Dosimetric effect of rotational setup errors in stereotactic radiosurgery with HyperArc for single and multiple brain...

In stereotactic radiosurgery (SRS) with single-isocentric treatments for brain metastases, rotational setup errors may cause considerable dosimetric effects. We assessed the dosimetric effects on HyperArc plans for single and multiple metastases.

For 29 patients (1-8 brain metastases), HyperArc plans with a prescription dose of 20-24 Gy for a dose that covers 95% (D_95% ) of the planning target volume (PTV) were retrospectively generated (Ref-plan). Subsequently, the computed tomography (CT) used for the Ref-plan and cone-beam CT acquired during treatments (Rot-CT) were registered. The HyperArc plans involving rotational setup errors (Rot-plan) were generated by re-calculating doses based on the Rot-CT. The dosimetric parameters between the two plans were compared.

The dosimetric parameters [D_99% , D_95% , D_1% , homogeneity index, and conformity index (CI)] for the single-metastasis cases were comparable (P > 0.05), whereas the D_95% for each PTV of the Rot-plan decreased 10.8% on average, and the CI of the Rot-plan was also significantly lower than that of the Ref-plan (Ref-plan vs Rot-plan, 0.93 ± 0.02 vs 0.75 ± 0.14, P < 0.01) for the multiple-metastases cases. In addition, for the multiple-metastases cases, the Rot-plan resulted in significantly higher V_10Gy (P = 0.01), V_12Gy (P = 0.02), V_14Gy (P = 0.02), and V_16Gy (P < 0.01) than those in the Ref-plan.

The rotational setup errors for multiple brain metastases cases caused non-negligible underdosage for PTV and significant increases of V_10Gy to V_16Gy in SRS with HyperArc.