The aim of this study is to investigate the dosimetric effect in the penumbra region for the multi-leaf collimator (MLC) fields with stepping patterns. Various irregular MLC fields with different stepping patterns were designed. Radiographic films were used to measure the beam profiles (cross-plane (X1-X2 jaw) and in-plane (Y1-Y2 jaw)) for different MLC field configurations of varying lengths of protruded leaves adjacent (on one or both sides) to the beam profile axis. 6 MV photon beams produced by the Varian 21 EX linear accelerator with 120-leaf Millennium MLC were used. The results showed that the penumbra widths (10% - 90%) of the profiles (cross-plane) with 0.5 cm gap width increased with the protruded lengths of the neighbouring leaves, while the penumbra widths did not vary too much for the larger gap widths of 1 cm and 1.5 cm. The neighbouring leaf's influence on the profile was very insignificant, when the distance between the profile and neighbouring leaf edge was larger than 1 cm. Moreover, when both the upper and lower portions of leaves were moved out forming a gap for the profile, the dose at the "edge" region was decreased by about 20% and 6% when the protruded lengths of the leaves were 0.5 cm and 1 cm, respectively. It is important to understand the dosimetric effect in the penumbra region for irregular MLC fields with stepping leaf patterns, especially during the commissioning process for the conformal external beam and intensity modulated radiation therapy.
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