Room: Davidson Ballroom A
Purpose: To characterize a dual-layer stacked MLC system which composes of a distal and a proximal MLC bank with respect to the source. The MLC leaves are of 1 cm wide and offset by 5 mm between the two layers.
Methods: The characterizations include MLC transmission and leakage, dosimetric leaf gap (DLG), static field penumbra and leaf corner effect. All measurements were performed for the distal MLC bank only and for both the distal and proximal banks stacked. The MLC transmission and leakage were obtained using EPID in dosimetry mode. The static MLC-defined field penumbra was measured with a diode scanning across a 2x10cmÂ² field at multiple X-positions and depths of Dmax and 10cm. The DLG was measured using an ion chamber with the sweeping gap method for distal bank only, since only distal bank participates in modulation at the time of this study. EPID was also used to capture the field junctions of abutting fields defined by MLC and quantify field corner effect.
Results: The measured average leaf transmission were 0.026%Â±0.006% for both banks closed, and 0.245%Â±0.041% with only the distal bank closed. DLG were 0.13mm for distal bank. The static field measurements showed similar penumbra at different X-positions for the distal bank only vs. for both banks, measuring 2.64Â±0.11mm vs. 2.78Â±0.13mm at Dmax, and 3.84Â±0.24mm vs. 3.78Â±0.13mm at 10cm. The abutting fields measurements revealed additional leakage at the corners of the distal MLC leaves when viewed from BEV, which resulted ~2.80% increase in transmission. Such leaf-corner leakage disappeared when using both leaf banks.
Conclusion: We presented the characterizations of a dual-layer MLC system for a jaw-less treatment delivery system. Difference in MLC characterizations were observed between using only distal MLC bank and both banks together. Further investigation is warranted for more accurate dose calculations in treatment planning system.
Funding Support, Disclosures, and Conflict of Interest: This research is partially supported by Varian Medical Systems.