(Sunday, 7/14/2019)
Room: ePoster Forums
Purpose: To evaluate the dosimetric performance of single-isocenter HyperArc (HyperArcâ„¢ Varian Medical Systems) treatment in comparison to multi-isocenter volumetric modulated arc therapy (VMAT) for single and fractionated stereotactic radiosurgery (SRS).
Methods: Twelve previously treated patients (1-7 metastases, total of 34 lesions) with multi-isocenter VMAT were re-planned using single isocenter HyperArc (HA) modality. Planning target volumes (PTV) ranged from 0.12 to 19.5cc and prescription doses were 22Gy in single fraction or 27Gy in 3 fractions based on PTV size. VMAT plans consisted of 4 to 5 non-coplanar arcs per lesion with beam angle and couch rotations manually selected. HA plans consisted of 4 fixed arcs with automatic isocenter placement and optimized collimator rotation per arc field. Dosimetric parameters analyzed included the Paddick conformity index (CI), homogeneity index (HI), gradient index (GI), radiation necrosis indicators for normal brain V12Gy, V18Gy (volume receiving 12Gy and 18Gy) and low dose spread into the normal brain (V4Gy).
Results: HA plans achieved a significantly higher CI (mean ± standard deviation) compared with VMAT plans (0.85 ± 0.08 vs. 0.81 ± 0.12, p < 0.01). HI was comparable between HA and VMAT plans (p = 0.09). In addition, there was no difference in the rapid dose falloff (GI) between HA and VMAT (p=0.88). V18Gy and V12Gy results were slightly better for HA compared to VMAT although not statistically significant (p=0.11). Low dose spillage into the normal brain (V4Gy) was non-significantly higher for HA plans vs. multi-isocenter VMAT plans (p=0.24).
Conclusion: HA plans provided higher dose conformity to the target and the same rapid dose falloff compared to VMAT plans. However, HA plans resulted in slightly larger volumes of normal brain receiving low doses. Treatment planning time and dose delivery are reduced for HA compared to multi-isocenter VMAT.
Funding Support, Disclosures, and Conflict of Interest: Research funding from Varian Medical Systems.
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