Relative electron dosimetry using the Scanditronix-Wellhöfer Beam Imaging System-2G

The Beam Imaging System 2G (BIS-2G) from Scanditronix-Wellhöfer is a two-dimensional (2D) charge-coupled device (CCD)-camera that measures the scintillation light produced by incident radiation. We examined the performance of the BIS-2G as a tool in quality control of patient boluses. In an attempt to simplify the production of the patient boluses, bolus edges were built as staircases and the dose distributions were measured and compared to the dose profiles below corresponding sloped bolus edges. Perspex plates covering half the irradiated field were used as generalized bolus edges. All BIS-2G measurements were performed using buildup of solid water while a diode measured corresponding dose profiles in a water phantom. Below the patient boluses, regions with doses < 95% and > 107% of the prescribed dose were defined. Below the edge, the relative doses measured by the BIS-2G were generally within 3% in dose and 3 mm in position compared to the diode measurements. Close to the field edge below the bolus, the BIS-2G measurements were in some cases as much as 7% lower in dose than the diode measurements. The BIS-2G measurements revealed hotspots below the patient boluses covering 1-16% of the total irradiated area. The highest point dose measured below the patient boluses ranged from 105% to 125% of the prescribed dose. For all bolus thicknesses, each edge in the staircase bolus caused a fluctuation in dose and increased the maximum dose compared to the sloped edge. For several cases, the maximum dose increased with 13% in relative dose, e.g., from 103% to 116%. The BIS-2G was found to be a useful tool in quality control of patient boluses, revealing large hot spots in the treatment volume for several patients. Bolus edges built as staircases cause considerable dose fluctuations and increase the maximum dose, and can therefore not be recommended.