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Measurement of Incident Electron Spots On TrueBeam

D Sawkey

D Sawkey1*, M Constantin1, S Mansfield1, J Star-Lack1, A Rodrigues2, Q Wu2, M Svatos1 (1) Varian Medical Systems, Palo Alto, CA, (2) Duke University Medical Center, Durham, NC

SU-E-T-553 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Lateral dimensions of the incident electron beam (spot sizes) are essential for accurate modelling of x-ray fields, especially for small field output factors and penumbrae. We present spot sizes measured on the Varian TrueBeam linac and propose an explanation for the observed shapes.

Methods: Spot were measured on 3 TrueBeam linacs using the 4X, 6X, 6FFF, 8X, 10X, 10FFF, and 15X beams. Two methods were used: a spot camera, and an edge CT reconstruction based on images of a thin plate with edge intersecting the beam at different collimator rotations. 2-dimensional Gaussian functions were fit to the data, with the spot orientation used as a fitting parameter.

Results: Spots were consistent for the 3 linacs. Sigmas of the fitted Gaussians were between 0.47 and 1.05 mm for all the beams. The 6X, 6FFF, and 15X beams had a distinct shape: the major axes were 30-40% larger than the minor axes, and the major axes were oriented 40 degrees from the inplane direction. The remaining spots were more symmetric, with major axes at most 15% larger than minor, and a similar orientation. The spot shapes could be explained by positing a helical electron motion at the entrance to the bend magnet, and using the known (non-achromatic) transport through the magnet. Fringe fields of the bend magnet, solenoid, or steering coils could produce the helical motion; the precise source is not known. Spots measured with the edge technique were 10% smaller in size than those measured with the camera. The 10FFF spots were 10% smaller than the 10X spots, as measured with the camera, but the 6X and 6FFF spots were similar to each other.

Conclusion: Representative spots for TrueBeam are presented. Values may be used for input to modelling of fluence output, for example using Monte Carlo methods.

Funding Support, Disclosures, and Conflict of Interest: Several of the authors are employees of Varian Medical Systems, as noted in the affiliations.

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