Session Item

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. 

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. 

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 the short time that the phantom is positioned in the ZAP system, the phantom does not adapt to the temperature of the internal space of the ZAP system.

The RUBY system can be used for a daily check on the ZAP system. It allows the combination of dose check and patient positioning system check in one workflow. This enables a fast and comprehensive check of the system in the morning.