ESTRO 37 Abstract book

S1122

ESTRO 37

NAL protocol, combined with 'Adaptive Maximum Likelihood”, has on the setup, the data was analysed both with and without the offline corrections applied. Thus, including the correction strategy, LBS under the best conditions possible was compared to SBS. Results For LBS, excluding all offline corrections, the amount of fractions where setup deviation exceeded the clinical setup deviation threshold for online correction of 4 mm was 44% / 31% / 36% in the vertical/longitudinal/lateral (vrt/lng/lat) direction, respectively. The corresponding result for SBS was 7.5% / 7.5% / 27% in vrt/lng/lat, respectively. Including offline corrections showed improvement in the setup for LBS, but SBS still provided a higher setup accuracy (see Figure 1). The amount of fractions where the setup deviation exceeded 4 mm when the offline corrections were included in the data was 22% / 21% / 28% for LBS and 7.5% / 6.5% / 20% for SBS in vrt/lng/lat, respectively.

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

Conclusion The results obtained show that SBS is a better method of positioning patients than LBS. The amount of setup deviations exceeding 4 mm was decreased in all directions for SBS compared to LBS. Although the offline correction strategy improved the positioning for LBS, SBS still showed a higher setup accuracy. EP-2048 Surface changes during the course of breast cancer radiotherapy: effect on the in-air outer margin. J. Seppälä 1 , A. Pandey 2 , A. Al-Gburi 2 , J.T.J. Honkanen 1 , T. Virén 1 , J. Heikkilä 1 , T. Koivumäki 3 , K. Vuolukka 1 1 Kuopio University Hospital, Cancer Center, Kuopio, Finland 2 University of Eastern Finland, Department of Applied Physics, Kuopio, Finland