ESTRO 2020 Abstract Book

S189 ESTRO 2020

OC-0352 Increased accuracy in reduced time – surface guided RT for hypofractionated prostate cancer patients A. Mannerberg 1 , M. Kügele 1,2 , S. Hamid 2 , K. Petersson 2 , A. Gunnlaugsson 2 , S.Å. Bäck 2 , S. Engelholm 2 , S. Ceberg 1 1 Lund University, Medical Radiation Physics, Lund, Sweden ; 2 Skåne University Hospital, Hematology- Oncology and Radiation Physics, Lund, Sweden Purpose or Objective Ultra-hypofractionated radiotherapy of prostate cancer treats patients in fewer number of fractions and with higher fractionation dose compared to conventional fractionation schemes. The treatment efficiency is maintained and there is no increased risk for late toxicity (Widmark et al 2019). To further reduce patient treatment time, flattening filter free (FFF) beams are used. The aim of this study was to: • Evaluate if surface guided radiotherapy (SGRT) could further reduce the total treatment time by reducing the patient setup time. Material and Methods A total of 40 prostate cancer patients were enrolled in this study. All patients received a hypofractionated (42.7 Gy / 7 fr) 6MV-FFF VMAT treatment plan. Twenty patients were positioned with LBS and 20 patients were positioned using surface based setup (SBS). For LBS, the patients were positioned by aligning skin tattoos with in-room lasers. For the SBS, the optical surface scanning system Catalyst TM (C- Rad Positioning AB, Uppsala, Sweden) was used. A deformable algorithm was used to calculate the patient isocenter position. A colour map projected onto the patient skin with an 8 mm tolerance was used for posture correction. The daily patient setup was verified by two orthogonal kV images and internal gold fiducial markers were used for the matching. For comparison purposes, the deviation from the planned position and the online position was evaluated by the total vector offset, V , for both LBS and SBS. The displacement from the planned position in the lateral (lat), longitudinal (lng) and vertical (vrt) direction, respectively, was used to calculate V (Eq. 1). • Investigate if SGRT can improve prostate patient setup accuracy compared to conventional laser based setup (LBS). In total, 280 verification images were evaluated. The patient setup time was extracted from the linac log files for both LBS and SBS. Results Results showed that the patient setup time decreased with 24% using SBS compared to LBS. The patient setup time was on average reduced with 42 s for each treatment fraction (p < 0.01, Mann Whitney U test). A statistically significant improvement in the total vector offset was observed for SBS (p < 0.01, Mann Whitney U test). The median total vector offset for LBS was 5.2 mm (0.41 - 17.3 mm) and the corresponding number for SBS was 4.6 mm (0.0 - 10.4 mm). The results show that with SBS the largest setup deviations are detected and eliminated. Also, the SBS results follow the colour map tolerance, which indicate that with narrower tolerance even better setup results will be obtained. However, this needs to be further evaluated. (Eq. 1)

Conclusion Using surface guided radiotherapy for ultra- hypofractionated prostate cancer patients reduced the patient setup time significantly with 42 seconds per treatment session. Furthermore, the initial setup accuracy was improved. OC-0353 Finding the contouring tool (manual vs automated segmentation) for prostate patients in RayStation F. Vallejo Castañeda 1 , M. Hinse 1 1 Centre intégré de cancérologie de Laval, Département de radio-oncologie, Laval, Canada Purpose or Objective To find the best contouring tool between manual-expert contours exported from Pinnacle vs automated contours done in RayStation (RS) using either an ATLAS base (ABS) or a model base (MBS) segmentation. Material and Methods A total number of 43 patients was used for the study. From them, 26 were used to construct 3 ABS models with a different specification and number of patients each (ABS15, ABS19 and ABS26). The remaining 17 patients were used for validation. Every validation patient was imported into RS and further sets of contours were created (ABS and MBS). Each ABS had expert contours of the bladder, rectum, and the femurs. ABS19 was the model applied to all validation patients. It was further used to compare it to ABS26 to test if increasing the number of patients to create a model would further improve the contours. ABS15 was the model for which a selection of patients with an specific bladder volume between 100-200cc was created. We wanted to test if the final contour of an organ could be improved by being in the same volume range as the model. From the validation patients; 7 were selected for which the volume of the bladder was in the range of ABS15. For them 2 further bladder contours were created using ABS19 and ABS15. Finally, the metrics used to compare our results were the mean DICE similarity coefficient between the expert vs. automated model and the time it took to run each model. Results For the bladder, MBS contours gave better results than ABS19 (0.91±0.06 vs 0.76±0.15). For the rectum the opposite was observed where ABS19 was better than MBS (0.72±0.08 vs 0.55±0.20). For the femurs comparable results were obtained with both models (0.95±0.01 (ABS19) vs 0.93±0.01 (MBS)). For the 7 patients selected to test ABS15, no significant increase in the mean DICE index for the bladder was obtained (0.75±0.11(ABS15) vs