ESTRO 2021 Abstract Book

S1496

ESTRO 2021

isocentric FLASH-RT treatment. It can be used to estimate the motion prior to treatment and to assure that the patient remains in the correct position during beam delivery.

PO-1768 Comparative study of two imaging systems for total body irradiation D. Gensanne 1 , S. Linca 2 , B. Dubray 1 1 Centre Henri Becquerel, Radiothérapie, Rouen, France; 2 Centre Henri Becquerel, Radiotherapie, Rouen, France Purpose or Objective During total body irradiation, the protection of lungs is performed by customized shielding blocks. To verify their positions before the beginning of the treatment and for every patient positioning modifications, ERLM films are commonly used. But there is no way to get intrafraction information, what is the main drawback of this kind of detector. In order to monitor the patient position during the irradiation, the Theraview Total Body Imager (TBI) was developed by Cablon Medical ®. Performances of the system are evaluated and an example of clinical imaging obtained is presented and discussed. Materials and Methods Irradiation is performed on a Novalis Truebeam Stx (Varian®) using a 6MV photons beam. Gantry is positioned to 270°, the field size is 40x40cm and the collimator position is fixed to 45°. In clinical used, the patient is placed at 4m from the radiation source and the imaging system is positioned behind the patient. For evaluating the performance of the TBI system, the Las Vegas phantom is used. The phantom is placed to 4m to the source and images are acquired with the two imaging systems, Theraview TBI and ERLM Kodak® films, respectively. 30 MU are delivered to get the images, corresponding to the optimal setting relative to doses and contrast to noise ratio for the Theraview TBI. Phantoms images obtained with the two imaging systems are compared in terms of Contrast to Noise Ratio (CNR) and number of visible inserts. According to the same acquisition technique, images obtained in a clinical use are acquired and compared. Results From a qualitative point of view, Las Vegas images obtained with the ERLM films are greater than that obtained with the Theraview TBI. These results are confirmed with the quantitative analysis given in the following table. Theraview TBI ERLM Film Such results are confirmed in clinical used but the image quality of the Theraview TBI still acceptable to visualize lungs and the shielding block positions. Conclusion Despite a lower image quality than ERLM Films, the Theraview TBI give the possibility to monitor the patient motion and the shielding block positions during the treatment. The advantage of the system is to detect intrafraction movements of the patient and to fine-tune the block position to better protect lungs. Moreover, as the Theraview TBI gives real-time images, the patient workflow is simplified and it is less time consuming. Further improvements have been done since this study by Cablon to increase the spatial resolution and the contrast to noise ratio. This new system is called TBI Cnergy (Cablon Medical ®). PO-1769 Dosimetric consequences arising from beam interruptions with Catalyst – Deep inspiration breath hold F. Orozco Martínez 1 , R. Morís 1 , R. Colmenares 1 , M. Cámara 1 , A.B. Capuz 1 , J.D. García 1 , D. Sevillano 1 , P. Retorta 1 , M.J. Béjar 1 , D. Prieto 1 , P. Galiano 1 , F. García 1 1 Hospital Universitario Ramón y Cajal, Medical Physics, Madrid, Spain Purpose or Objective The assessment of the dosimetric impact of treatment beam interruptions with Catalyst system in breast deep inspiration breath hold (DIBH) patients. Materials and Methods We analysed this effect by means of two different methods: radiochromic films (Gafchromic RTQA2) and SRS MapCheck (SunNuclear). For this purpose, we defined a perpendicular rectangular radiation beam which was delivered on the radiochromic film and the SRS MapCheck detector, both of them mounted on the QUASAR platform (figure 1). Number of visibly inserts 10 15 Dimension of the lower detectable insert (mm) 2 1 CNR 0.85 à 2.25 1.75 à 3.75