Abstract Book

S1119

ESTRO 37

mainly due to differences in bladder and rectal filling. To account for target volume shape and position variations an appropriate IGRT strategy has to be applied. In proton therapy the corrections for the resulting changes in dose conformality are typically more critical than in photon therapy. This in silico planning study was performed to evaluate different robust optimization (4D) IMPT strategies using full and empty bladder CT scans for locally advanced cervical cancer patients in terms of target coverage and OAR dose. Material and Methods CT scans of five patients were retrospectively selected for this study. The ITV 45 was defined as GTV cervix + 1cm with the addition of uterus, upper vagina, and lymph nodes. IMPT plans were created using two lateral and two posterior oblique fields in RayStation version 5.0 with 5 mm setup and ±3% range robust optimization parameters. In addition, the 4D optimization option available in RayStation was used to create plans which were optimized on two CT sets simultaneously: the plan CT and a selected repeat CT having typically a different bladder volume. Six types of plans were created in total: a full bladder single-CT-optimized IMPT plan (F), a full bladder 4D optimized plan where the repeat CT was chosen such that the bladder volume difference between the two sets was less than 100 cc (F100), and the same type of plan but with the bladder volume difference in the interval of 100-200cc (F200). A corresponding set of plans was also created with the empty bladder planning CT: (E), (E100), and (E200), respectively. Each plan type was evaluated on each of the repeated CT scans and the target coverage of the ITV 45 as well as OAR doses (whole bowel, bone marrow, rectum, bladder) were computed. Plans were optimized to cover 98% of the ITV with 95% of the dose (voxelwise min evaluation), while an effort was made to minimize the whole bowel and bone marrow dose. Results The 4D optimized plans resulted in better target coverage than the non-4D optimized plans (see Figure 1A). Overall, V95 of the ITV45 was highest for the F200 and E200 plans (98.1 and 98.3, respectively). The target coverage was adequate for all planning strategies as long as the bladder volume difference between the plan CT and the evaluation CT was maintained within 100cc. Empty bladder plans resulted in slightly higher whole bowel dose while the bone marrow dose was slightly lower (Figure 1B) , as a result of decrease in the radiological equivalent pathway.

Conclusion As long as the bladder volume differences are maintained within ±100cc, robustly optimized IMPT plans using a single CT scan provided adequate target coverage. With larger bladder volume differences 4D optimized plans with empty bladder plan CTs were the most robust. EP-2047 Surface guided radiotherapy increases setup accuracy for locoregional breast cancer patients A. Mannerberg 1,2 , M. Kügele 1,2 , L. Berg 1,2 , A. Edvardsson 1 , S. Alkner 2,3 , S. Ceberg 1,2 1 Lund University, Medical Radiation Physics- Department of clinical sciences, Lund, Sweden 2 Skåne University Hospital, Department of Hematology- Oncology and Radiation Physics, Lund, Sweden 3 Lund University, Skåne University Hospital- Department of Clinical Sciences Lund- Oncology and Pathology, Lund, Sweden Purpose or Objective The aim of this study was to investigate if the patient setup could be improved for breast cancer patients with nodal involvement, using surface guided radiotherapy (SGRT) compared to the common clinical setup method involving lasers and skin markings. Material and Methods Ten patients were positioned according to the conventional laser and skin markings based setup (LBS) and eleven patients were positioned with surface based setup (SBS), using the Catalyst TM system (C-Rad Positioning AB, Uppsala, Sweden). All patients were initially positioned in supine position with their arms raised over their head. Orthogonal verification images were acquired according to the 'No Action Level” (NAL) correction strategy for both groups to evaluate the setup accuracy. The tolerance for online correction was 4 mm. For LBS the patients were positioned with laser and skin markings aligning and for SBS the patients were positioned according to the Catalyst TM system’s positioning modality cPosition, with numbers indicating an isocenter offset as close to zero as possible. The setup deviations arising from matching the verification images acquired during the treatment course to the reference images were compared between LBS and SBS. In total, 127 verification images for patients positioned with LBS and 93 verification images for patients positioned with SBS were acquired. The cumulative probability was analysed for both groups. To investigate what impact the NAL protocol, combined with 'Adaptive Maximum