Application of a glass rod detector for the output factor measurement in the CyberKnife.

A radiophotoluminescent glass rod detector has recently become commercially available. We evaluated the feasibility of the commercial glass rod as a new detector for measuring output factors in the CyberKnife. The glass rod detector was irradiated in a water phantom using a holder stand, which was specially designed for this study. The holder was composed of a PMMA tube with an attached vertical bar for the glass rod detector. The measured output factors obtained with the glass rod detector were compared with measurements made with a pinpoint ionization chamber, a diode, and a radiochromic film. The measured relative output factors obtained with the glass rod detector agreed with other detectors within 1.0% for collimator sizes larger than 20mm. However, it was observed that the differences between the output factors measured with the glass rod detector and those obtained with the pinpoint chamber increased rapidly as the collimator size decreased. The relative output factors measured with the diode were consistently higher than those obtained using other detectors for the collimators sizes less than 10mm in diameter. The glass rod detector results were in good agreement with those obtained from the radiochromic EBT film over the entire range of collimator sizes.