Systematic studies on the beam stability and the transient effect of a siemens ONCOR impression accelerator

Abstract

The measurements described here were performed in preparation of an electron linear accelerator ONCOR Impression (Siemens, Concord, USA) for gated irradiation. The gating signal was derived from the system based on a pressure sensor belt (AZ-733V, Anzai Medical, Tokyo, Japan). In a first series of measurements, the stability of the gated beam was studied. For this, dose distributions delivered to a phantom at rest in gated as well as in static accelerator mode were compared. The duty cycles, field sizes and radiation modalities were varied in a wide range. The experiments revealed a maximum depth dose deviation (on the central beam) of less than 0.8 % and a maximum deviation in the lateral dose distribution of 3 %. The transient effect during switching on and off the beam influences the duration of the irradiation, in particular for gated treatment with low duty cycle. This effect is a special feature of each accelerator and depends on the radiation modality. Therefore, the transient effect of the used accelerator was measured and analyzed. A pronounced delay between switching on the irradiation and reaching the dose equilibrium was found for 6 MV X-rays. For periodically moving targets, dose blurring depends on the amplitude of the movement and on the duty cycle. The latter influences substantially the treatment time, thus it is required to find a compromise between precision and duration of the irradiation. Using a moving phantom the dose blurring was analyzed in dependence on the duty cycle. © 2009 Springer-Verlag.