ESTRO 36 Abstract Book

S955 ESTRO 36 _______________________________________________________________________________________________

Table 1 shows RLIC results for AC and EPID. AC data is given by phantom software. It can be noticed that the coincidence for both isocenters is lower for the EPID, this can be explained because AC just take into account one direction in each gantry angle .

EP-1736 Radiation and lasers isocenters coincidence with ArcCheck phantom F. Tato de las Cuevas 1 , J. Yuste Lopez 1 1 Hosp. Univ. de Canarias, Medical Physics Dept., Santa Cruz de Tenerife, Spain Purpose or Objective One tool of Machine QA module of ArcCheck phantom ( AC ) software checks Radiation and Lasers Isocenters Coincidence ( RLIC ). The purpose of this work is to evaluate the precision and accuracy of this software tool, comparing it to the same test made with EPID (Electronic Portal Imaging Device). Material and Methods The LINAC is an Elekta Synergy with Agility MLC and 6 MV energy. The RLIC with ArcCheck phantom ( AC ) are obtained following the instructions of the software manual. The measurements are done in continuous gantry movement and for discrete gantry angles. Measurements are made at 9 º collimator angle for a 1x25 cm field. A series of measurements were made also in 99º to see the MLC effect, as Agility head has not backup jaws. The AC displacements from laser isocenter in two directions are made in order to check software sensitivity. RLIC are made with EPID, positioning a Bearing Ball ( BB ) in the lasers isocenter of a 5x5 cm field and acquiring Images from 0º to 360º gantry angles in 45 º steps. The radiation center of the squared field and the center of the BB are calculated with a MATLAB in-house software. BB center is calculated with sub-pixel accuracy in each direction, 3 profiles are obtained and fitted to Gaussian curves, and the mean maximum of the 3 curves is calculated. Radiation field center is obtained calculating the 50% pixel value of a vertical and horizontal profile. The difference between BB center and radiation field center are computed for each gantry angle for in-plane and cross-plane directions. The RLIC for EPID measurements are computed using these values. Results The RLIC results obtained with AC for each gantry are compared with EPID in the first figure. The mean distance over all gantry angles, for AC (for 9 and 99 º collimator degrees) and EPID are: 0.3, 0.6 and, 0.7 mm, respectively. The AC results are just distance (because this phantom is not capable of give deviation in in-plane direction for each gantry angle). The results for AC for 9º are higher than for 99º because of the irregular MLC radiation field limit exposed for 9º to the AC diodes. The RLIC for EPID are given in in-plane and cross-plane directions, the distance for each gantry angle is calculated from both directions and show a bigger mean value than for AC, because of being calculated in just one direction in this phantom.

The sensitivity of AC for RLIC is fairly good taking into account the uncertainties of measurement 1 mm between laser positions. Conclusion ArcCheck software is capable of give a fairly accurate measurement of the laser and radiation isocenters coincidence, taking into account to add about 0.5 mm displacement in X and Z directions. EP-1737 Efficient troubleshooting of accelerator faults using the TrueBeam Log Viewer software application L.H. Praestegaard 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective In case of an accelerator fault, the identification of the root cause often takes much longer time than the correction of the fault itself (for example replacement of a component or a calibration). Accordingly, the uptime of an accelerator very much depends on an efficient troubleshooting process. In addition, an overview of existing faults is essential for an efficient planning of service tasks. Material and Methods The TrueBeam Log Viewer application is an in-house software application developed in C#. For the Varian TrueBeam system all faults occurring during beam on produces an event. With the application all events, for user-selected accelerators and data interval, can be listed including fault details, treatment plan details, mechanical axes, imaging parameters, and imaging arm positions at the time of the event (see figure 1). For each event the TrueBeam system produces a number of node records, each with detailed information about system parameters versus time just before the fault occurred (for example all MLC positions and motor currents at 500 time steps of 10 ms). All node records are readily available in the application from the list of events. In addition, the application can generate an event alarm, including the corresponding event data, each time an event occurs for user-selected accelerators.