Correcting for electron contamination at dose maximum in photon beams.

Data are presented to allow the photon beam quality specifier being used in the new AAPM TG-51 protocol, %dd(10)x, to be extracted from depth-dose data measured with a 1 mm lead foil either 50 cm or 30 cm from the phantom surface. %dd(10)x is the photon component of the percentage depth dose at 10 cm depth for a 10x10 cm2 field on the surface of a phantom at an SSD of 100 cm. The purpose of the foil is to remove the unknown electron contamination from the accelerator head. Monte Carlo calculations are done: (a) to show these electrons are reduced to a negligible level; (b) to calculate the amount of electron contamination from the lead foil at the depth of dose maximum; and (c) to calculate the effect of beam hardening on %dd(10). The analysis extends the earlier work of Li and Rogers [Med. Phys. 21, 791-798 (1994)] which only provided data for the foil at 50 cm. An error in the earlier Monte Carlo simulations is reported and a more convenient method of analyzing and using the data is presented. It is shown that 20% variations in the foil thickness have a negligible effect on the calculated corrections.