ESTRO 38 Abstract book

S926 ESTRO 38

Measurements were performed in a water phantom (PTW MP3 Phantom Tank) with vertical beam geometry. The source-to-surface distance was 100 cm and the field size 10 x 10 cm 2 . The chambers were situated at depth 10 cm in the centre of the field and integration time for collecting the charge (Q) was 60 s. Two readings of 100 MU set in 300MU/min rate were collected at each voltage for each measurement of ks. Nine different polarization voltages (V) were used (+50V, +100V, +150V, +200V, +250V, +300V, +350V, +400V). The chambers were at a fixed position and polarization voltage was changing giving enough time elapsed for the chamber to achieve The ks value was determined; firstly plotting the data 1/Q and 1/V for each ionization chamber in a Jaffé curve (Eq. 1) in order to confirm their linearity and so the reliability of Boag’s model for the beam in the range of voltages. A second value of ks was determined by means of two- voltage method (Eq. 2) measuring at polarization voltage +100V and +400V. This method assumes the linear dependence of 1/Q and 1/V. Value of ks was also determined by theoretical calculation taking into account the effects of initial recombination, diffusion and volume recombination (Roos and Derikum, 2000)(Eq.3) stabilization. Measurement

Conclusion Ks value is specific for each ionization chamber and need to be estimated for absolute dosimetry measurements. Two-voltage method should be applied only after verification of the linearity of 1/Q and 1/V in the range of polarization voltage used. Jaffé curve must be measured for obtaining greater accuracy. EP-1719 Automated data processing and BigData in radiation therapy O. Schmidt 1 , N. Ballmann 1 , C. Bert 1 , R. Fietkau 1 1 University Hospital Erlangen, Radiation Oncology, Erlangen, Germany Purpose or Objective The wide range of technologies and systems in radiation therapy produces a vast amount of heterogenic data sets. These are neither easy accessible nor processable for automated use in terms of clinical, technical or organizational issues. Therefore an IT-infrastructure has been developed which integrates the different parts of the treatment process in a single setup without a secondary data storage system. This project evaluates the effort, feasibility and validity of a single setup automatization approach with the oncology-information-system (OIS) Mosaiq (Elekta AB, Stockholm). Material and Methods The Mosaiq database is key to organizational issues and technical treatment delivery data in its role as record & verify system. Therefore a direct read-only access to the database was granted by Elekta. Topics of recurring interest can be displayed in a web application, accessible on every workstation in the institution, whereas non- regular issues are processed in a program within the infrastructure and given templates. Results The actual usage per treatment series of the different radiation techniques, as recorded in Mosaiq, was of interest for the year 2017. The distribution resulted in 3D- conformal (44%), VMAT (26%), StepNShoot-IMRT (22%) and Arc (8%) as shown in fig.1. Another issue was the identification of patient groups, which could be treated at the Linac VERO (Brainlab GmbH, Munich) without major changes to the treatment plan in order to increase the workload of that specific Linac considering the technical limitations (max. field size (x, y<15 cm), energy level (6 MeV) table rotation (<35°)). The evaluation was performed for the treatment parameters of all delivered series at the department in the given period. The result

Results Jaffé plots of the three chambers studied are shown in Figure 1. Farmer ionization chamber shows the highest linearity. Table 1 presents ks values determined by different approaches: Jaffé plot, two-voltage method and theoretical calculation, for each chamber. In theoretical calculation, the lowest ks is the one obtained from the PinPoint chamber, since it has smaller electrode distance. However, in the empirical methods ks obtained are larger, as well as deviation. This can be explained by the major instability of the measurements compared to the bigger chambers.