SU-E-T-385: Accelerated Beam Delivery with MLC Gaps in IMRT Fields

By default, the 4-mm wide leaf-pairs of the Elekta Beam Modulator MLC not used for field shaping are moved to under the primary collimator to minimize the leakage dose. The trade-off is the increased beam delivery time in the case of IMRT. This study examines the dosimetric impact and improved delivery efficiency by allowing the unused leaf-pair as 2mm gap to stay within the PTV aperture during beam-on.

The capability of the treatment planning system (Pinnacle) in modeling the central and off-axis dose through 2mm MLC gaps, respectively, was investigated. EBT film measurement performed in a solid water phantom was compared with the computed dose. For two representative IMRT cases, the dosimetric features of treatment plans generated with and without MLC gaps in the field were compared. Similarly, the delivery times for 13 IMRT cases with and without MLC gaps were compared.

The peak dose of the leaf gap computed with Pinnacle was dependent on dose grid resolution. The difference between a 0.5mm and 2mm dose grid was about 16.5% for 6MV and 15MV beam respectively. Using a 0.5mm dose grid, the maximum difference between measured and calculated dose was < 1% for a central axis gap, and < 3% for an off-axis gap. The maximum dose through a 2mm×4mm leaf gap was estimated < 10cGy for a typical IMRT plan. DVH comparison showed an increase of target dose up to 2% for the IMRT plans with gap, with minimal dose change to all critical structures. For the 13 IMRT cases, the beam delivery times were reduced by 27+/-9.7%, from 8.7+/-4.5 minutes to 6.5+/-4.0 minutes.

The beam delivery is accelerated by utilizing minimal gaps in the IMRT fields. The increased dose to the target is acceptable and is minimal to all critical structures.