Evaluation of Peripheral Dose for Varian Trilogy Linear Accelerator

Abstract Objective To measure and evaluate the peripheral dose(PD) for Trilogy linear accelerator in different setup condition and investigate the feasibility of the diode dosimetric system to measure the peripheral dose.Methods Peripheral dose were measured using a CC13 ionization chamber and the diode dosimetric system in a set of solid water phantom. Measurements were performed for different depths, field sizes, physical and virtual wedge, radiation beam energy and up at distance of 1cm to 31cm beyond the field edges. PD is separated into PDleakage and PDscatter by measure peripheral dose with or without scattering phantom. CRIS phantom was used for this research with the diode dosimetric system at the interest points of the breast, thyroid, and lens.Results All the measure data were normalized to isocenter. The measured PD decreases exponentially as a function of distance up to 31cm from the edge. PD shows no significant relevant to depth and it increases with the increased field size. As the physics wedge angle increase, PD increases about 1%, but enhanced dynamic wedge decreased 2-3% compared with open field. As the beam energy increase, PD decreased. All PD data difference less than 1% between CC13 ionization chamber and diode. The PD of CRIS phantom for Volume Modulated ARC Therapy (VMAT) is minimum and the mean dose for breast、thyroid and lens is 6.72 mGy、2.90 mGy and 2.37 mGy respectively.Conclusion The diode dosimetric system provides an sufficient assessment in peripheral regions of 6MV X-ray beam. PD changes because of field size、depth、beam energy etc and the assessment of PD would be helpful to evaluate the dose received by the relevant critical structures near the treatment field. Furthermore it is advantaged to use external shielding for critical organs.