ESTRO 37 Abstract book

S1131

ESTRO 37

Deviations of the surface-based method were compared to CBCT corrections as gold standard. A theoretical Catalyst TM setup error was calculated between the translational deviations of the surface scanner and the laser positioning. One-sided t-tests were used for statistical analysis. Results Between 10/2016 and 06/2017 a total of 1902 fractions in 110 patients (66 male and 44 female) were evaluated. The distribution of fractions was as follows: Head: 689, Thorax: 460, Abdomen: 630 and Extremities: 123. The data analysis revealed total deviations of 0.09 ± 2.03 mm for the lateral axis, 0.07 ± 3.21 mm for the longitudinal and 0.44 ± 3.08 mm vertical axis for the Catalyst TM system, compared to -0.6 ± 3.54 mm lateral, 0.53 ± 3.47 mm longitudinal and 0.19 ± 3.49 mm vertical for the laser positioning compared to CBCT. The calculated theoretical setup error was -0.15 ± 4.21 mm lateral, 0.46 ± 4.47 mm longitudinal and -0.25 ± 4.54 mm vertical. Stratifying data by anatomic regions, the lowest positional deviations were found in the intra-cranial treatments. Larger deviations occurred in the thoracic and abdominal region (see Fig. 1). A statistical comparison using Two one-sided t-tests (Test references: Head ± 0.5 mm, Thorax ± 1.0 mm, Abdominal ± 1.5 mm and Extremities ± 2.0 mm, Alpha = 0.05) showed a general concordance of the two methods (p ≤ 0.036), excluding the vertical direction of the abdominal region (p = 0.198), where Catalyst TM achieved a higher precision (Mean value of deviation -0.59 mm vs. 0.68 mm for the laser).

Electronic Poster: Physics track: Adaptive radiotherapy for inter-fraction motion management

EP-2060 Daily dose calculation using CBCT images can trigger treatment adaptation for lung cancer patients L. Hoffmann 1 , A.A. Khalil 2 , M.M. Knap 2 , U.V. Elstrøm 1 , L.P. Kaplan 3 , D.S. Møller 1 1 Aarhus University Hospital, Department of medical physics, Aarhus, Denmark 2 Aarhus University Hospital, Department of oncology, Aarhus, Denmark 3 Aarhus University, Department of physics and astronomy, Aarhus, Denmark Purpose or Objective Anatomical changes appear during the RT course for some lung cancer pts. Treatment adaptation may be required to avoid target under dosage. The work load is high requiring manual inspections of daily images to identify pts benefitting from adaptation. We investigated the reliability of automatic structure transfer and dose calculation using CBCT images obtained during treatment as a trigger for adaptation. Material and Methods Six pts treated with 66Gy/33fractions (F) or 45Gy/30F were setup using daily CBCT matching on the primary tumour. The pts were treated according to an adaptive strategy and 3 of the pts were adapted during treatment. Two surveillance CT scans (sCT) were acquired at F10 and F20. Target was contoured on the sCTs by a radiation oncologist. Deformable image registrations (DIR) from planning CT (pCT) to each of the CBCTs were made in MIM software and synthetic CT scans (synCT) were created. Target was propagated from pCT to the synCTs and the CBCTs. The treatment plan was transferred to the sCT, synCT and CBCT using the online registration and dose was calculated in the Eclipse TPS. Separate stoichiometric HU calibration curves were used for CT and CBCT. Target coverage at F10 and F20 was compared in terms of mean dose and V95% PTV . Results Comparison with sCT at F10 and F20: The absolute difference in V95% PTV between sCT and synCT or CBCT is shown in Fig 1.Three pts had no anatomical changes and reproducible positioning. In these pts, the mean [SD] PTV dose differed by 0.0Gy [0.2] between sCT and synCT/CBCT and V95% PTV deviated <1%. Pts 4-6 experienced anatomical changes. V95% PTV deviated up to 15% between sCT and synCT/CBCT. For pts 4 and 5, the deviations were due to the performance of the DIR in the presence of large anatomical changes. For pt 6, the deviation was due to positional changes. Full RT course: Pts 1-3 required no adaptations and V95% PTV calculated on synCT was >96% for all fractions. Pts 4-6 were adapted based on visual inspection of daily CBCTs and recalculation on the daily synCTs showed minimum V95% PTV of 80% (pt4), 78% (pt5) and 87% (pt6). Pt 4, had changes in an atelectasis located at tumour site and target under-dosage was picked up by the synCTs. Fig 2 shows the daily dose calculated on synCT for pt 5 and 6. In pt 5, pleural effusion appeared at F22. A clear under dosage of PTV is seen on synCT in agreement with the findings on sCT even though deviations between sCT and synCT are seen on the absolute level. Pt 6 had interfractional changes in tumour and node position resulting in under-dosage of PTV-N seen on both sCT and synCT and adaptation was triggered by both sCT and synCT calculations. Conclusion Dose calculation on daily synCT or CBCT is an effective tool to evaluate the need for adaptive treatment planning. In pts with negligible anatomical changes, dose

Conclusion In our experience of 110 cases, real time optical surface system Catalyst TM seems to be a reliable and feasible image-guided system for patient's positioning without any additional radiation exposure. Overall, the system showed good results in therapeutic areas with fixed tumor-surface relations. From the head to thoracic and abdominal regions a decrease in accuracy was observed in Catalyst TM data as well as in the laser based positioning. A wider variability in the means of tumor movement relative to the surface could be the cause. Overall both methods performed within a comparable range and with reasonable clinical accuracy.