Monte Carlo investigation of electron beam output factors versus size of square cutout

A major task in commissioning an electron accelerator is to measure relativ e output factors versus cutout size (i.e., cutout factors) for various elec tron beam energies and applicator sizes. We use the BEAM Monte Carlo code [ Med Phys. 22, 503-524 (1995)] to stimulate clinical electron beams and to c alculate the relative output factors for square cutouts. Calculations are p erformed for a Siemens MD2 linear accelerator with beam energies, 6, 9, 11, and 13 MeV. The calculated cutout factors for square cutouts in 10X10 cm(2 ), 15X15 cm(2), and 20X20 cm(2) applicators at SSDs of 100 and 115 cm agree with the measurements made using a silicon diode within about 1% except fo r the smallest cutouts at SSD=115 cm where they agree within 0.015. The det ails of each component of the dose, such as the dose from particles scatter ed off the jaws and the applicator, the dose from contaminant photons, the dose from direct electrons, etc., are also analyzed. The calculations show that in-phantom side-scatter equilibrium is a major factor for the contribu tion from the direct component which usually dominates the output of a beam . It takes about 6 h of CPU time on a Pentium Pro 200MHz computer to simula te an accelerator and additional 2 h to calculate the relative output facto r for each cutout with a statistical uncertainty of 1%. [S0094-2405(99)0140 5-4].