ESTRO 36 Abstract Book

S474 ESTRO 36 2017 _______________________________________________________________________________________________

which may result in under-dosage of target. We investigated the applicability of a decision suppo rt system developed for photon therapy in a proton therapy setting. Material and Methods Twenty-three consecutive NSCLC patients stage 1B to IV treated with adaptive photon therapy were retrospectively planned using intensity modulated proton therapy. The adaptive protocol was based on geometrical measures of target positioning and large anatomical changes as e.g. atelectasis, as observed on daily CBCT scans. Two surveillance CT-scans were acquired during the treatment course. The consequences of anatomical changes were evaluated by recalculation of the proton plans on the surveillance scans. The CTV receiving 95% of the prescribed dose was analyzed. Proton treatment plans were scaled to prescribed doses of 70, 74 or 78Gy, to investigate if full CTV coverage at 95% of 66Gy = 62.7Gy could be maintained by increasing the prescribed dose. Results Fourteen (61%) patients needed adaptations when treated with protons, given that 95% of the CTV must be covered by 95% of the dose. In comparison, no patients needed adaptation when treated with photons using this criterion. Figure 1 shows CTV coverage for all patients. For proton therapy, the adaptive protocol was found to identify patients with large target under-dosage (six patients, group A). Additionally, under-dosage was observed for another eight patients (group B) with non-rigid changes up to 15mm in the positioning of the bones. The median decrease in coverage for all patients was 92.8% [48.1- 100%]. Robust optimization reduces the decrease in target coverage, but does not eliminate the under-dosage, see Fig.2.All patients in group B would be treated sufficiently when prescribing 74Gy with all CTVs receiving 95% of 66Gy. For patients in group A, only two patients would be treated sufficiently with a 78Gy prescription. A geometric decision support protocol as the present is thus mandatory in order to maintain target coverage of the patients in group A. When increasing the prescribed dose, the maximum dose to important normal tissue such as the oesophagus, trachea, bronchi, and heart increases and may thus be the dose limiting factor. Conclusion Large anatomical changes can be corrected for by an adaptive protocol. Non-rigid positioning erro rs are not identified by the geometrical criteria used for photo ns but can be compensated by an increase in the prescribed dose keeping in mind that this requires additional attention to organs at risk. Robust optimisation reduces, but does not eliminate the risk of under-dosage. Daily imaging and treatment adaptation for a high fraction of patients is mandatory in proton therapy for loco-regional lung cancer.

PO-0877 Proton therapy of oesophageal cancer is more robust against anatomical changes than photons D.S. Møller 1 , M. Alber 2 , T.B. Nyeng 1 , M. Nordsmark 3 , L. Hoffmann 1 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 (pre- operative 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 (IMPT CR ), robust optimization of the CTV and full coverage of the PTV (IMPT PR ) and no robust optimization, but full coverage of the PTV (IMPT P ). 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 IMPT PR . Recalculation on the surveillance scans demonstrated that 7/26 (27%) IMRT plans showed CTV coverage < 99%. For