Dosimetric comparison between gated and steady photon and electron beams delivered by the Siemens ONCOR linear accelerator.

Objective: The gated irradiation technique is a sophisticated tool for focusing the prescribed dose to periodically moving target volumes while sparing the surrounding healthy tissue. Thus, gated irradiation is predominantly applied to organ motion caused by breathing. The tumour is irradiated in a specific phase of its motion. This requires to switch on and off the beam periodically. The beam-on time may change in a certain range depending on the individual breathing curve, site specific conceptions and the tumour dose conformity required. The latter will be increased for a short beam-on time, however, causing an increase in the number of irradiation cycles for reaching the prescribed dose. Prior to the therapeutic application of gated irradiation, the dose distributions delivered for photons and electrons in steady and gated mode in static phantoms were measured.

Materials: Photon and electron beams were delivered by a Siemens ONCOR Impression linear accelerator. The gating signal was provided by a commercial respiratory gating system based onto a pressure belt feedback mechanism in combination with a respiratory phantom (AZ-733, Anzai Medical). For measurements in stationary phantom linear diode array BMS 96 (Schuster Medizinische Systeme GmbH) was used to measure beam profiles and a solid water phantom (RW3, PTW) with ionization chamber (0.6 cm3 Farmer, PTW) were used to measure the depth dose at different depths. Respiratory frequency were set to 10 and 15 min-1 and beam-on time to 1, 2 and 3 s. Different field sizes (5 × 5, 10 × 10 and 20 × 20 cm2) for 6 and 15 MV photons and 10 × 10 cm2 for 9 and 12 MeV electrons were used.

Results: Depth doses maximum deviation for gated photon and electron beams is up to 0.8 % in comparison to non-gated beams. The maximum deviation between gated and non-gated dose profiles has been found to be less than 3 % and is located in the penumbra region. Deviations are randomized and do not depend on the photon and electron energy. Minor dependence of field size was observed but in most cases, the results for the 10 × 10 cm2 field were more accurate. None respiratory frequency dependence was noticed as well.

Conclusion: To evaluate more realistic cases with respect to therapy the dosimetric and geometric verification using a moving phantom will also been done.