ESTRO 2022 - Abstract Book

S27

Abstract book

ESTRO 2022

Figure 2 shows that the general behaviour of the contribution to the dose of the different components is similar. Overall, the impact to the dose in SV is reduced when the material of each component is modelled as liquid water. Conclusion Small components of the chamber impact k B,Q considerably. Therefore, chamber design and, to a lesser extent, choice of material affects k B,Q , and an accurate model of the chamber components and its further validation are important for correct calculations of k B,Q .

MO-0053 A comprehensive daily QA routine at the ZAP-X system based on the RUBY phantom

K. Buesing 1 , J. Harmsen 2 , P.D. Klassen 3 , H.K. Looe 4 , B. Poppe 4 , D. Poppinga 5

1 Carl-von-Ossietzky University , University Clinic for Medical Radiation Physics, Oldenburg, Germany; 2 Practice for Radiation Therapy Nordhorn-Meppen, Radiation Therapy Meppen, Meppen, Germany; 3 St. Bonifatius-Hospital , Department of Neurosurgery, Lingen, Germany; 4 Carl-von-Ossietzky University, University Clinic for Medical Radiation Physics, Oldenburg, Germany; 5 PTW Freiburg, PTW, Freiburg, Germany Purpose or Objective The ZAP X system is a novel system for stereotactic radiotherapy of brain lesions. It is based on a gyroscopically mounted linear accelerator system with 3 MV nominal photon energy with fixed collimators between 4 mm and 25 mm field size. The aim of the work is the establishment of a fast and comprehensive daily check which combines dosimetry and patient positioning in one workflow. Materials and Methods In order to establish the daily check, a planning CT of a RUBY head phantom (PTW Freiburg, Germany) with RUBY System QA MultiMet insert (PTW Freiburg, Germany) and blind plugs was performed once. For the planning CT, the standard patient immobilization systems were used and a patient mask was prepared for the head phantom. An isocenter was positioned at the central detector position and a treatment plan was calculated using the 25 mm cone size and the “north pole” position (equivalent to 0° at a conventional linear accelerator). The treatment plan contains 500 MU comparable to the standard ZAP dosimetry routine in-air. During the daily routine, the RUBY system was positioned on the couch with the patient mask and a Semiflex 3D ionization chamber (PTW Freiburg, Germany) was positioned in the central detector position. The other two detector positions were filled by blind plugs. The daily routine was started with the automated patient positioning workflow. Thereby, the phantom is aligned by the dedicated workflow based on kV images. After patient positioning, the treatment is started and the dose is measured with the ionization chamber and a UNIDOS electrometer (PTW Freiburg, Germany). The required couch displacement as well as the measured dose is documented daily. As a comparison, the routine free in air provided by ZAP was also performed daily with a Semiflex 3D chamber. Results Both methods, dose measurement in air and the RUBY based workflow, showing constant dose values within 1%. In contrast to the ZAP routine, the RUBY-based workflow can also be used to check patient positioning daily. However, the temperature correction for dose measurement must be observed for the RUBY phantom. For the daily check, the RUBY phantom was stored in the treatment room and the temperature in the treatment room was used as the correction temperature. During