ESTRO 38 Abstract book

S1066 ESTRO 38

of motion observed at simulation. The mean absolute paddle height modification required was 7.4 mm (range 4- 13 mm). 3 cases were modified prior to the first fraction, 2 later in treatment. Of the patients treated without compression, 3 required coaching to improve breathing to match liver motion limits based on 4DCBCT. Conclusion 4DCBCT IGRT identified 5/13 cases where modifications to abdominal compression were required to achieve liver and OAR motion equivalent to simulation. For patients unsuitable for abdominal compression, 3/6 cases were identified where coaching was required. 4DCBCT is a valuable tool in ensuring the planned dose is successfully delivered. EP-1955 Increased accuracy in setup position by using surface scanning M. Garcia 1 , J. Dekker 2 1 Dr. Bernard Verbeeten Institute, Radiotherapy, Tilburg, The Netherlands ; 2 Dr. Bernard Verbeeten Institute, Medical Physics, Tilburg, The Netherlands Purpose or Objective The radiotherapy lung patients are traditionally setup using permanent tattoos en laser. During the setup the positioning of the arms can be difficult to position. Depending on the GTV and PTV repositioning can be necessary. The objective of this study was to investigate the accuracy and reproducibility of the arm and body position using surface scanning, compared with our standard positioning method using lasers and pointguards. Material and Methods  Twenty healthy test subjects participated in this study.  Initial position: The test subjects were first positioned on the treatment couch according to our standard setup protocol for lung patients. The body and arms were marked in the Z (vertical), Y1, Y2 (longitudinal), and X (lateral) direction. The marked direction are showed in figure 1

Results A tolerance margin of 3 mm (millimeters) was applied as a criterion for an accurate setup position. Results:  Body position: The deviation in X, Y and Z direction was within the tolerance margin for 78% of the measurements using the standard positioning method, compared to 75% using the surface scanner.

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Y 2 longitudinal body deviation: With 59% of the measurements, the positioning using laser alignment and pointguards showed a deviation that was smaller than or equal to 3 mm. When using the surface scanner, this occurred in 76% of the measurements. Arm position: In 27% of the measurements, the deviation was within the tolerance margin when using the standard method, while this was increased to 42% when using the surface scanner. The results of the arm positioning are showed in figure 2. Setup time: The average setup time was 1 minute and 7 seconds for the standard positioning. This was increased to 2 minutes and 17 seconds for the setup using the surface scanner.

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Positioning: Two positioning methods were compared, both using absolute table positions 1 : - Standard method: Positioning with laser alignment and pointguards. - Surface scanner: Positioning with a Catalyst™ surface scanner. Analysis: For both positioning methods, the deviation of the markings in the Z, Y1, Y2, and X directions was measured. To establish the efficiency of both positioning methods, the setup time per positioning method was also measured.

Conclusion The positioning method using the surface scanner showed an improved body (Y2 direction) position, especially with regard to the arm position.As the positioning method with the surface scanner entails a new positioning technique, it took relatively longer to set up the patient than with the positioning standard method using laser alignment and pointguards. 1 Kruijf de, W., & Martens, R. (2015). Reducing patient posture variability using the predicted couch position EP-1956 Image quality of in-treatment 4D-CBCT obtained at various doses in VMAT for SBRT: a phantom study Y. Shimohigashi 1 , Y. Doi 1 , M. Maruyama 1 , Y. Yotsuji 1 , Y. Kai 1 , R. Toya 2 1 Kumamoto University Hospital, Radiological Technology, Kumamoto, Japan ; 2 Kumamoto University Hospital, Radiation Oncology, Kumamoto, Japan Purpose or Objective In-treatment four-dimensional cone beam computed tomography (4D-CBCT) is a useful tool for assessing the intra-fractional location of a moving tumor and determining planning target volume (PTV) margin. However, in-treatment 4D-CBCT projection data acquired during volumetric modulated arc therapy (VMAT) depends

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