Session Item

On a linac, the position of the isocentre is used as the reference, e.g. for calibration of MLCs and jaws. Typically, the lowest energy defines the reference isocentre for linacs with multiple energies available. Thus a simple and fast test is needed to ensure that all energies agree with this reference. The current study presents a novel and simple phantom that fast but accurately can measure the spot position using the electronic portal imaging device (EPID) for both FF (flattening-filter) and FFF (flattening-filter-free) energies.

Two brass spheres (BSs) with a diameter of 5 mm and a central hole of diameter 0.5 mm were glued to a string passing through the holes approximately 40 cm apart. Four such strings were placed in the corners of a 6×6 cm square inside a polymethyl methacrylate (PMMA) box to prevent airflow-induced movements. An additional line with one BS was placed centrally between the other BSs, Fig. 1(a). Using the room lasers, the box was placed with the central BS approximately at the isocentre (exact position not important).

The BSs were irradiated with 20 MU while acquiring EPID images at gantry angles from 356° to 4° in one-degree steps. Lines on the EPID images through each pair of BS will pass through the point of vertical beam direction. Thus, the crossing point of multiple lines is the vertical beam spot position. A MATLAB code was used to analyse the images for BS positions and perform cross-point calculations, Fig. 1(b). Variation in spot position was measured as the difference between the measured cross-points (U, V) and the 6 MV FF reference (Uref, Vref).

Variation in spot position from four independent phantom setups on different days was performed at two Elekta linacs. The data for the Versa HD accelerator is seen in Fig. 2. Overall, the deviations are within two pixels. A pixel represents ~0.4 mm spot translation and 1.1 mm change in field edge position since the MLC is calibrated at the reference position and placed one-third between the target and the patient. The 10 MV FF beam was more stable than 6 MV FFF and 10 MV FFF, which might be related to the lack of servo-steering of the FFF beams in the U (lateral) direction - in the other direction servo-steering is available for all energies. The error bars in Fig. 2(a) are the standard deviation of the repeated imaging series. For all energies, the gantry averaged standard deviation was <0.23 and <0.16 pixels in the U and V direction, respectively. The largest variation was seen for 10 MV FFF.

The increased uncertainty of the crossing point of the FFF beam might partly be related to image noise since the EPID frame rate is constant; thus, a higher dose rate and fixed number of MUs result in fewer frames (~factor 4).

A novel phantom able to detect the relative position of the beam spot was developed. The phantom is easy to set up at the linac, does not need precise alignment and provides precise measurements of the beam spot position of multiple beam energies.