Dosimetric performance of two linear accelerator-based radiosurgery systems to treat single and multiplebrain metastases

Abstract

Objective: To evaluate and compare the dosimetric plan quality for noncoplanar volumetric arc therapy of single and multiple brain metastases using the linear accelerator-based radiosurgery system HyperArc and a robotic radiosurgery system.

Methods: 31 tumors from 24 patients were treated by stereotactic radiosurgery using the CyberKnife system. CT images, structure sets, and dose files were transferred to the Eclipse treatment planning system for the HyperArc system. Dosimetric parameters for both plans were compared. The beam-on time was calculated from the total monitor unit and dose rate.

Results: For normal brain tissue, the received volume doses were significantly lower for HyperArc than for CyberKnife_G4 and strongly correlated with the planning target volume (PTV) for cases of single brain metastasis. In addition, the difference in volume dose between CyberKnife_G4 and HyperArc was proportional to the PTV. For multiple brain metastases, no significant difference was observed between the two stereotactic radiosurgery systems, except for high-dose region in the normal tissue. In low dose for brain minus PTV, when the maximum distance among each target was above 8.0 cm, HyperArc delivered higher dose than CyberKnife_G4. The mean ± SDs for the beam-on time were 15.8 ± 5.3 and 5.6 ± 0.8 min for CyberKnife_G4 and HyperArc, respectively (p < .01).

Conclusion: HyperArc is best suited for larger targets in single brain metastasis and for smaller inter tumor tumor distances in multiple brain metastases.

Advances in knowledge: The performance of HyperArc in comparison with CyberKnife_G4 was depended on defined margin and tumor distances.

Figure 1.

Dose distributions for CK and HA with target volumes of 0.1 (A, B) and 4.4 cm³ (C, D) in single metastasis cases. The colored lines represent different isodose lines. Cyan and white represent the low-isodose lines of 3.0 and 6.0 Gy, respectively. At target volume of 4.4 cm³, the isodose volume for HA is clearly smaller than that for CK. CK,CyberKnife; HA, HyperArc; PTV, planning target volume.

Figure 2.

(a) PTV according to GI. Blue represents CyberKnife, and orange represents HyperArc. GI is expressed in logarithmic scale. The GI for CyberKnife and HyperArc suitably fitting with the PTV. The dashed lines represent regression curves. (b) The relationships between PTV and the differences. The volume of the PTV is expressed in logarithmic scale. The red dotted line represents X coordinate of 0.03. GI, gradient index; PTV, planning target volume.

Figure 3.

 Box plots of (A) V₃, (B) V₆, and (C) V₁₂ for brain minus PTV in CyberKnife (CK) and HyperArc (HA). CK is represented in blue, and HA is represented in orange. CK, CyberKnife; HA, HyperArc; PTV, planning target volume.

Figure 4.

Relations between PTV and brain doses at (A) V₃, (B) V₆, and (C) V₁₂. CK is represented in blue, and HA is represented in orange. The relations are fitter using linear regression with coefficient of determination R². The regression dotted line slopes for HA are lower than those for CK in each case. As the difference in dose becomes larger, the PTV increases. Triangles represents the estimated dose with regression for HA when 1 (gray) and 2 mm (yellow) margins in every direction are added to the PTV. For V₃ and V₆, the 2 mm margin added to the PTV matches the regression line for the estimated dose in HA and CK, whereas in V₁₂, the matching occurs for the 1 mm margin. CK, CyberKnife; HA, HyperArc; PTV, planning target volume.

Figure 5.

 Dose distribution for HyperArc at GTV-to-PTV margin increments of 1 mm for (A) small and (B) large targets. Each colored line represents an isodose line, where cyan and white represent low-isodose lines at 3.0 and 6.0 Gy, respectively. GTV, gross tumor volume; PTV, planning target volume.

Figure 6.

Dose distributions for CK and HA with target volumes of (A, B) 0.5 and (C, D) 0.7 cm³ in cases of multiple metastases. The maximum distance between each tumor were (A, B) 4.7 and (C, D) 9.2 cm. Each colored line represents an isodose line, where cyan and white represent low-isodose lines at 3.0 and 6.0 Gy, respectively. CK, CyberKnife; HA, HyperArc; PTV, planning target volume.