Manual Normal Tissue Objectives (mNTO) for Brain Metastases Using Non-Coplanar VMAT and Hyperarc Techniques
Abstract
Purpose
HyperArc generally provides steeper dose gradients for multiple brain metastases (mets). For a single brain met, utilizing the VMAT technique based on two-orthogonal Arcs with optimal mNTO is an alternative to HyperArc. The purpose of this study was to determine the best mNTO values based on the dose distribution for Brain mets SRS.
Methods
HyperArc in Eclipse remains the preferred solution for multiple brain mets due to its superior automation and steeper gradients. However, if HyperArc is unavailable, or only a single brain Lesion exists, utilizing a two orthogonal Arcs VMAT technique with optimal mNTO is a clinical alternative to HyperArc. Our previous way to reduce intermediate dose for two orthogonal Arcs VMAT was to use three control rings around GTV to assist optimization. To match our previous practice on Brain mets SRS with three control rings. We optimized mNTO instead of three rings to get the very similar dose distribution.
Results
10 patients with brain metastases with different lesion volumes, receiving 15Gy-22Gy prescriptions were re-planned with HyperArc, two orthogonal Arcs VMAT with three rings, and two orthogonal Arcs VMAT with mNTO without rings. Two orthogonal Arcs VMAT with three rings, with mNTO without rings, vs. HyperArc for single-Lesion SRS were planned and compared for isodose distribution, gradient index, planning speed, treatment time. mNTO with varying priority, distance from target border, start dose, end dose, and dose fall-off values were evaluated and optimal mNTO was determined.
Conclusion
mNTO with a priority of 100; a fall-off of 0.5 mm⁻¹; distance from target border: -0.5mm to 0mm; start dose: 100%; and end dose: 20% – 30; is the most effective alternative for achieving radiosurgical quality dose fall-off in standard VMAT.