Purpose: Single-isocenter stereotactic radio surgery (SRS) to multiple targets allows patients to be treated significantly faster than multi-isocenter SRS but is more susceptible to rotational uncertainty. Currently, in single-isocenter SRS, the isocenter is placed at the geometric center of all the lesions. Large targets and those closer to the isocenter are less influenced by rotational uncertainty. The aim of this work is to optimize the placement of the isocenter to minimize the effect that a rotational error would have on target coverages.
Methods: Targets from three previous patients with multiple brain metastases with 2, 4 and 5 targets were used to optimize isocenter placement in single-isocenter SRS. The targets were converted to equivalent spheres with a second sphere being calculated for each target representing dose coverage. The intersecting volume of each target sphere with its respective dose sphere was calculated after a 1° rotational was applied with respect to isocenter placement during optimization iteration. The optimization function sought to minimize the difference in coverage between targets while maximizing the coverage of each individual target using the weighted Tchebycheff technique.
Results: The average variation in target coverage across targets with the optimized isocenter was 1.04%, 10.71% and 9.07%, while placing the isocenter at the geometric center resulted in 1.38%, 16.49% and 12.86% for 2, 4 and 5 target patients respectively. For all cases, the average difference in coverage across targets was reduced with the optimized isocenter.
Conclusion: Small targets and those furthest from the isocenter are more influenced by rotational uncertainty in single-isocenter SRS. Optimization may be used to place the isocenter in such a way that target coverage is less perturbed by rotational uncertainty. Absolute coverage can be adjusted by dose normalization but minimizing variation in coverage reduces the extent to which particular targets suffer greater losses than others.
Funding Support, Disclosures, and Conflict of Interest: NIH/NCATS, TL1 TR002647