Experimental evaluation of a 2D and 3D electron pencil beam algorithm

Verification of electron beam treatment-planning algorithms in the presence of heterogeneities can be very difficult. Using controlled geometries to minimise physical uncertainties in geometric alignment and composition, a large number of measurements were made to test the performance of a 2D and 3D electron pencil beam algorithm. A Therados RFA-3 beam-scanning system interfaced to a microcomputer was used to measure the dose distributions. The geometric arrangement consisted of single and double rods 1 cm in diameter situated just below the surface of a unit density phantom. The electron densities (relative to water) of the rods ranged from 2.12 (aluminium) to 1.29 (soft bone analogue), and their length could be varied between 1 cm and 10 cm. Measured isodose distributions beyond the inhomogeneities were compared with those predicted theoretically. Calculations were performed on a VAX-11/780 using 2D and 3D implementations of the Hogstrom electron pencil beam algorithm. The authors report on the nature of this 3D implementation and assess the magnitude of discrepancies between calculation and measurement for 10 MeV and 18 MeV electron beams, and for the variety of phantom compositions and geometries identified above.

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