ESTRO 38 Abstract book

S1099 ESTRO 38

different adaptation workflows and their impact on target coverage and OAR sparing. Material and Methods A 55 year old patient with a single recurrent pararectal lymphnode metastasis of a prostate cancer was treated with the Unity 1.5 T MR-Linac (Elekta AB, Stockholm, Sweden) with 35/30 Gy to the GTV (10 mm³)/PTV in five fractions using eight step-and-shoot IMRT beams. The reference plan and all adapted plans were created with Monaco 5.4 (Elekta AB. Stockholm. Sweden). For daily online adaptation, the adapt-to-position (ATP) workflow was applied consisting of an isocenter shift for each segment followed by a segment weight optimization. Later, OARs were delineated offline on the daily pretreatment MR (T2w 3D). Retrospectively, the online adapt-to-shape (ATS) workflow was simulated, with a complete reoptimization starting from the fluence map based on the daily anatomy. In addition, a workflow consisting of using original segments (OS) only was simulated, in which the shift of the patient was not taken in account. The different workflows were compared with respect to the rectum D 1.4ccm , which corresponds to the D 2% of the rectum in the reference plan, as well as PTV and GTV D 98% . Plan conformality was evaluated based on V 20Gy of the total patient volume. To estimate the applied cumulative dose, the DVH parameters were averaged over all fractions. Results Table 1 shows the daily shift of the patient with respect to the PTV position in the reference plan. Table 2 shows the evaluated DVH parameters for each workflow. The rectum D 1.4ccm for the reference plan was slightly lower on the daily contoured and accumulated parameters (22.9 Gy) compared to the initial planning CT (23.5 Gy). The OS workflow shows a strong decrease of tumor coverage. Even for the minimal absolute shift (fx 5, 0.45 cm) the PTV D 98% dropped from 29.2 Gy to 21.6 Gy. In the ATP workflow tumor coverage was maintained for all fractions , with minimal differences in the averaged PTV D 98% of 29.2 Gy and 28.9 Gy for the reference plan and ATP, respectively, and equivalent OAR-sparing. The ATS workflow shows decreased rectum D 1.4ccm . with respect to the reference plan, equivalent tumor coverage with respect to PTV D 98% (28.7 Gy) and GTV D 98% (34.6 Gy) and the highest conformality (V 20Gy = 18.1 Gy).

Results The clinically delivered (adapt to shape) plans show the highest target coverage with an average PTV V 35Gy of 99.2 ± 1.5% [range, 96-100%] (Figure 2). The clinical prescribed dose criteria were met for all fractions. The adapt to position strategy would result in the lowest PTV coverage with an average PTV V 35Gy of 87.3 ± 6.8% [range, 72-99%], which is below dose criteria for three fractions for patient 1 and all fractions for patients 2 and 3. With the CBCT- linac back-up plans the PTV V 35Gy was on average 94.5 ± 2.1% [range, 90-97%] which would be below criteria for 1, 5 and 2 fractions for patient 1, 2 and 3, respectively. For both the adapt to position and CBCT-linac back-up plans, small violations would have occurred for the ureter for patients 1 and 2, with a maximum D max of 40.9Gy.

Conclusion Evaluation of the first clinical SBRT treatments of pelvic lymph node oligometastases on the 1.5T MR-linac using the full re-planning approach yields beneficial DVH parameters compared to conventional treatment. The adapt to position option yields equal or worse DVH parameters compared to conventional treatment. This emphasizes the dosimetric benefit of online contour adaptation. EP-2007 Analysis of MRgRT treatment adaptation strategies with a high-field MR-Linac M. Nachbar 1 , D. Mönnich 1,2,3 , C. Marks 2 , A. Stolte 2 , O. Dohm 2 , D. Thorwarth 1,3 , D. Zips 2,3 , C. Gani 2 , S. Boeke 1,3 1 University Hospital Tübingen, Section for Biomedical Physics- Department of Radiation Oncology, Tuebingen, Germany ; 2 Department of Radiation Oncology, University Hospital and Medical Faculty- Eberhard Karls University Tübingen, Tübingen, Germany ; 3 German Cancer Consortium DKTK, partner site Tübingen- and German Cancer Research Center DKFZ, Heidelberg, Germany Purpose or Objective MR-Linac systems are a promising technology with the capability of online plan adaptation. Here we evaluate the first treatment at our institution and compare

Conclusion Here we show with our initial experience that virtual couch shift with ATP is feasible to deliver high-precision MRgRT in a difficult-to-treat situation. Given the higher