Film planar dose verification of "step and shoot" intensity modulated radiation therapy fields

The aim of this study was to quantify the rounded leaf end effect of multileaf collimator (MLC) from a sequence of segmented fields within an intensity modulated radiation therapy (IMRT) delivery by the step and shoot technique. This field sequence was designed to highlight and quantify the match line effect. The observation of this study included: (1) The dosimetry characteristics of MLC (2) Match line effect (3) Dose verification of MLC segmented fields. The Kodak X-Omat V film was used as a dosimeter in this study. During this study, all the films were orientated perpendicular to the beam central axis. In the determination of optical density-dose calibration curve, the film was placed between solid water slabs at dmax under a 6MV photon beam which was produced using a Clinac 2100C (Varian Medical System). The dosimetry characteristics of MLC were divided into two parts: interleaf transmission/leakage measurement and rounded leaf end transmission/leakage measurement. Also two sequences of small MLC fields with different offset distances were customised to produce multiple match lines. Then film and computed planar dose maps were compared for five, seven and ten segment step and shoot IMRT fields.