Dosimetric evaluation and clinical application of virtual mini-multileaf collimator.
One of the major concerns with multileaf collimators (MLC) is the jagged field edge that produces a larger penumbra compared with that produced by a Cerrobend block. The dosimetric undulation of the MLC can be minimized by replacing an existing MLC with a mini-MLC, an expensive replacement, or by software implementation, which essentially converts a regular MLC into a virtual mini-MLC. In this study, the dosimetry in the penumbra region of a virtual mini-MLC replacing the Cerrobend block is investigated for clinical applications. HD270, a software program implemented by Siemens (Concord, CA), combines the use of an MLC and a table translation perpendicular to the leaf plane to produce a smooth field edge, thus reducing isodose undulation. Three different step resolutions are available: 5 mm, 3 mm, and 2 mm. Using film dosimetry, the penumbra regions are studied at two different depths for clinical blocks and corresponding MLC setup, as well as HD270 with different resolutions for both 6-MV and 15-MV x-ray beams. The dose delivery time for HD270 on auto-sequencing mode is compared with the use of Cerrobend blocks. The clinical applications of HD270 in head-and-neck (head and neck) and prostate treatments are investigated. For single-field irradiation, the 80-20% penumbra widths for both the 45 degrees block and the circular block are reduced with HD270 compared with MLC for both 6 and 15 MV at different depths. At 2-mm resolution, the scalloping isodose lines (IDLs) with MLC completely disappear, although the penumbra is still larger than the Cerrobend block. On the other hand, the difference in dose undulations between 2-mm and 3-mm resolution is small. In the head and neck irradiation, the 80-20% widths with HD270 are 1 to 2 mm less than MLC, but they are still 2 mm wider than with a Cerrobend block. The 50% IDL is reduced by 2 mm with HD270 compared with MLC, which provides safety near spinal cord. Dose-volume histogram (DVH) calculations for the different shielding techniques indicate that the HD270 improves the spinal cord dose distribution significantly compared with MLC. A similar improvement in dose undulation is observed for the prostate case. In the dose region, >60% of the prescribed dose, there is approximately 10% less irradiated volume for the rectum when HD270 (3 mm resolution) is employed compared with MLC. The treatment time was compared with that from the Cerrobend block, and it was found that even at 3-mm resolution, there is a 20% reduction in treatment time in a head and neck treatment; with a 2-mm resolution, there is a 15% increase in time. The isodose undulation due to MLC can be significantly reduced with the HD270. Clinical application with HD270 for head and neck and prostate irradiation provides a smaller penumbra region compared with MLC, although it still gives a larger one compared with the Cerrobend block. In the clinical cases presented in this study, the 3-mm resolution is the most effective in improving the penumbra and delivery time. The HD270 implementation is a versatile and cost-effective solution for reducing MLC undulation.