Accurately Evaluating Settling Responses of Ionization Chambers Used in Radiation Therapy Depend on the Accelerating Tube

摘要

The growing use of multiple small fields in radiotherapy treatments such as intensity-modulated radiation therapy has increased the importance of small-field dosimetry. In this study, we investigate the settling response of a set of ionization chambers exposed to 4, 6, and 10 MV stereotactic radiotherapy X-ray fields. Previous studies had reported that lack of pre-irradiation could result in settling response errors of up to several percentage points. While the use of ionization chambers does contribute to this behaviour, the most obvious factors affecting the settling response appear to be the area of the insulator, the material used for the central collector electrode, and the accelerating tube. The results of this study show that Farmer-style ionization chambers with electrode connections that are guarded up to an active air volume settle quickly (within five minutes); moreover, the changes in their responses are small. On the other hand, small ionization chambers exhibit settling times of 11-24 min. In this study, the settling times of small ionization chambers were found to be dependent on the accelerating tube.