires online plan adaptations to correct for setup errors due to the fixed couch position. The aim of our study was to validate the size and direction of such plan adaptations (simple dose shifts) and evaluate dosimetric differences for rectum cancer patients.
protons, IMPTPR plans were the most robust and only 4/26 (15%) decreased in coverage to below 99%. The CTV coverage for all patients and plans are shown in Fig 1. The most common anatomical changes are lateral target deformations, enlargement of mediastinum and changes in diaphragm position. The posterior proton plans are sensitive to target deformations, while the multiple photon fields are sensitive to all three types of changes (see Fig. 2).
PO-0877 Proton therapy of oesophageal cancer is more robust against anatomical changes than photons D.S. Møller1, M. Alber2, T.B. Nyeng1, M. Nordsmark3, L. Hoffmann1 1 Aarhus University Hospital, Department of Medical Physics, Aarhus C, Denmark 2 Heidelberg University Hospital, Department of Radiation Oncology, Heidelberg, Germany 3 Aarhus University Hospital, Department of Oncology, Aarhus C, Denmark Purpose or Objective Anatomical changes such as changes in the mediastinum and the diaphragm position are seen in oesophageal cancer patients during the course of radiotherapy. Field entrance through areas with a high risk of changes is often unavoidable with intensity modulated photon radiotherapy (IMRT) if target conformity and reduction of dose to especially lungs and heart is pursued. Delivery of proton therapy is highly sensitive to anatomical changes, but using only one posterior field may avoid high risk entrances. We investigate the sparing of normal tissue and the potential gain in robustness towards anatomical changes using intensity modulated proton therapy (IMPT) instead of IMRT. Material and Methods Twenty-six consecutive patients with medial or lower oesophageal or gastroesophageal junction(GEJ) cancer treated with IMRT (5-8 fields) were retrospectively planned with IMPT using one posterior beam. The fractionation schedules were either 41.4 Gy/23fx (preoperative regime, 22 patients) or 50Gy/27fx (definitive regime, 4 patients). To ensure dose coverage of the CTV for photon plans, a PTV (5 mm AP, 5mm LR, 8 mm CC) was used to account for uncertainties in planning and delivery. For protons, three different strategies were pursued. Robust optimization of the CTV (IMPTCR), robust optimization of the CTV and full coverage of the PTV (IMPTPR) and no robust optimization, but full coverage of the PTV (IMPTP). Robust optimization was performed accounting for 3mm isocenter shifts and 3% density uncertainty. IMRT and IMPT plans were compared in terms of dose to lungs and heart. For all patients, an additional surveillance CT-scan was obtained at fraction 10 and used for recalculation of both IMRT and IMPT plans, analysing the percentage of CTV receiving 95% of the prescribed dose. Results Using IMPT instead of IMRT reduced the lung and heart dose significantly regardless of the IMPT strategy (p<0.001 using a Wilcoxon signed rank test). The mean lung and heart doses decreased from sample median = 8.7Gy [1.6;16.3] and 17.1Gy [1.1;24.1] using IMRT to 2.2 Gy [0.5;8.5] and 9.1 Gy [0;15.5], using IMPTPR. Recalculation on the surveillance scans demonstrated that 7/26 (27%) IMRT plans showed CTV coverage < 99%. For
Conclusion Treating oesophageal cancer with protons has the advantage of decreasing dose to organs at risk and at the same time it improves the robustness towards common anatomical changes. Frequent imaging is still needed to identify patients with target deformations requiring adaptive treatment planning.
PO-0878 Plan adaptation on the MR-Linac: first dosimetric validation of a simple dose shift R. Koopman1, A.J.A.J. Van de Schoot1, J. Kaas1, T. Perik1, T.M. Janssen1, U.A. Van der Heide1, J.J. Sonke1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Patient positioning on the MR-Linac (MRL; Elekta AB, Stockholm) requires online plan adaptations to correct for setup errors due to the fixed couch position. The aim of our study was to validate the size and direction of such plan adaptations (simple dose shifts) and evaluate dosimetric differences for rectum cancer patients.