Dosimetric comparison of fractionated radiosurgery plans using frameless Gamma Knife ICON and CyberKnife systems with linear accelerator–based radiosurgery plans for multiple large brain metastases

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For patients with multiple large brain metastases with at least 1 target volume larger than 10 cm3, multifractionated stereotactic radiosurgery (MF-SRS) has commonly been delivered with a linear accelerator (LINAC). Recent advances of Gamma Knife (GK) units with kilovolt cone-beam CT and CyberKnife (CK) units with multileaf collimators also make them attractive choices. The purpose of this study was to compare the dosimetry of MF-SRS plans deliverable on GK, CK, and LINAC and to discuss related clinical issues.


Ten patients with 2 or more large brain metastases who had been treated with MF-SRS on LINAC were identified. The median planning target volume was 18.31 cm3 (mean 21.31 cm3, range 3.42–49.97 cm3), and the median prescribed dose was 27.0 Gy (mean 26.7 Gy, range 21–30 Gy), administered in 3 to 5 fractions. Clinical LINAC treatment plans were generated using inverse planning with intensity modulation on a Pinnacle treatment planning system (version 9.10) for the Varian TrueBeam STx system. GK and CK planning were retrospectively performed using Leksell GammaPlan version 10.1 and Accuray Precision version for the CK M6 system. Tumor coverage, Paddick conformity index (CI), gradient index (GI), and normal brain tissue receiving 4, 12, and 20 Gy were used to compare plan quality. Net beam-on time and approximate planning time were also collected for all cases.


Plans from all 3 modalities satisfied clinical requirements in target coverage and normal tissue sparing. The mean CI was comparable (0.79, 0.78, and 0.76) for the GK, CK, and LINAC plans. The mean GI was 3.1 for both the GK and the CK plans, whereas the mean GI of the LINAC plans was 4.1. The lower GI of the GK and CK plans would have resulted in significantly lower normal brain volumes receiving a medium or high dose. On average, GK and CK plans spared the normal brain volume receiving at least 12 Gy and 20 Gy by approximately 20% in comparison with the LINAC plans. However, the mean beam-on time of GK (∼ 64 minutes assuming a dose rate of 2.5 Gy/minute) plans was significantly longer than that of CK (∼ 31 minutes) or LINAC (∼ 4 minutes) plans.


All 3 modalities are capable of treating multiple large brain lesions with MF-SRS. GK has the most flexible workflow and excellent dosimetry, but could be limited by the treatment time. CK has dosimetry comparable to that of GK with a consistent treatment time of approximately 30 minutes. LINAC has a much shorter treatment time, but residual rotational error could be a concern.

ABBREVIATIONS CBCT = cone-beam CT; CI = Paddick conformity index; CK = CyberKnife; GI = gradient index; GK = Gamma Knife; HI = homogeneity index; IMRT = intensity-modulated radiation therapy; LINAC = linear accelerator; MF-SRS = multifraction SRS; MLC = multileaf collimator; OBI = on-board image; PIV = prescription isodose volume; PTV = planning target volume; QA = quality assurance; SF-SRS = single-fraction SRS; SRS = stereotactic radiosurgery; TC = target coverage; TPS = treatment planning system; TV = target volume; VMAT = volumetric modulated arc therapy.

Article Information

Correspondence Eun Young Han: The University of Texas MD Anderson Cancer Center, Houston, TX.

INCLUDE WHEN CITING Published online April 5, 2019; DOI: 10.3171/2019.1.JNS182769.

Disclosures The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

© AANS, except where prohibited by US copyright law.



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    Dose distribution of GK, CK, and VMAT plans in the coronal planes for patient 5. Figure is available in color online only.

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    Box-and-whisker plot for V4Gy (A), V12Gy (B), and V20Gy (C) values for LINAC, GK, and CK plans. The whiskers shows the minimum and maximum values and the 3 horizontal lines in each box represent the first quartile, second quartile (median), and third quartile, respectively. Figure is available in color online only.

  • View in gallery

    Beam-on time for LINAC and GK and beam delivery (beam-on and tumor tracking) time for CK plans. Figure is available in color online only.


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