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

Abstract

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 5mmleaf-width MLC (VCSD ). VCSD dose distributions using MLC leaf gaps of 2-5mmwere generated and isodose sphericity metrics, peak dose gradients, optimal normalization ranges, and achievable field widths were compared to those of 5.0-12.5mmdiameter 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 VCSD technique was performed. VCSD and physical cones sphericity metrics agree within 3.5% and VCSD plans achieved a dose gradient of 21.3%mm-1 , comparable to 10.0-12.5mmdiameter physical cones. Normalization within the 50%-77% range preserves the optimal dose gradient within 2%⋅mm-1 and enables the treatment of 5-11mmdiameter 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). VCSD 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 5mm.Validationmeasurementsshowed100% of points passing a 2% / 0.5mmgamma test for all plans. The VCSD technique efficiently generates spherical dose distributions for the treatment of small brain metastases. Characteristics of the VCSD dose distributions are sufficiently comparable to those of physical cones to support VCSD as an alternative for the treatment of spherical PTVs as small as 5mmin diameter.