  Purpose: A novel rotating gamma stereotactic radiosurgery (SRS) system
(Galaxy RTi) with real-time image guidance technology has been developed for
high-precision SRS and frameless fractionated stereotactic radiotherapy (SRT).
This work investigated the dosimetric quality of Galaxy by comparing both the
machine treatment parameters and plan dosimetry parameters with those of the
widely used Leksell Gamma Knife (LGK) systems for SRS. Methods: The Galaxy RTi
system uses 30 cobalt-60 sources on a rotating gantry to deliver non-coplanar,
non-overlapping arcs simultaneously while the LGK 4C uses 201 static cobalt-60
sources to deliver noncoplanar beams. Ten brain cancer patients were unarchived
from our clinical database, which were previously treated on the LGK 4C. The
lesion volume for these cases varied from 0.1 cm3 to 15.4 cm3. Galaxy plans
were generated using the Prowess TPS (Prowess, Concord, CA) with the same dose
constraints and optimization parameters. Treatment quality metrics such as
target coverage (%volume receiving the prescription dose), conformity index
(CI), cone size, shots number, beam-on time were compared together with DVH
curves and dose distributions. Results: Superior treatment plans were generated
for the Galaxy system that met our clinical acceptance criteria. For the 10
patients investigated, the mean CI and dose coverage for Galaxy was 1.77 and
99.24 compared to 1.94 and 99.19 for LGK, respectively. The beam-on time for
Galaxy was 17.42 minutes compared to 21.34 minutes for LGK (both assuming dose
rates at the initial installation). The dose fall-off is much faster for
Galaxy, compared with LGK. Conclusion: The Galaxy RTi system can provide dose
distributions with similar quality to that of LGK with less beam-on time and
faster dose fall-off. The system is also capable of real-time image guidance at
treatment position to ensure accurate dose delivery for SRS.
