Dose measurements compared with Monte Carlo simulations of narrow 6 MV multileaf collimator shaped photon beams

Small fields where electronic equilibrium is not achieved are becoming incr easingly important in clinical practice. These complex situations give rise to problems and inaccuracies in both dosimetry and analytical/empirical do se calculation, and therefore require other than conventional methods. A na tural diamond detector and a Markus parallel plate ionization chamber have been selected for clinical dosimetry in 6 MV photon beams. Results of simul ations using the Monte Carlo system BEAM/EGS4 to model the beam geometry ha ve been compared with dose measurements. A modification of the existing com ponent module for multileaf collimators (MLCs) allowed the modeling of a li near accelerator SL 25 (Elekta Oncology Systems) equipped with a MLC with c urved leaf-ends. A mechanical measurement method with spacer plates and a l ight-field edge detection technique are described as methods to obtain geom etrical data of collimator openings for application in the Monte Carlo syst em. Generally a good agreement is found between measurements and calculatio ns of depth dose distributions and deviations are typically less than 1%. C alculated lateral dose profiles slightly exceed measured dose distributions near the higher level of the penumbras for a 10 x 2 cm(2) held, but agree well with the measurements for all other cases. The simulations sue also ab le to predict variations of output factors and ratios of output factors as a function of field width and field-offset. The Monte Carlo results demonst rate that qualitative changes in energy spectra are toe, small to explain t hese variations and that especially geometrical factors affect the output f actors and depth dose curves and profiles. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01109-8].