Dosimetric comparison between gated and stationary photon and electron beams delivered by the Siemens ONCOR Avant-Garde linear accelerator.

Objective: The gated irradiation technique is a sophisticated tool for focusing the prescribed dose to periodically moving target volumes whilst 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 duty cycle may vary in a certain range (about 17 to 75%). It depends on the individual and site specific time dependent elongation and the tumour dose conformity required. Obviously the latter will be increased at a low duty cycle, which, however, increases the number of irradiation cycles for reaching the prescribed dose. Transient effects during switching on and off the beam may influence the dose and its homogeneity. Prior to the therapeutic application of gated irradiation of bronchial carcinoma the dose distributions delivered for photons and electrons in steady and gated mode have been measured.

Methods: Photon and electron beams were delivered by an ONCOR AvantGarde linear accelerator (Siemens). The gating signal was provided by a commercial respiratory gating system based on a pressure belt feedback mechanism (AZ-733, Anzai Medical, Tokyo, Japan) in combination with a respiratory phantom. Lateral profiles of dose were measured by means of a linear diode array BMS 96 (Schuster Medizinische Systeme GmbH, Forchheim, Germany). Depth distributions within a solid water phantom (Piacryl RW3, PTW Freiburg, Germany) and output factors were determined using a Farmer ionization chamber (0.6 cm3) of PTW, Freiburg, Germany. Respiratory frequency values were set to 10 and 15 min^-1 (i.e. 6 and 4 s, respectively) and beam-on time values of 1, 2 and 3 s were chosen. Different field sizes (5 × 5, 10 × 10 and 20 × 20 cm²) were used for 6 and 15 MV photons and 10 × 10 cm² for 9 and 12 MeV electrons. The dose values and output factors of the gated beam delivery have been evaluated relative to those of the constant mode at 50, 100 and 200 MU for photons and 100 and 200 MU for electrons.

Results: Output factors and depth doses maximum deviations for gated photon and electron beams are up to 0.8 % in comparison to non-gated beams. The difference in symmetry and flatness of the dose profiles is less than 3 %. Deviations increase with decreasing beam-on time, decrease with increasing depth, are randomized and do not depend on the photon and electron energy. Minor dependence on the field size was observed but in most cases the results for the 10 × 10 cm² field were more accurate. None respiratory frequency dependence (i.e. 15 min^-1 or 10 min^-1) was noticed as well.

Conclusion: All this measurements were performed in a stationary phantom and they are acceptable for clinical applications. To evaluate more realistic cases with respect to therapy the geometric and dosimetric verification in moving phantoms will be done.