Room: Exhibit Hall | Forum 3
Purpose: To demonstrate a novel technique using a multi-level MLC (MLMLC) device to create a single arc VMAT plan using differential transmission through each leaf bank level, enabling a complex fluence at each gantry position, using only a single set of fixed leaf positions per gantry position. Single-level MLC (SLMLC) VMAT only allows for a simple, on-or-off fluence at each gantry position.
Methods: A MLMLC VMAT prostate plan was optimized, simulated by 72 static gantry beams. Fluences were sent to a program that made a single set of fixed leaf positions per planar fluence using a 3-level MLC device. Each level of the MLMLC device has a different thickness to help efficiently modulate the beam. Leaf widths were 1cm. MLMLC Leaves are able to inter-digitate in both directions perpendicular to leaf travel. The set of leaf positions were converted into compensators of equivalent thickness. Field weights were matched to an equivalent static gantry IMRT plan. Dose was calculated. A single arc, SLMLC VMAT plan using 0.5cm leaves and 179 control points was optimized using the same DVH objectives. Note that the SLMLC plan optimized leaf positions for an optimal 3D dose while the MLMLC plan optimized leaf positions to match an optimal fluence per field. Each plan was normalized to the same target mean dose. Parameters from both plans were compared.
Results: The total MU, mean rectal dose and bladder dose for the MLMLC vs SLMLC plans were 3102MU vs. 2281MU, 1040cGy vs. 978cGy, and 733cGy vs. 697cGy.
Conclusion: While MLMLC plan creation was demonstrated, it did under-perform compared to the SLMLC plan. This may be explained by significant differences between the processes for MLMLC versus SLMLC plan creation. Future works are being prepared to improve the quality of MLMLC plan creation.