Technical Note: Investigation of the dosimetric impact of stray radiation on the Common Control Unit of the IBA Blue Phantom2

Abstract Purpose This technical note aims to investigate the dosimetric impact of stray radiation on the Common Control Unit (CCU) of the IBA Blue Phantom2 and the measured beam data. Methods Three CCUs of the same model were used for the study. The primary test CCU was placed at five distances from the radiation beam central axis. At each distance, a set of depth dose and beam profiles for two open and two wedge fields were measured. The field sizes were 10 × 10 cm2 and 30 × 30 cm2 for the open fields, and 30 × 30 cm2 and 15 × 15 cm2 for the 30° and 60° wedges, respectively. The other two CCUs were used to cross check the data of the primary CCU. Assuming the effect of stray radiation on the data measured at the farthest reachable distance 4.5 m is negligible, the dosimetric impact of stray radiation on the CCU and consequently on the measured data can be extracted for analysis by comparing it with those measured at shorter distances. Results The results of three CCUs were consistent. The dosimetric impact of stray radiation was greater for lower energies at larger field sizes. For open fields, the data variation was up to 4.5% for depth dose curves and 7.1% for beam profiles. For wedge fields, the data variation was up to 9.3% for depth dose curves and 10.6% for beam profiles. Moreover, for wedge field profiles in the wedge direction, they became flatter as the CCU was placed closer to the primary radiation beam, manifesting smaller wedge angles. Conclusion The stray radiation added a uniform background noise on all measured data. The magnitude of the noise is inversely proportional to the square of the distance of the CCU to the primary radiation beam, approximately following the inverse square law.

Three-dimensional (3D) water scanners are routinely used for commissioning and quality assurance (QA) of radiotherapy linear accelerators and treatment planning systems. [1][2][3] The Blue Phantom 2 (IBA Dosimetry GmbH, Schwarzenbruck, Germany) is widely used for measurement and analysis of the radiation beams of medical linear accelerators. 4,5 It consists of a Common Control Unit (CCU) and a water phantom with three-dimensional servo. The CCU integrates a controller and two independent electrometers. It also has built-in pressure and temperature sensor interfaces which automatically apply the temperature and pressure correction factor. 6 In the User's Guide, the manufacturer states that "the CCU is a sensitive electronic device that can be affected by stray radiation. In order to prevent significant influence of scattered radiation on the electronics and to increase the lifetime of the CCU, it has to be placed at a minimum distance of 3 m from the radiation field border." 6 Many users consider this recommendation as a protection against radiation damage to the CCU. Few realize that the distance of the CCU to the primary radiation beam can significantly affect the measurement results if the recommendation of minimum distance is not followed. We discovered inadvertently that if the CCU is positioned close to the primary radiation beam, it can result in as high as 10.6% of discrepancies in the measurement results. If users are unaware of this adverse effect and use the tainted measurement results for commissioning treatment planning systems, it can result in substantial systematic errors. It also can cause inconsistency and confusion in linear accelerator commissioning and annual quality assurance. 4,5,7 The purpose of this study is to provide a systematic assessment of the adverse effect of the CCU when it is placed at various distances from the primary radiation, and to make users aware of the dosimetric impact of stray radiation on the measurement results if the manufacturer's recommendation is not followed.

2.A | Data measurement
Three CCUs of the same model were used in the study. One unit CCU1, served as the primary unit, was used to collect all the data; and the other two, CCU2 and CCU3, were used to cross check the results of the primary unit to rule out the possibility that the adverse effect was isolated to a specific CCU. During the data collection, the CCUs were placed on a stand (82 cm above the floor) rather than on the couch to minimize the ambient scattering. It also allowed us to position the CCU to a farther distance up to 4. All data were measured in water with the IBA Blue Phantom 2 using a Varian TrueBeam STx linac. Both the field and the reference detectors were 0.125 cc cylindrical ion chambers (Model CC13, IBA Dosimetry GmbH, Schwarzenbruck, Germany). Two photon energies 6 and 15 MV were used for the data measurement. During the measurement, the reference detector was placed above the jaws so that its signal was not affected by field sizes. The field detector relative to the reference detector was normalized to 100% only once using the 6 MV photon beam with the field detector placed in the central beam at the depth of 1.5 cm of a 10 × 10 cm 2 open field, while the CCU was placed at the distance S = 0.5 m. This normalization was then kept unchanged throughout the data measurement at all distances, field sizes, and energies.

2.B | Data processing and analysis
The data were processed and analyzed with OmniPro Accept 7.5 (IBA Dosimetry GmbH, Schwarzenbruck, Germany). The dosimetric impact of stray radiation on the CCU at various distances was reflected on the measured depth dose curves and beam profiles.
The data measured at different distances were compared and examined. For open fields of 30 × 30 cm 2 and 10 × 10 cm 2 field F I G . 1. Setup of the Common Control Unit (CCU) during the data measurement. The CCU was placed on a stand at 82 cm above the floor. The stand was then positioned at various distances S from the radiation beam. S is the distance between the rear edge of the CCU and the radiation beam central axis. The geometries used to measure the data were gantry = 0°, collimator = 0°, and SSD = 100 cm.     ger with lower energy, as lower energy X-rays generate more scatter radiation. But for the field size 10 × 10 cm 2 , this effect was less prominent for both energies, as shown in Figs. 3(b) and 3(d).  that is, the effect of stray radiation on the CCU was greater for lower energies at larger field sizes. Moreover, the dose variations were negligibly small (<0.4%) for distances beyond 3.0 m.

3.B | Dosimetric impact of stray radiation on the depth dose and beam profiles of wedge fields
For wedge fields, we present only the data measured for 6 MV photon beam because the data measured for 15 MV showed similar trend but the distance effect was less prominent. Figures 5(a) Table 1 and the fitting curve is plotted in Fig. 6(b). The vertical axis is the averaged noise level N and the horizontal axis is the inverse square of an effective distance S' = S + c.
The effective distance was used due to the fact that both the virtual source of the stray radiation and the exact location in the CCU affected by the stray radiation were unknown. The solid dots are the averaged extracted noise levels and the dashed line is the fitted curve. It should be noted that the difference between the profiles measured at 3.0 and 4.5 m was small (0.37%), suggesting that the vendor's recommendation of placing the CCU at least 3.0 m from the primary radiation is sufficient to mitigate such an effect. The mechanism of the stray radiation effect on the CCU is unknown.
Further investigation is warranted to find the cause of this effect. To further reduce the stray radiation effect, we recommend that the manufacturer better shield the CCU from stray radiation, or allow to place the CCU outside the treatment vault.

| CONCLUSION
Stray radiation can have a significant impact on the performance of the CCU, manifested as a uniform background noise added to the measured data. The averaged magnitude of the added background noise level is inversely proportional to the square of the distance of the CCU to the primary radiation beam, approximately following the inverse square law. The adverse dosimetric impact on the measured depth dose and beam profiles can be substantial if the recommended minimum distance is not met. It is important that users are aware of the impact of the CCU effect and always follow the manufacturer's recommendation to place the CCU at a minimum distance of 3 meters from the primary radiation beam.

CONFLI CT OF INTEREST
The authors declare no conflict of interest.