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The Effect of MLC Leaf Width in Single Isocenter Multi-Target Radiosurgery with VMAT

Z Abisheva*, J Adamson , S Floyd , J Salama , F Yin , W Giles , Duke University Medical Center, Durham, NC


(Tuesday, 7/31/2018) 1:15 PM - 1:45 PM

Room: Exhibit Hall | Forum 3

Purpose: The benefit of small MLC leaf widths (2.5mm) are well-documented for conformal radiosurgery techniques; however, MLC leaf width has not been extensively studied for modulated techniques such as VMAT. This study examines the effect of leaf width on dosimetric quality of single isocenter, multiple target radiosurgery plans with VMAT.

Methods: 10 patients with 3-10 intracranial brain metastases were selected who were originally treated with HDMLC (2.5 mm) were replanned using standard MLC (5 mm). The same treatment geometry was used with 3-5 VMAT arcs with flattening filter free 6 MV photons and dose 18-20Gy in single fraction (n=8), or 5 fractions of 5-5.5Gy (n=2). Conformity index, low and moderate isodose spill (V30% and V50%) were selected for analysis and V12 Gy was also analyzed for single fraction cases.

Results: When plans were replanned using the larger MLCs, the MU changed from 5792±2324 to 5672±2302 (p=0.2), conformity index increased by 2.7%±4.1% (p < 0.05), V30% and V50% increased by 25.9%±16.7% and 20.8%±15.2% (p < 0.05 for both) respectively, and V12 Gy increased by 19.3%±15.5% (p < 0.01). In none of the single fraction cases did the change in V12Gy cause sufficient concern of radionecrosis to warrant fractionating the treatment.

Conclusion: Using 5mm MLCs lead to minor increase in conformity index and moderate increases in low and moderate isodose spill. It remains to be seen whether changes to planning techniques could help to mitigate these effects.

Funding Support, Disclosures, and Conflict of Interest: Dr. Adamson reports ownership of Clearsight RT LLC which is unrelated to this study.


Stereotactic Radiosurgery, Radiation Dosimetry, MLC


TH- External beam- photons: VMAT dose optimization algorithms

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