Radiosurgical treatment of solitary brain metastases using virtual cones with a standard multileaf collimator

PURPOSE: The virtual cone has been previously introduced as a novel technique for generating small, spherical dose distributions using a high‐definition multileaf collimator (MLC) for functional radiosurgery applications. There has been no reported investigation into adapting this technique to a standard MLC for the treatment of solitary intracranial metastases as an alternative to physical stereotactic cones. This study characterizes the virtual cone technique adapted to a standard 5 mm leaf‐width MLC (VC(SD)). METHODS: VC(SD) dose distributions using MLC leaf gaps of 2–5 mm were generated and isodose sphericity metrics, peak dose gradients, optimal normalization ranges, and achievable field widths were compared to those of 5.0–12.5 mm diameter physical cones. Target sizes feasible to treat were identified and planned for comparison against established techniques using Paddick conformity index (PCI) and dose volume metrics. End‐to‐end validation of the VC(SD) technique was performed. RESULTS: VC(SD) and physical cones sphericity metrics agree within 3.5% and VC(SD) plans achieved a dose gradient of 21.3% mm(−1), comparable to 10.0‐12.5 mm diameter physical cones. Normalization within the 50%–77% range preserves the optimal dose gradient within 2%⋅mm(−1) and enables the treatment of 5–11 mm diameter planning target volumes (PTVs). Mean PCI for virtual and physical cones was 0.957 and 0.949, which compared favorably against conformal arc and VMAT (0.899 and 0.926). VC(SD) outperformed conformal arc and VMAT for all dose volume metrics, and the mean 50% dose volume differed from physical cones by < 0.5cc for PTVs as small as 5 mm. Validation measurements showed 100% of points passing a 2% / 0.5 mm gamma test for all plans. CONCLUSIONS: The VC(SD) technique efficiently generates spherical dose distributions for the treatment of small brain metastases. Characteristics of the VC(SD) dose distributions are sufficiently comparable to those of physical cones to support VC(SD) as an alternative for the treatment of spherical PTVs as small as 5 mm in diameter.