Characteristics of a scintillator-based daily quality assurance device for radiation oncology beams

A plastic scintillator based device marketed as a "DCT 444" is investigated for daily quality assurance (QA) of radiation beams. Variations in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of gantry, collimator, and patient support assembly (PSA) rotations were investigated at photon energies of 6, 10, and 18 MV and electron energies in the range of 5-20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation damage were also investigated in kilovoltage and megavoltage beams. Results indicate that the DCT response is linear with dose and dose rate, but shows directional dependence for all beams. The response is dependent on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated through 0-36 degrees, the DCT response varied within +/- 4.5% and +/- 7.2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, deviations up to +/- 11% were noted. The DCT response is symmetric across 0 degrees and peaks at +/- 60% PSA angle for all accelerators and beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviation as high as 30% was observed when DCT was placed on a thick polystyrene phantom. The long term stability studied over 25 months showed varied signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2-4 weeks). The lightweight DCT device is linear with dose and dose rate, stable, and easy to operate. It stores ten readings that can be downloaded to a personal computer. This study indicates that DCT-444 is a suitable QA tool for central axis dose measurement. However, proper precaution is needed as this device exhibits directional and positional dependence. For a meaningful QA result of a radiation unit, the daily QA setup geometry should be identical to that used for calibration of the DCT device.