Evaluation of dosimetric impact and plan robustness of MLC motion and setup errors in SRS for multiple brain
Huijun Xu1, Hong Zhang2, Weiguang Yao1
1Department of Radiation Oncology, University of Maryland School of Medicine, Baltimore, Maryland, USA.
Background:
HyperArc is an effective technique for treating multiple small brain metastases using high-definition, precise radiation. However, the small size of individual lesions makes treatment vulnerable to inherent inaccuracies from beam-limiting devices such as multi-leaf collimators (MLCs), as well as patient setup errors. These uncertainties must be thoroughly evaluated prior to administering a single-fraction, high-dose HyperArc treatment to ensure optimal clinical outcomes.
Purpose:
To comprehensively evaluate the robustness of HyperArc VMAT plans for multiple brain metastases (MBT) against (i) high-definition MLC leaf positional errors and (ii) patient rotational and translational setup variations.
Methods And Materials:
A retrospective analysis of 34 MBT plans from 30 patients with 123 lesions treated using HyperArc VMAT plans was performed. Prescription doses (21-24 Gy) targeted PTVs (GTV + 1 mm), with each plan addressing 2-7 brain metastases (< 2 cm). D100% for GTV and V12Gy for normal brain were re-evaluated under simulated (i) 20 MLC positional errors (0.2-1 mm) and (ii) 26 patient rotational (Yaw/Pitch/Roll by 1°) and 26 translational errors (SI/LR/AP by 1 mm). The impact of target distance to isocenter (DTI), target volume, and plan modulation factor on plan robustness was assessed. Replans with larger PTV margins, 1.5 and 2 mm, were investigated for a proper PTV margin for the worst-case scenarios.
Results:
Per our cohort, D100% of GTV is highly sensitive to both MLC and patient errors. Their dosimetric effects follow linear regressions: 1 mm shift reduces GTV D100% by 8%; 1 mm symmetric MLC openings increase GTV D100% by > 22%, while 1 mm symmetric closings decrease it by > 25.8% per 1 mm; Patient rotation/translation impacts GTV D100% by 0.04-1.28% per 1° or ∼7% per 1 mm. Normal brain V12Gy is only sensitive to MLC errors, increasing slightly with shifts and significantly with symmetric openings (+5cc/1 mm), but decreasing with closings (-2.77cc/1 mm). DTI shows minimal correlation with MLC shifts but is affected by patient setup uncertainties. Large targets amplify dose sensitivity to symmetric MLC errors, whereas plan modulation factor shows no significant impact. A minimum 1 mm margin is required to absorb these uncertainties, with 2 mm margins recommended in the worst cases to ensure treatment accuracy.
Conclusions:
Overall, the target dose and V12Gy for HyperArc VMAT plans are more significantly affected by symmetric MLC opening and closing errors than by patient rotational setup errors. The pronounced effects of these errors highlight the importance of rigorous plan robustness verification, accounting for target size, margins, and DTI.
