(Saturday, 4/7/2018) 10:30 AM - 12:30 PM
Room: Marquis Ballroom 5-8
Purpose: To evaluate the physical and dosimetric characteristics of the SX1 stacked and staggered dual-layer multi-leaf collimator (MLC) on the Halcyonᵀᴹ system.
Methods: For physical characterization, we evaluated the MLC positioning accuracy due to varying gantry angles, collimator angles, leaf speed, and over time, using the EPID imager measurements. The temporal stability of the MLC positioning accuracy was derived from Machine Performance Check (MPC) data over 3 months. For dosimetric characterization, we evaluated the leaf transmission, leaf-end effect, as well as the tongue-and-groove effect. Leaf transmission was measured with NIST-traceable PTW 30013 chamber, by comparing a field with distal, proximal and both MLC layers fully-closed, to an open field. Leaf-end effect was measured using the same chamber with sweeping gap fields of varying gap sizes defined by the distal MLCs. The tongue-and-groove effect was investigated using the imager with fields defined by interspersed odd and even leaves for the different banks, gantry angles, collimator angles and offsets.
Results: The maximum MLC position deviation was within ±0.1 mm from the baseline of MPC measurements over 3 months, with the proximal exhibiting a greater deviation range compared to the distal MLCs. Picket-fence deliveries with both the distal-and-proximal-combination had reduced inter-leaf, through-leaf and leaf-end transmission compared to delivery with distal-only. We observed a gantry angle dependency for MLC positioning accuracy, and minimal collimator angle dependency. The measured leaf transmission factor was 0.0041 for distal-only, 0.0040 for proximal-only, and negligible for distal-and-proximal-combination (nominal 0.0047). The measured leaf end effect was -0.0187 cm (nominal 0.01 cm). The tongue-and-groove effect showed minimal gantry angle and collimator angle dependency. A noticeable systematic deviation between measurements and treatment planning system handling of the tongue-and-groove effect warranted further investigation.
Conclusion: We have comprehensively described the performance characteristics of the SX1 dual-layer MLC on the Halcyonᵀᴹ system.
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