Exhibit Hall | Forum 7
Purpose: Due to its fast, safe and effective treatment, frameless HyperArc module has generated a global clinical interest for linac-based SRS for multiple brain lesions and fractionated-SRS. While delivering a single high-dose to a small brain lesion via highly-modulated HyperArc VMAT plan, inherent risk of small-field dosimetry errors can compromise treatment delivery accuracy. We proposed MLC aperture-shape controlled HyperArc to minimize this effect.
Methods: Fourteen SRS patients with a single small brain lesion were retrospectively selected, with an average target size of 3.3±3.1cc(0.7–10.5cc). Initially, clinical treatment plans were generated via fully-automated HyperArc module with Q-fix mask, Encompass support-device with a 6MV-FFF beam and SRS NTO-prescribed 24 Gy to target volume. For the new strategy, patients were re-planned using a modified HyperArc VMAT: fitting MLC aperture-to-target with very high MLC aperture-controller priority, calculating DCA-dose prior to HyperArc VMAT (d-HyperArc) optimization with manually assigned NTO. Identical planning geometry and parameters were used. Plan quality, delivery accuracy and efficiency were compared.
Results: For similar target coverage, conformity and dose gradient, d-HyperArc provided average higher dose to gross tumor by 1.5Gy(p=0.009).These insignificant differences were observed: dose to optic pathway, brainstem, MBD, V8Gy, and V12Gy. d-HyperArc provided significantly lower total MUs by 1870(p=0.0003) and smaller modulation factor 0.78(p=0.003), and reduced beam-on time by 1.52min(p=0.0002) minutes, on average (maximum, 2.61 min). Beam delivery accuracy was improved by 3%, on average(p<0.001), maximum 7% in some cases for d-HyperArc plans. Independent Monte Carlo calculation showed d-HyperArc plans agreed to TPS within 2.5% compared to 4.5% for original HyperArc VMAT plans.
Conclusion: d-HyperArc via MLC aperture-shape controller and DCA-dose provided better plan quality, with substantial reduction in total MUs, beam modulation and beam-on time. d-HyperArc QA results showed mitigating small-field dosimetry errors were critical to reduce patient setup uncertainties-suggesting improved accuracy of single-dose HyperArc delivery to small brain lesions.
Stereotactic Radiosurgery, Setup Errors, MLC