Dosimetry of a linear accelerator under respiratory gating

In the present study, the impact of respiratory gating on the beam characteristics of a linear accelerator is investigated. The main focus is put oil the influence of the duty cycle. Measurements mere performed oil a linear accelerator type Oncor (Siemens) with photon energies 6 MV and 15 MV, equipped with the Anzai gating system AZ-733V. Depth dose curves and beam profiles were found not to be significantly altered by gating even for duty cycles down to 5% for realistic respiration frequencies (dose variations all <2.5%). However, for very small duty cycles, the absolute dosimetery changes significantly (dose deviations >10%), leading to clinically relevant underdoses. The crucial parameter for the the dosimetry is the number of monitor units per gate. Our results imply that treatment planning for respiratory gating call he performed on the basis of data obtained under ungated operation if and only if the absolute gates sizes during treatment are sufficiently large. The limiting values for the gate sizes have to be determined individually for each accelerator.