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.