Comparison of different detectors in output factor measurement for the CyberKnife system

Abstract

Objective To compare five different detectors in output factor (OF) measurement for the CyberKnife (CK) system, and to select a suitable detector. Methods OFs for 12 different sizes of CK collimators were measured by EBT3 films and 5 different commercially available detectors, consisting of diode detectors PTW 60017 and PTW 60018, ionizing chamber detectors PTW 31010 and PTW 30013, and diamond detector PTW 600019.OF was compared between different detectors and different measurement orientations. Results When the size of collimator was larger than 30 mm, the OF deviation among five detectors was less than 1%.When the size of collimator was smaller than 30 mm, however, the OF deviation among five detectors became large and obviously increased with the decrease in the size of collimator. With a OF deviation less than 2%, PTW 60019 achieved the best agreement with films. Compared with films, diode detectors gave slightly higher OFs, while ionizing chamber detectors gave much smaller OFs. The OF measurement was also affected by measurement orientation. PTW 60019 gave a smaller OF in the direction parallel to the central axis than in the direction perpendicular to the central axis of the radiation field, while PTW 31010 had an opposite result. Conclusions When the size of collimator is larger than 30 mm, PTW 31010, PTW 60017, PTW 60018, and PTW 60019 can be directly used for the OF measurement. When the size of collimator is smaller than 30 mm, correction is needed for the OF measurement using the above detectors. PTW 30013 is not suitable for the OF measurement in the small radiation field. Key words: Small field; Output factor; Detector; Cyberknife