(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: The error detection capabilities of 2D-arrays were investigated and compared.
Methods: Measurements were carried out at a Siemens Artiste linac with 6 MV. MLC leaf bank errors of 1, 2 and 3 mm were introduced to the x1 leaf bank of a 15x15 cm² field and multi-segment IMRT fields. The 2D-arrays OCTAVIUS 729 (PTW-Freiburg) and MapCHECK 2 (Sun Nuclear) were placed in a RW3 (solid water) slab phantom in a depth of 3 cm with the isocenter plane at their effective depth of measurement. The change in gamma index passing rate with respect to the introduced x1 leaf bank error was investigated when comparing the erroneous measurements to the reference measurements with the same 2D-array.
Results: With increasing leaf bank error a steeper decline in the local 3 mm/ 3% passing rate was investigated for the OCTAVIUS 729 compared to the MapCheck 2. While passing rates for a 3 mm leaf bank error were 76.7 % and 82.7% for the OCTAVIUS 729 and the MapCHECK 2 array respectively when measuring a head and neck IMRT field, the passing rates were 73.5 % and 81.3 % respectively for a prostate IMRT field. While a 2 mm leaf bank error in a 15 x 15 cm² field was detected by all OCTAVIUS 729 detectors adjacent to the x1 collimator edge using a 3 mm/3% gamma criterion, 1/3 of the MapCHECK detectors adjacent to the x1 leaf bank failed to register the collimator error.
Conclusion: The steeper decline in gamma index passing rate with increasing leaf bank error for the vented ionization chamber array can be explained by its bigger detector dimensions and therefore higher fill factor compared to the diode array, while its volume averaging effect does not seem to influence the gamma index analysis of the investigated field setups.
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