Radiation exposure during head repositioning for multiple shot gamma knife treatment with APS

Leksell Gamma Knife (LGK) stereotactic radiosurgery is performed all over the world. Two-hundred-one narrow Co-60 photon beams are focused at a point; hence, many shots are required to cover a complicated or large target volume. The automatic positioning system (APS) enables us to deliver many shots without entering the treatment room. The treatment couch moves out 23 cm to a defocused position during the automatic head repositioning. Since the patient’s head stays inside the LGK unit, additional exposure to the patient is expected. This paper presents the results of radiation dose measurements during the head repositioning (intra-shot dose) when using the APS system. We built a 16-cm diameter solid phantom for the APS system. The phantom holds one ionization chamber and nineteen TLD chips for dose measurements. We created a treatment plan, for which there were nine shots with the 18-mm helmet and the shots were placed at the nodes of 3x3 grids in the horizontal plane crossing the beam focus point. The phantom moved by 2 cm between shots eight times. Measurements showed that the total intra-shot dose to the center of the phantom was 14 cGy for Co-60 sources producing a dose rate of 310.7 cGy/min. The intra-shot dose per repositioning varied from 0.4 cGy to 4.7 cGy (the average was 1.74 cGy), depending on the shot locations relative to the measurement point. The additional radiation dose during the couch in and out was 0.3 cGy. The results showed that the intra-shot repositioning with the APS system adds small but not negligible dose to the dose expected for the manual repositioning method.