Role of multileaf collimator in replacing shielding blocks in radiation therapy

To facilitate the use of multileaf collimator (MLC) in field shaping, we te sted 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 perfor med for a single field as well as a multiple field setup. For the single fi eld 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 deg rees with the horizontal axis. The blocks were then replaced with MLC leave s. 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 correspondin g MLC setup. For multiple fields, the same sets of blocks were set up on th e 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 exami ned by comparing the dose-volume histogram (DVH) of the normal tissues in t he penumbral regions for the different setups. To correlate the effect of M LC with a change in the block margin, difference dose-volume histograms (DD VH) 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 correlati on obtained was used to predict the effect on the penumbra region of the ML C setup for the lateral fields of a patient irradiated with a four-field se tup. The calculations were carried out with 15 MV x-rays. For the single fi eld setup, dose undulation is largest for the 50% isodose line (IDL) as ref lected in the largest increase in the 50% to 20% isodose region compared wi th the 90% to 10% and the 80% to 20% regions. The increase in the penumbral width is largest for the 20 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 simila r to an increase in the block margin. For 15 MV x-rays, the increase varies inversely with the angle of the blocks, from > 6 mm increase in block marg in for the 20 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 irradi ation with MLC is similar to changing the margin of the blocks. For the pos terior 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 min for th e straight portion of the block to about 6 mm in the superior portion of th e block. Characterization of an MLC setup replacing a Cerrobend block is ne cessary 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 possib le to estimate with reasonable accuracy the effect of MLC replacing a Cerro bend block. (C) 2001 Wiley-Liss, Inc.