Effects of Moving the Irradiation Isocenter From the Lesion to the Center of the Brain on High-Definition Dynamic Radiosurgery Planning With Volumetric-Modulated Arcs for Single Brain Metastases

Background This study was conducted to examine the effects of moving the isocenter (IC) position from the lesion to the center of the brain on stereotactic radiosurgery (SRS) planning with volumetric-modulated arcs (VMA) using the High-Definition Dynamic Radiosurgery (HDRS) platform, a combination of the Agility^® multileaf collimator (MLC) (Elekta AB, Stockholm, Sweden) and the Monaco^® planning system (Elekta AB), for single brain metastases (BMs). Methodology The study subject included 36 clinical BMs with the gross tumor volume (GTV) ranging from 0.04 to 48.09 cc (median 7.91 cc). Two VMA plans were prepared for each GTV under identical conditions except for the location of the IC as follows: the center of each GTV (IC_GTV) vs. the center of the brain (IC_Brain). The same prescription dose was uniformly assigned to each GTV <i>D</i> _{V-0.01 cc}, the minimum dose to a GTV minus 0.01 cc (<i>D</i> _>95%), for GTV >0.20 cc and <i>D</i> _95% for GTV ≤0.20 cc. Results The GTV values on the dose-volume histograms (DVHs) were different from the same contoured volumes, and the differences were considerably different between the IC_GTV and the IC_Brain, ranging from -0.07 to 0.14 cc. The inter-IC distances significantly decreased as the depths of the GTV increased. The total calculation times (tCT) and total monitor units (MU) per fraction were significantly longer and higher in the IC_Brain than in the IC_GTV, respectively. The MU differences significantly increased as the IC distances increased. The dose 2 mm inside the GTV boundary was significantly higher in the IC_Brain than the IC_GTV, while there were no significant differences in the other metrics relevant to the dose conformity and gradients outside and inside the GTV boundary. The lack of significant differences in the dose distributions was mainly attributed to some of the leaf widths of essentially <5 mm, variable by the dynamic involvement of diaphragms in the segment formation, and the different combinations of MLC angles with 0º, 45º, and 90º. Conclusions HDRS with VMA created similar dose distributions for SRS of single BMs, irrespective of the IC position, except for the steepness of dose increase just inside the GTV boundary. However, moving the IC from the lesion to the center of the brain significantly increased the tCT and total MU per fraction and was associated with a substantial change in the GTV value ​​on the DVH. Positioning the IC to the center of each lesion is generally recommended for VMA-based HDRS of single BMs.