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To facilitate the use of multileaf collimator (MLC) in field shaping, we tested the hypothesis that the changes in the penumbra due to MLC replacing a Cerrobend block can be related to a change in the margin of the block. We also investigated if it is possible to estimate the effect of MLC replacing a block in terms of a change in the block margin. Calculations were performed for a single field as well as a multiple field setup. For the single field setup, blocks with equal areas were drawn at the four corners of a 16 x 20 cm(2) field at angles of 20 degrees, 40 degrees, 60 degrees, and 80 degrees with the horizontal axis. The blocks were then replaced with MLC leaves. For 6 MV x-rays, dose profiles in the penumbra regions of the blocks at 5- and 10-cm depths were compared with those obtained with the corresponding MLC setup. For multiple fields, the same sets of blocks were set up on the anterio-posterior (AP-PA) pair of a four-field setup. The margins of the blocks were increased (i.e., block shaved) in 1 mm steps to a maximum of 6 mm. The similarity between MLC and the change in the block margin was examined by comparing the dose-volume histogram (DVH) of the normal tissues in the penumbral regions for the different setups. To correlate the effect of MLC with a change in the block margin, difference dose-volume histograms (DDVH) of the normal tissues relative to the original block were compared for the MLC setup with those for the changes in the block margin. The correlation obtained was used to predict the effect on the penumbra region of the MLC setup for the lateral fields of a patient irradiated with a four-field setup. The calculations were carried out with 15 MV x-rays. For the single field setup, dose undulation is largest for the 50% isodose line (IDL) as reflected in the largest increase in the 50% to 20% isodose region compared with the 90% to 10% and the 80% to 20% regions. The increase in the penumbral width is largest for the 20 degrees block when replaced by the MLC and is smaller as the angle increases. The increase in the penumbral width also increases with depth. The effect of replacing a Cerrobend block with an MLC is similar to an increase in the block margin. For 15 MV x-rays, the increase varies inversely with the angle of the blocks, from6 mm increase in block margin for the 20 degrees block to about 1 mm for the 80 degrees block. In the clinical example, replacing the blocks in the lateral fields of a four-field irradiation with MLC is similar to changing the margin of the blocks. For the posterior block, MLC is similar to a 1- to 2-mm increase in the margin of the block, whereas for the anterior block the effect is similar to 1 mm for the straight portion of the block to about 6 mm in the superior portion of the block. Characterization of an MLC setup replacing a Cerrobend block is necessary for adequate coverage of target volume. The effect of MLC replacing a Cerrobend block is similar to a change in the block margin. It is possible to estimate with reasonable accuracy the effect of MLC replacing a Cerrobend block.