Preliminary dosimetric comparison between fixed and rotating source stereotactic radiosurgery systems

Abstract

Purpose: The Akesis Galaxy RTi (AK) is a novel rotational ⁶⁰ Co-based cranial stereotactic radiosurgery (SRS) system. While similar systems have been compared against the fixed-source Leksell Gamma Knife (GK) system using stylized phantoms, dosimetric plan quality with realistic anatomy has yet to be characterized for this or any other rotating system versus GK. This study aims to benchmark AK dosimetric performance against GK by retrospectively replanning previously-treated GK patients at our institution.

Methods: Thirteen patients, previously treated on a GK Icon, were re-planned on the AK treatment planning system using the same prescription doses and isodoses as the original GK plans. The cohort includes patients treated for brain metastases, schwannomas, pituitary adenomas, trigeminal neuralgias, and arteriovenous malformations. Plans are evaluated with target coverage metrics (D_min , D_mean , D_95% , V_150% ) and dose conformality indices: Radiation Therapy Oncology Group conformity index (CI), selectivity, Paddick CI (PCI), gradient index (GI).

Results: AK plans use fewer shots and larger collimation compared to GK plans, resulting in statistically significant reductions in treatment time (p = 0.047) by as much as 88.4 minutes while maintaining comparable target V_100% . For most metastatic cases, GK produces higher D_min (16.0-25.9 vs. 12.5-24.3 Gy, p = 0.008) while AK produces higher V_150% (0.03-14.92 vs. 0.02-11.59 cc, p = 0.028). For non-metastatic cases, GK provides superior CI (p = 0.025) and GI (p = 0.044). No statistically significant differences were found in the remaining metrics.

Conclusion: This cohort demonstrates that the AK system is able to achieve largely comparable dosimetric results to GK, typically with shorter treatment times. Further investigation with a larger cohort is underway.

Keywords: Gamma Knife; rotating source gamma system; stereotactic radiosurgery.