ESTRO 2021 Abstract Book

S1310

ESTRO 2021

Results The result of this study is a patient-specific phantom of human arm part consisting of 12 cuts. The phantom contents volumes corresponding to the different tissues, which are made with different printing parameters. After phantom CT-investigation, the obtained data were compared with initial patient’s data. It is shown that the difference of average CT-indices for each area is less than 20 HU that is comparable with CT-indices uncertainty in conventional CT-scans of patients. Conclusion In this study, a patient-specific phantom of human arm part was manufactured using 3D-printing. The phantom's CT-data are in a good agreement with initial data. Thus, the study prove the possibility of patient- specific phantoms production using 3D-printing. These phantoms are prospective for radiotherapy plan verification in difficult clinical cases. PO-1585 A fast and robust procedure for independent regular Linac output quantification D. Hoffmans 1 , A.J. van de Schoot 1 , R. Tiggelaar 1 , T.H. Pepping 1 , M.J. Simons 1 , M.A. Admiraal 1 , N. van Wieringen 1 , J. Wiersma 1 1 Amsterdam University Medical Centers, Department of Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective Current protocols for output quantification are robust in design but require substantial time and careful handling of the experimental setup. The setup even requires additional care for bore-systems like the ETHOS (Varian Medical Systems) treatment machine, where the treatment isocenter is obscured by the bore. The ETHOS does have a daily output check provided by the Machine Performance Check (MPC). The objective of this study was to design a simple and rigid phantom to quickly, reproducibly and independently measure the output of the ETHOS, to monitor the long-term output stability and to benchmark the results to MPC for three ETHOS machines. Materials and Methods We rigidly positioned a Baldwin Farmer ionization chamber (BF) centrally in a rectangular PMMA block (fig 1). Additionally, we placed a thermometer inside the block. The block is mounted on a plate with three notches in a triangular configuration, to reproducibly mount the block at the far end of the ETHOS couch (fig 1). Using MV imaging we determined the couch coordinates for which the BF is positioned in the isocenter. To establish a charge to dose factor (at standard pressure and temperature) for the BF-Block we determined both the output of the ETHOS according to NCS18 protocol as well as the pressure and temperature corrected reading of the BF for the reference field size. We weekly measured the output on three ETHOS machines by mounting the BF-Block on the treatment couch and moving the table to the predetermined coordinates. Two machines were monitored over a timespan of 9 months, the third machine during 6 months. MPC acquires a large number of parameters to assure that the performance of the ETHOS is within tolerance from a previous set baseline. The signal of the MV imager is used to provide data on the output and to determine that an output change is within a hard-coded tolerance of +/- 4%. We compared MPC data on the output change with the measurement results. This work is supported by the Russian Science Foundation, project No. 19-79-10014.

Results Accurate positioning of the phantom could be verified by MV imaging. All three ETHOS machines showed a gradually increasing output of approximately 0.1% per month (fig 2). The output variation shows a distinct relation with the local pressure of 0.15% per 10mBar. The output of two treatment machines was adjusted during the timespan to stay within our tolerance of +/- 1%. These adjustments were retrospectively removed from all data shown in figure 2. Output deviations reported by MPC show dependency on atmospheric pressure but do not show a gradually increasing output over multiple months. The +/- 4% MPC limit for output change was never violated.