CHARACTERISTICS OF A SCINTILLATOR-BASED DAILY QUALITY ASSURANCE DEVICE FOR RADIATION ONCOLOGY BEAMS

A plastic scintillator based device marketed as a ''DCT 444'' is inves tigated for daily quality assurance (QA) of radiation beams. Variation s in DCT responses were studied for three identical devices with beam, energy, distance, dose, dose rate, and orientations. The effects of g antry, collimator, and patient support assembly (PSA) rotations were i nvestigated at photon energies of 6, 10, and 18 MV and electron energi es in the range of 5-20 MeV from three different linear accelerators. The short and long term linearity, reproducibility, and radiation dama ge were also investigated in kilovoltage and megavoltage beams. Result s indicate that the DCT response is linear with dose and dose rate, bu t shows directional dependence for all beams. The response is dependen t on its orientation and the angle of rotations of collimator, gantry, and PSA of the linear accelerator. When the collimator is rotated thr ough 0-360 degrees, the DCT response varied within +/- 4.5% and +/- 7. 2% for the 6 MV and 18 MV beams, respectively. With PSA rotation, devi ations up to +/- 11% were noted. The DCT response is symmetric across 0 degrees and peaks at +/- 60 degrees PSA angle for all accelerators a nd beam energies. The effect of backscattering medium on DCT response is noted for all beams but significant for kilovoltage beams. A deviat ion as high as 30% was observed when DCT was placed on a thick polysty rene phantom. The long term stability studied over 25 months showed va ried signal losses possibly due to radiation damage. However, there is no noticeable signal loss over a short period of time (2-4 weeks). Th e lightweight DCT device is linear with dose and dose rate, stable, an d 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 geo metry should be identical to that used for calibration of the DCT devi ce. (C) 1996 American Association of Physicists in Medicine.