Dosimetric comparisons of different hypofractionated stereotactic radiotherapy techniques in treating intracranial tumors > 3 cm in longest diameter

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The authors sought to compare the dosimetric quality of hypofractionated stereotactic radiosurgery in treating sizeable brain tumors across the following treatment platforms: GammaKnife (GK) Icon, CyberKnife (CK) G4, volumetric modulated arc therapy (VMAT) on the Varian TrueBeam STx, double scattering proton therapy (DSPT) on the Mevion S250, and intensity modulated proton therapy (IMPT) on the Varian ProBeam.


In this retrospective study, stereotactic radiotherapy treatment plans were generated for 10 patients with sizeable brain tumors (> 3 cm in longest diameter) who had been treated with VMAT. Six treatment plans, 20–30 Gy in 5 fractions, were generated for each patient using the same constraints for each of the following radiosurgical methods: 1) GK, 2) CK, 3) coplanar arc VMAT (VMAT-C), 4) noncoplanar arc VMAT (VMAT-NC), 5) DSPT, and 6) IMPT. The coverage; conformity index; gradient index (GI); homogeneity index; mean and maximum point dose of organs at risk; total dose volume (V) in Gy to the normal brain for 2 Gy (V2), 5 Gy (V5), and 12 Gy (V12); and integral dose were compared across all platforms.


Among the 6 techniques, GK consistently produced a sharper dose falloff despite a greater central target dose. GK gave the lowest GI, with a mean of 2.7 ± 0.1, followed by CK (2.9 ± 0.1), VMAT-NC (3.1 ± 0.3), and VMAT-C (3.5 ± 0.3). The highest mean GIs for the proton beam treatments were 3.8 ± 0.4 for DSPT and 3.9 ± 0.4 for IMPT. The GK consistently targeted the lowest normal brain volume, delivering 5 to 12 Gy when treating relatively smaller- to intermediate-sized lesions (less than 15–20 cm3). Yet, the differences across the 6 modalities relative to GK decreased with the increase of target volume. In particular, the proton treatments delivered the lowest V5 to the normal brain when the target size was over 15–20 cm3 and also produced the lowest integral dose to the normal brain regardless of the target size.


This study provides an insightful understanding of dosimetric quality from both photon and proton treatment across the most advanced stereotactic radiotherapy platforms.

ABBREVIATIONS CBCT = cone-beam CT; CI = conformity index; CK = CyberKnife; DSPT = double scattering proton therapy; GI = gradient index; GK = Gamma Knife; GTV = gross tumor volume; HI = homogeneity index; IMPT = intensity modulated proton therapy; LINAC = linear accelerator; MLC = multileaf collimator; MU = monitor unit; PTV = planning target volume; RBE = relative biological effectiveness; SRS = stereotactic radiosurgery; SRT = stereotactic radiotherapy; VMAT = volumetric modulated arc therapy; VMAT-C = coplanar arc VMAT; VMAT-NC = noncoplanar arc VMAT.

Article Information

Correspondence Yongrui Bai: Renji Hospital, School of Medicine, Shanghai Jiao Tong University, Shanghai, China.

INCLUDE WHEN CITING Published online March 22, 2019; DOI: 10.3171/2018.12.JNS181578.

Disclosures S.F.D. reports receiving an honorarium from Medtronic.

© AANS, except where prohibited by US copyright law.



  • View in gallery

    Example case showing dose distribution differences from GK (A), CK (B), VMAT-C (C), VMAT-NC (D), DSPT (E), and IMPT (F) in the axial (top row), coronal (middle row), and sagittal (bottom row) planes.

  • View in gallery

    Box-and-whisker plots for CI (A), GI (B), HI (C), normal brain volume receiving 5 Gy (V5; D), normal brain volume receiving 12 Gy (V12; E), and integral dose (F) to the normal brain for all of the cases planned for the GK, CK, VMAT-C (V-C), VMAT-NC (V-NC), DSPT, and IMPT treatments.

  • View in gallery

    The ratios of normal brain volume receiving 5 Gy (V5; A), 12 Gy (V12; B), and integral dose (C) for all of the modalities relative to GK with respect to different target sizes. Volumes are in cm3.


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