Daily Elekta Unity beam checks with the on-board MV imager during the first year of clinical use
Guido Hilgers,
The Netherlands
PO-1734
Abstract
Daily Elekta Unity beam checks with the on-board MV imager during the first year of clinical use
Authors: Guido Hilgers1, Marijke Ikink1, Ilona Potters1, Danny Schuring1
1Radiotherapiegroep, Medical Physics, Arnhem/Deventer, The Netherlands
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Purpose or Objective
To report on the daily beam checks of our Unity MR-Linac (Elekta, Stockholm, Sweden) with the on-board MV imager (MVI) during the first year of clinical use.
Material and Methods
For one year – from July 1, 2021 to June 30, 2022 – daily beam checks were performed. Each day, the MVI was irradiated with 100 MU using an 18 cm × 8 cm field at a gantry angle of 0 degrees, and an image was acquired. From the image, profiles through the beam center in AB (IEC1217: X) and TG (IEC1217: Y) directions were extracted. The beam center was defined as the mean isocenter pixel of the MVI. See Fig. 1 for a map of block positions (X1, X2), leaf bank positions (Y1, Y2) and profile directions.
Figure 1: Block positions, leaf bank positions and profile directions.
Field sizes and symmetries in both directions were determined using the methods described in the pre-publication of NCS report 33 (2020). This report uses a model-based approach to obtain the field edges from a four parameter fit on both sides of the profile. The field size is defined as the distance between the inflection points of these fits. The symmetry is defined as the difference between the areas under the left and right sides of the profile divided by the average of these areas.
Results
In total, 208 daily beam checks were performed. See Table 1 for both the results and the differences of the mean values with respect to the nominal values.
Table 1: Measured positions, field sizes and symmetries.
| Direction | Item | Mean [Max – Min] | Mean - Nominal |
| AB | X1 (mm) | -89.6 [-89.8 – -89.5]
| 0.4
|
| X2 (mm) | 90.2 [90.0 – 90.4]
| 0.2
|
| Field size (mm) | 179.8 [179.7 – 179.9]
| -0.2
|
| Symmetry (%) | 0.3 [0.2 – 0.4]
| 0.3
|
| TG | Y1 (mm) | -40.1 [-40.2 – -40.0]
| -0.1
|
| Y2 (mm) | 39.8 [39.6 – 40.1]
| -0.2
|
| Field size (mm) | 80.0 [79.8 – 80.3]
| 0.0
|
| Symmetry (%) | -0.5 [-0.6 – -0.4]
| -0.5
|
The block and leaf bank positions were well within the tolerance level of ±1 mm defined by the Elekta MR-Linac Consortium (Roberts et al., Med. Phys. 2019). Consequently, the field sizes were also in close agreement with the nominal field sizes and well within the defined tolerance level of ±2 mm. The variation in Y2 positions (0.5 mm) was slightly larger than the variation in Y1 positions (0.2 mm) due to the second of the two MLC re-calibrations that were performed during the first year. This re-calibration caused a systematic shift of approximately +0.2 mm (≈ 1 pixel) in the mean Y2 position. Note that this is also reflected in the variation in TG field sizes. The symmetries were close to 0.0% as can be expected, since the MVI applies a flood field calibration to the acquired images.
Conclusion
The daily beam checks of our Unity MR-Linac with the MVI showed that block positions, leaf bank positions and field sizes were extremely stable. It was also shown that they were well within the tolerance levels of the Elekta MR-Linac Consortium, which suggests that it is possible to be more stringent.