Influence of initial pencil beam parameters on large non-applicator electron field profiles calculated using Monte Carlo methods

Purpose: To investigate the influence of the initial pencil beam width on large non‐applicator electron field profiles calculated using the BEAMnrc/EGSnrc Monte Carlo code. For a Varian 2100C linear accelerator, significant discrepancies of up to 10% in the shoulder of the profile were found between measured and calculated profiles for 40×40 cm2 electron fields collimated using x‐ray jaws alone, when using the standard electron spot size of 1.5 mm. A small angular variance of the electron pencil‐beam at the x‐ray target would be equivalent to increasing the electron pencil‐beam spot size at the primary scattering foil, which would be expected to affect the electron fluence profile exiting the linear accelerator. Method and Materials: Cross‐beam profiles at the depth of maximum central‐axis dose were measured and calculated for electron beams with a field size of 40×40 cm2 collimated by the x‐ray jaws without an electron applicator. The electron spot size at the position of the x‐ray target was modeled as a mono‐energetic mono‐directional Gaussian with a full‐width half‐maximum (FWHM) that varied from 1.5 to 5 mm. For each energy, measured and calculated profiles were compared to determine the optimal FWHM. Results: Adjusting the FWHM of the source greatly affected the shoulder of the calculated off‐axis profiles, reducing discrepancies to ±4%. The optimal FWHM ranged 2.5 to 4.5 mm and was energy dependent. Assuming that the correct spot size at the x‐ray target is 1.5 mm, this is equivalent to an angular spread at the x‐ray target of 8.8 to 20.9 mrad. Conclusion: The angular spread of the electron pencil‐beam at the x‐ray target is significant and varies with energy, mostly influencing the shoulders of large non‐applicator electron field profiles. The effect of the initial angular spread on other dosimetric data will be the subject of future investigations. © 2005, American Association of Physicists in Medicine. All rights reserved.