Session Item

Daily quality assurance (QA) of the radiation beam on linear accelerators (linacs) used for radiotherapy is important for ensuring a high quality of treatment delivery to the patient at all times. Frequently QA has to be reliable, simple and fast to carry out. Linac build-in systems such as the MV imaging controller (MVIC) of the Elekta Unity MR linac has the advantages of easy to use and with no risk of collision during usage. This project was initiated by an abrupt change in the MVIC image analyses, Fig. 1, after two years of daily QA on an Elekta Unity using the MVIC. The abrupt change occurred around day 35 shown in the Fig. 1 after Elekta adjusted slightly the microwave energy by reducing the Charging Capacitive Power Supply (CCPS) value from 1200 to 1195 V. The CCPS parameter sets the energy of the microwaves. The aim of this work was to investigate the reliability of daily QA using the MVIC and how to interpret observed changes.

A PTW 1D water phantom with two PTW Farmer chambers type T30006 located in a water phantom on the beam axis at depth 10 and 20 cm. The detector at depth 10 cm was located at the isocentre of the linac. An electrical pulse counter was connected to the linac. The gantry was at 90 degree, the field size was 22×10 cm, and 100 MU was delivered in each beam. The CCPS value was varied in steps of 5 V in the linac control software. For each MVIC exposure the electrometer readings, and pulse countings were registered. The scaling factors and intensities of the MVIC images were extracted to calculate the output, flatness and symmetry parameters of the beam. 

In Fig. 2(a) the deviation in output is shown as function of reference field calibration for different settings of the CCPS. A change of 5 V in CCPS results in approximately 1% decrease in output. This is slightly less than what was observed in Fig. 1 which was around 1.5%. The magnetron feedback software was disabled in the measurements shown in Fig. 2 while it was enabled in Fig. 1. Also, an energy change might be the explanation of the small difference in output compared in Fig. 1 and 2(a) for a 5 V change of CCPS.  In Fig. 2(b) the flatness and symmetry properties of the beam are shown for different settings of the CCPS. The CCPS value affect the energy which is seen as a change in flatness. The MVIC panel was able to detect small energy changes. An increase in energy (TPR_20,10)  results in a decrease in flatness.

The MVIC panel of the Elekta Unity linac was found being a valuable, efficient and a reliable tool to detect small changes in output as well as in energy. It is a strength that the MVIC panel is in fixed position all time relative to the linac beam thereby reducing setup uncertainties. Therefore, the MVIC system may detect changes before observed by other external systems such as water phantoms used for routine QA.