(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To investigate the distance dependence of the common control unit (CCU) of the IBA Blue Phantom² to the primary radiation beam.
Methods: Three CCUs of the same model were used for the study. The primary test CCU was placed at 0.5 m, 1.0 m, 2.0 m, 3.0 and 4.5 m away from the radiation beam central axis. At each distance, a set of PDDs and profiles at depth 10 cm for two open and wedge fields were measured. The field sizes were 10 x 10 cm² and 30 x 30 cm² for two open fields; 30 x 30 cm² and 15 x15 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 data measured at the farthest distance 4.5 m received negligible stray radiation, the effect of stray radiation can be extracted for analysis by comparing it with those measured at shorter distances.
Results: The results of three CCUs were consistent. The closer the CCU was to the primary radiation beam, the stronger the impact on the measured data. The impact was greater for lower energies at larger field sizes. For wedge fields, the profiles in the wedge direction looked flatter, manifesting a smaller wedge angle as the CCU was closer to the primary radiation beam. The variation of the measured data due to the distance was up to 10% for PDDs and profiles. The distance dependence of the CCU follows approximately an inverse square law.
Conclusion: The stray radiation added a uniform background radiation dose to all measured data. The closer the CCU was to the primary radiation beam, the greater the added dose. It is recommended to place the CCU at a minimum distance of 3 meters from the primary radiation beam.
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