Local Control After Single-Isocenter Dynamic Conformal Arc SRS for Brain Metastases

Brain metastases occur in up to 30% of patients with advanced cancer and remain a major clinical challenge. While WBRT was historically the standard treatment, it provided limited survival benefit and significant neurocognitive toxicity, leading to increasing use of stereotactic radiosurgery (SRS). Recent advances with single-isocenter, multi-target (SIMT) dynamic conformal arc (DCA) techniques on modern linear accelerators have enabled efficient treatment of numerous lesions within a single session, though concerns regarding geometric accuracy persist. This study aimed to evaluate local control outcomes and influencing factors following DCA-SIMT radiosurgery in patients with multiple brain metastases.

We retrospectively analyzed 195 brain metastases treated using single-isocenter, multi-target dynamic conformal arc (DCA-SIMT) stereotactic radiosurgery on a Varian TrueBeam LINAC between August 2018 and September 2020. Treatment planning was performed with Brainlab Elements MultiMets software, version 2.0 and image guidance with ExacTrac. Local control was assessed on MRI according to BM-RANO criteria, while radiation-induced contrast enhancements (RICE) were identified using multiparametric MRI. The median follow-up duration was 12 months. Statistical analyses included chi-square and ROC analyses, with p < 0.05 considered statistically significant.

A total of 195 brain metastases in 37 patients were analyzed. Local control at 6 months was achieved in 93% of lesions, with complete or partial response in 82%. Distance-to-isocenter (DTI), gradient index (GI), and target volume (GTV/PTV) were not associated with local control. In contrast, conformity index (CI) < 1.42 predicted better treatment response (AUC = 0.698, p = 0.0006). Margin expansion ≥ 0.5 mm was associated with improved local control (p = 0.049), while higher margins did not further improve outcomes. Prescription dose showed no significant impact. The addition of immunotherapy or targeted therapy within 4 months post-SRS significantly increased the likelihood of radiographic response (OR = 2.55, p = 0.030), with the strongest association observed in lung adenocarcinoma patients (p < 0.001).

DCA-SIMT stereotactic radiosurgery achieves high local control in patients with multiple brain metastases. Conformity index, minimal margin expansion, and systemic therapy influenced outcomes, whereas distance-to-isocenter and dose did not. Further validation is needed to optimize margins in high-DTI scenarios.