Dosimetric quality and delivery efficiency of robotic radiosurgery for brain metastases: Comparison with C-arm linear accelerator based plans

The incidence of brain metastases is increasing and various treatment modalities exist for brain metastases. The aim of this study was to investigate the dosimetric quality and delivery efficiency of robotic radiosurgery (CyberKnife) for multiple brain metastases compared with C-arm linear accelerator (linac) based plans. C-arm linac based plans included intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) and non-coplanar VMAT with 1, 3 and 5 non-coplanar arcs, respectively (NC1, NC3 and NC5). For 20 patients, six plans with a prescription dose of 30 Gy in three fractions were generated. The gradient index (GI), conformity index (CI), maximum dose (D-max) of organs at risk (OARs), normal brain tissue volume (V-3 Gy-V-24 Gy), monitor units (MUs) and beam on time (BT) were evaluated. The GI of CyberKnife plans (3.60 +/- 0.70) was lower than IMRT (6.21 +/- 2.26, P < 0.05), VMAT (6.04 +/- 1.93, P < 0.05), NC1 (5.16 +/- 1.71, P < 0.05), NC3 (5.02 +/- 1.59, P < 0.05) and NC5 (5.03 +/- 1.72, P < 0.05). The CI of the VMAT plans (both coplanar and non-coplanar) was larger than IMRT and CK plans. The D-max for most OARs of the CyberKnife plan was lower than the C-arm linac based plans, although some differences were not statistically significant. The normal brain tissue volume of CyberKnife plan was lower than the C-arm linac based plans, and the normal brain tissue volume of non-coplanar VMAT plans was lower than IMRT and VMAT plans at high-moderate dose level. However, the MUs and BT of CyberKnife plans was more than C-arm linac based plans. CyberKnife plan was better than C-arm linac based plans in protecting normal brain tissue and OARs for patients with multiple brain metastases. C-arm linac based plan with non-coplanar arc provided better protection of normal brain tissue than coplanar plan. However, the BT of CyberKnife plan was longer than C-arm linac based plans.