To determine the peripheral dose (PD) of a Trilogy accelerator under different conditions and the feasibility of PD measurement using the semiconductor diode ionization chamber.

In a solid water phantom, a CC13 air-filled ionization chamber and a semiconductor diode ionization chamber were used for PD measurements with different distances (13 measurement locations within 1-31 cm), depth (3, 5, 15 cm), field sizes (10, 20, 30 cm), wedge (W15, W45, VW15, VW45), and beam energy (6, 18 MV). The relationship of PD with PD_leakage and PD_scatter was determined by removing the scatter phantom. Simulating the patients with cervical cancer undergoing radiotherapy, a CIRS phantom received volumetric modulated arc therapy (VMAT), step-shoot intensity-modulated radiotherapy (IMRT), and sliding-window IMRT to measure PDs of the breast, thyroid, and lens. All the data were normalized to the isocenter.

PD was gradually reduced with the increase in distance (13.41% at 1 cm from the edge to 0.25% at 31 cm from the edge). With a fixed distance from the edge of the radiation field, there was no significant difference in PD between different depths. A radiation field with a size of 30 cm had a PD about two-fold higher than that with a size of 10 cm. PD increased with the increase in the physical wedge angle and increased by 1% compared with the open field; PD decreased with the increase in the virtual wedge angle and decreased by 2-3% compared with the open field. PD decayed from 13.35% at 1 cm to 0.23% at 31 cm under 6 MV X-ray and from 11.06% at 1 cm to 0.20% at 31 cm under 18 MV X-ray. D_scatter was dominant in the regions close to the edge of radiation field and decreased from 62.45% at 1 cm to 5.71% at 25 cm. In all measurements under 6 MV X-ray, the maximum proportion difference between CC13 ionization chamber and diode ionization chamber was less than 1%. PDs of the breast, thyroid, and lens were 6.72, 2.90, and 2.37 mGy in VMAT mode, 7.39, 4.05, and 2.48 mGy in step-shoot IMRT mode, and 9.17, 4.61, and 3.21 mGy in sliding-window IMRT mode, respectively.

For the measurement of PDs, the CC13 air-filled ionization chamber and semiconductor diode ionization chamber have good consistency and feasibility under 6 MV X-ray. In clinical practice, the understanding of the relationship of PD with different radiation conditions helps to reduce the doses to organs at risk. Shielding and protective techniques can further reduce dose deposition.

Peripheral dose; Diode; Ionization chamber; Solid water; Simulation phantom