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A Practical Method for Evaluation of Electron Virtual Source Position

Z Huang

Z Huang1*, W Jiang1 , B Stuart1 , S Leu1 , T Liu2, Y Feng1 , (1) East Carolina University, Greenville, North Carolina, (2) Houston Methodist Hospital, Sugar Land, Texas


SU-F-T-71 (Sunday, July 31, 2016) 3:00 PM - 6:00 PM Room: Exhibit Hall

Since electrons are easily scattered, the virtual source position for electrons is expected to locate below the x-ray target of Medical Linacs. However, the effective SSD method yields the electron virtual position above the x-ray target for some applicators for some energy in Siemens Linacs. In this study, we propose to use IC Profiler (Sun Nuclear) for evaluating the electron virtual source position for the standard electron applicators for various electron energies.

The profile measurements for various nominal source-to-detector distances (SDDs) of 100-115 cm were carried out for electron beam energies of 6-18 MeV. Two methods were used: one was to use a 0.125 cc ion chamber (PTW, Type 31010) with buildup mounted in a PTW water tank without water filled; and the other was to use IC Profiler with a buildup to achieve charge particle equilibrium. The full width at half-maximum (FWHM) method was used to determine the field sizes for the measured profiles. Backprojecting (by a straight line) the distance between the 50% points on the beam profiles for the various SDDs, yielded the virtual source position for each applicator.

The profiles were obtained and the field sizes were determined by FWHM. The virtual source positions were determined through backprojection of profiles for applicators (5, 10, 15, 20, 25). For instance, they were 96.415 cm (IC Profiler) vs 95.844 cm (scanning ion chamber) for 9 MeV electrons with 10x10 cm applicator and 97.160 cm vs 97.161 cm for 12 MeV electrons with 10x10 cm applicator. The differences in the virtual source positions between IC profiler and scanning ion chamber were within 1.5%.

IC Profiler provides a practical method for determining the electron virtual source position and its results are consistent with those obtained by profiles of scanning ion chamber with buildup.

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