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Dosimetric Impact of Simultaneous and Separate Planning with Fixed/Iris Collimators on Brain Doses in Multiple Brain
Hatice Tataroglu1, Ela Delikgöz Soykut1, Mustafa Özer1
1Department of Radiation Oncology, Samsun Training and Research Hospital, Samsun, Turkey.
Background:
CyberKnife® stereotactic radiosurgery enables frameless, image-guided treatment of multiple brain metastases. Collimator type (Fixed vs. Iris) and planning strategy (Simultaneous vs. Separate) may influence low-dose normal brain exposure, plan quality, and treatment delivery efficiency.
Objectives:
The objective of the study was to compare the dosimetric performance of different collimator types and planning strategies in patients with three or four brain metastases under standardized planning conditions.
Materials And Methods:
Twenty patients were retrospectively replanned using four treatment configurations: Fixed Separate, Fixed Simultaneous, Iris Separate, and Iris Simultaneous. All plans were generated using identical prescription doses and planning objectives. Dosimetric evaluation included normal brain V8, V10, V12 (volumes receiving ≥8 Gy, ≥10 Gy, and ≥12 Gy), mean brain dose (Vmean), Paddick conformity index (PCI), and gradient index (GI). Comparisons were performed using Friedman and Wilcoxon signed-rank tests. Estimated treatment time was recorded for all plans.
Results:
Significant differences were observed for V8 (P = 0.039), Vmean (P = 0.019), and PCI (P < 0.001), whereas V10 (P = 0.180), V12 (P = 0.103), and GI (P = 0.420) showed no significant differences among the planning approaches. Median dosimetric values tended to favor Iris Simultaneous planning for reducing low-dose normal brain exposure. The significant PCI difference was primarily attributable to lower conformity in the Iris Separate plans. Total planning target volume demonstrated a near-significant positive association with V12 (P = 0.057). Iris-based plans were associated with shorter estimated treatment times.
Conclusion:
Under standardized planning conditions, Iris Simultaneous planning demonstrated favorable trends in reducing low-dose normal brain exposure while improving treatment delivery efficiency. Although conformity differed significantly among planning approaches, mainly because of lower PCI values in the Iris Separate plans, dose gradient characteristics remained comparable. These findings suggest that planning strategy may influence conformity more than dose fall-off in CyberKnife treatment of multiple brain metastases. Nevertheless, given the limited sample size, these results should be interpreted cautiously and validated in larger prospective studies.
