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SRS Plan Quality and Treatment Efficiency: VMAT Vs Dynamic Conformal Arcs

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S Gros

S Gros*, A Plypoo , A Sethi , Loyola Univ Medical Center, Maywood, IL


SU-I-GPD-T-570 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To compare the dosimetry and treatment efficiency of VMAT and Dynamic Conformal Arc SRS planning.

Methods: Twenty-three SRS plans from twelve patients (1-4 lesions per patient) previously treated with 6MV FFF and VMAT were retrospectively replanned with Dynamic Conformal Arcs using: (1) original arc angles (DCA1), and (2) optimized arc angles (DCA2). All DCA plans followed the original prescription dose and achieved similar target coverage (within 0.5%) as VMAT plans. All SRS plans used 3 to 6 ipsilateral incident arcs. Plans were analyzed and compared based on the following quality parameters: total MU, target conformity index (CI100), low-dose spillage (CI50), and normal tissue irradiated (V12Gy, V18Gy). The dose distribution within the target was assessed by the Hotspot Location Index, (HLI) = ‘hotspot distance to target center of mass’ / ‘target equivalent sphere radius’.

Results: DCA plans required fewer MUs (ave: 23%, max: 56%) than VMAT plans. Such MUs reduction would shorten beam-on time by 8-57%. VMAT plans were more conformal (CI100 = 1.18±0.14) than DCA1 (1.34±0.14) and DCA2 (1.27±0.12). In contrast, low-dose spillage, CI50, was better for DCA1 (4.36±1.05) and DCA2 (4.12±0.81) compared to VMAT (5.26±2.01), which is attributable to the lower total MUs in DCA plans. Most (>79%) of the DCA plans irradiated less normal tissue (V12Gy and V18Gy) compared to VMAT. HLI qualitatively increased with increasing target equivalent sphere radius. Larger targets (PTV>13 cc) showed a preference for VMAT for all dosimetric parameters, except total MUs and V12Gy.

Conclusion: While VMAT based SRS plans consistently resulted in superior target dose conformity, DCA plans yielded comparable target dose conformity with reduced low-dose spillage for most small centrally located lesions. Therefore DCA-based SRS plans optimize clinical SRS workflow via faster treatment planning and delivery, consistent treatments (reducing the likelihood of intra-fraction motion), while eliminating the need for VMAT QA.

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