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Treatment Planning Study of Volumetric Arc Therapy for Spine Stereotactic Radiosurgery Using Flattening Filter Free Beams

K Chin

K Chin*, N Wen, Y Huang, J Jin, I Chetty, S Ryu, Henry Ford Health System, Detroit, MI

MO-F-108-5 Monday 4:30PM - 6:00PM Room: 108

Purpose: To evaluate the potential usage of flattening filter-free (FFF) with volumetric modulated arc therapy (VMAT) in the treatment of spine radiosurgery.

Methods: VMAT plans were retrospectively generated for six patients treated with intensity modulated radiotherapy (IMRT) according to the RTOG 0631 protocol. The VMAT plans were developed in the Eclipse treatment planning system on a Varian Truebeam equipped with a Millennium 120 MLC system, using the flattening-filter-free mode with a maximum dose rate of 1400 MU/min. The plans were prescribed to either 16 or 18Gy in one fraction to cover at least 90% of the target. Planning target volume (PTV) coverage, conformity index (CI), dose to spinal cord, distance to fall off between the prescription and maximum isodose line of the cord, as well as plan delivery efficiency were evaluated.

Results: PTV coverage was better for VMAT compared to IMRT plans. The ratios between VMAT and IMRT for PTV coverage were 1.06 for D99% and 1.01 for D95%. The hot spots were comparable; the ratios of D10% and D5% were both 0.994. The VMAT plans were more conformal than the IMRT plans (CI of 1.13 vs. 1.28, p= 0. 0004). The VMAT plans also had shorter distance to fall off between the prescription and max cord isodose line (0.28 vs. 0.38 cm, p=0.0006). The average cord dose is 4.72 Gy in the VMAT plans, relative to 5.77 Gy in the IMRT plans. Finally, the VMAT treatments required slightly less MUs than the IMRT plans (8236 vs 9410, p=0.116).

Conclusion: The VMAT plans had better PTV coverage compared to the IMRT plans. Furthermore, VMAT allowed for lower mean dose to the spinal cord, a sharper dose gradient between the PTV and the cord, and required fewer MUs, thus reducing the treatment time.

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