KEY POINTS
- The retrospective simulation study included 34 HyperArc plans from 30 patients, covering 123 brain metastases. Each plan treated two to seven lesions smaller than 2 cm using single-fraction SRS, usually 21–24 Gy, with a 1 mm GTV-to-PTV margin.
- Investigators simulated 20 systematic MLC error scenarios from 0.2 to 1.0 mm, along with 26 combinations of 1° rotational errors and 26 combinations of 1 mm translational errors. GTV D100% and normal-brain V12Gy were recalculated for every scenario.
- A unilateral 1 mm MLC shift reduced GTV D100% by 8.6%. Symmetric closing produced a 25.3% reduction per millimetre, while symmetric opening increased GTV D100% by 23.3% per millimetre, all p < 0.001.
- Translational errors reduced GTV D100% by 9.0% with one degree of freedom, 16.6% with two, and 23.7% with three, corresponding to approximately 7.3% additional loss per added axis.
- Rotational effects increased with target distance from the common isocentre. For each additional millimetre of distance, GTV D100% decreased by approximately 0.1%, 0.2%, and 0.3% under one-, two-, and three-axis rotational errors, respectively.
- Normal-brain V12Gy was comparatively insensitive to setup errors but changed with symmetric MLC deviations. A 1 mm opening increased V12Gy by approximately 5 cm³, while a 1 mm closing reduced it by 2.77 cm³.
- A 1 mm PTV margin was adequate for many routine scenarios, and increasing it to 2 mm raised normal-brain V12Gy by less than 1 cm³ and serial-organ doses by less than 0.3 Gy. However, even a 2 mm margin could not reliably compensate for large symmetric MLC closings or worst-case multidirectional setup errors.
CLINICAL TAKEAWAY
HyperArc target coverage is highly vulnerable to systematic MLC calibration errors and residual translational offsets, particularly for small peripheral lesions treated in one fraction. Rigorous MLC quality assurance, six-degree-of-freedom correction, and individualized margin review are more important than simply expanding every target to 2 mm.