Photon scatter in intensity modulating filters evaluated by first Compton scatter and Monte Carlo calculations and experiments in broad beams.

High-atomic-number materials may be used as intensity modulating filters for inverse radiation treatment planning with photon beams. Such filters, when placed in a bremsstrahlung beam, attenuate the primary fluence, but also produce scattered photons that will reach the patient. To account for such effects in the optimization of photon beam intensities a semiempirical method based on narrow and broad beam transmission measurements was used to quantify the number of scattered photons produced in these filters. The method was verified by performing analytical calculations based on first scatter and a Monte Carlo simulation in 6 and 18 MV photon beams. The resultant experimental transmission ratios agree with calculations by these methods within 2 per cent under the experimental conditions investigated. The semiempirical method can thus be used as a basis for preliminary decision-making to select the proper material for intensity modulating filters and can provide a fast method to perform independent quality checks of the calculation accuracy of dose planning systems. Change in beam penetration is of less concern when treatments of target volumes at smaller depths are of interest. A 10 g cm(-2) thick filter made of low-melting-point alloy produces a change in percentage depth dose of less than 2 per cent for depths larger than 10 cm independent of field size. Similarly the scatter correction modifies the dose distribution by less than 5-10 per cent in most cases.