Dosimetric comparison of the anisotropic analytical algorithm and Acuros XB for single-fraction cranial stereotactic radiosurgery with a 6 MV flattening filter-free beam.

Stereotactic radiosurgery delivers an ablative single-fraction dose to small intracranial targets, and the reported dose depends on the calculation algorithm. This exploratory case series quantified the change in reported dosimetry when plans clinically finalised with the anisotropic analytical algorithm (AAA) were recalculated with Acuros XB (AXB) in dose-to-medium mode. Seven consecutive brain metastases in six patients, treated with 6 MV flattening filter-free volumetric modulated arc therapy, were recalculated on the same CT dataset using identical arc geometry, control points, multileaf collimator sequences and monitor units, without re-optimisation or renormalisation. Dose-volume histogram parameters were compared for the planning target volume (PTV), normal brain and organs at risk, and plan quality with the RTOG prescription-isodose-to-target-volume (PITV) ratio, the Paddick conformity index (PCI) and the gradient index (GI). Paired comparisons used exact two-sided Wilcoxon signed-rank tests and are interpreted as exploratory. AXB reported lower PTV dose metrics than AAA for the same deliverable plans, with mean D95% falling from 21.66 to 21.10 Gy, D98% from 21.19 to 20.58 Gy and Dmean from 24.58 to 24.13 Gy (allp= 0.016). The mean paired D98% difference was 3.00 ± 0.80% of the prescription dose. The PITV ratio fell from 1.19 to 1.10 and GI rose from 3.23 to 3.43 (bothp= 0.016), whereas PCI did not differ (0.79 versus 0.82,p= 0.188). Normal-brain V12Gy and V10Gy were unchanged. Routine electronic portal imaging device dosimetry of the delivered plans gave a mean 2%/2 mm gamma passing rate of 98.5 ± 1.7%, documenting deliverability rather than validating either algorithm. These findings support careful review of target-dose metrics when an established AAA workflow is transitioned to AXB. The cohort is small and no independent reference dosimetry was available, so the study establishes neither clinical superiority nor an effect on tumour control or toxicity.