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. 2017;4(4):289-301.

Validity of the use of nose tip motion as a surrogate for intracranial motion in mask-fixated frameless Gamma Knife® Icon™ therapy

Gavin Wright  1 , Natalie Harrold  1 , Paul Hatfield  1 , Peter Bownes  1
Affiliations

Validity of the use of nose tip motion as a surrogate for intracranial motion in mask-fixated frameless Gamma Knife® Icon™ therapy

Gavin Wright et al. J Radiosurg SBRT. 2017.

Abstract

This study investigates the validity of monitoring nose movement, using an infrared stereoscopic camera system (HDMM), to evaluate intracranial movement during treatment with the Icon™-model Gamma Knife®.

Methods: The HDMM was validated by comparison against known displacements. Next, an anthropomorphic phantom was rotated to register nose displacements on the HDMM, which were compared to the displacements of seven intracranial locations determined by cone-beam CT (CBCT). Similarly, CBCT-calculated intracranial displacements were compared against HDMM-reported nose displacements for patients.

Results: HDMM-indicated displacements were accurate within 0.06mm mean. In the phantom, CBCT-calculated nose displacements agreed within 0.05mm (mean) of HDMM-reported nose displacements. In 16 instances intracranial displacements exceeded nose displacements; at the most extreme by 73% (2.76mm versus 1.59mm). Overall, intracranial anatomy displaced by 43% (mean) less than the nose. Patient data included no intracranial target displacements exceeding nose displacements.

Conclusions: Intracranial phantom and patient anatomy displaced by approximately half that of the nose, suggesting nose movement is generally a suitable surrogate for intracranial movement. The study constitutes the presentation of a simple, robust method that can be applied to determine the relationship between nose tip and intracranial motion in real patients undergoing frameless treatments on Icon™.

Keywords: Gamma Knife; Icon; cone beam CT; frameless stereotactic radiosurgery; intrafraction motion; mask; stereoscopic nose tracking.

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Conflict of interest statement

Authors’ disclosure of potential conflicts of interest Dr. Wright reports attendance at user group meetings of early Icon adopters and meetings organized and hosted by Elekta. All other authors reported no conflict of interest.

Figures

Figure 1
Figure 1
HDMM system, consisting of stereoscopic infrared camera (a) attached to the foot of the couch (b), with a line-of-sight to four immobile reflective markers (c) integral to the Icon™ head support (d)
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Figure 2
Figure 2
Typical workflow on Gamma knife® Icon™, indicating timings of CBCT scans and HDMM active monitoring within the treatment process.
Figure 3
Figure 3
(a) RANDO phantom setup on Icon™ with nose marker (white arrow), immobile reflective markers (hollow arrows) and stereotactic axes indicated. (b) Cerebellar point (99.6, 66.3, 106.0)mm. (c) Left trigeminal nerve root point (117.3, 106.1, 100.7)mm. (d) Superior point (92.3, 120.6, 21.4)mm. (e) Posterior point (98.9, 51.9, 50.4)mm. (f) Anterior point (85.1, 172.2, 59.5)mm. (g) Right lateral point (53.3, 118.6, 58.0)mm. (h) Left lateral point (156.0, 102.0, 64.0)mm.
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Figure 4
Figure 4
Magnitude of reflective marker displacement as induced by, and measured with, the validation tool, plotted against the corresponding displacement indicated by the HDMM system.
Figure 5
Figure 5
Displacement of the nose marker on the anthropomorphic head phantom as indicated by the HDMM system, plotted against nose point displacement calculated from co-registration of phantom’s CBCT scans. The dashed line represents the line of unity.
Figure 6
Figure 6
translation (Trans) and rotation (Rot) co-registration components resulting from the three investigated modes of phantom movement: (a) intended rotation about the z’ axis, (b) intended rotation about the x’ axis, and (c) intended rotation about the y’ axis.
Figure 7
Figure 7
Magnitude of displacement of intracranial anatomy at locations approximately corresponding to the cerebellar midline (cere), left trigeminal nerve root (tri) and extreme superior (sup), posterior (post), anterior (ant), left lateral (left) and right lateral (right), compared against the magnitude of displacement of the nose tip indicated by the HDMM system. Displacements are compared for the three investigated modes of phantom movement: (a) intended rotation about the z’ axis, (b) intended rotation about the x’ axis, and (c) intended rotation about the y’ axis. The line of unity (dotted line) is also included.
Figure 8
Figure 8
Displacement of points representative of the centre of intracranial target volumes, compared against displacements of the nose tip indicated by the HDMM system, from eleven patients undergoing frameless GK Icon™ therapy at our centre. The line of unity (dotted line) is also included.
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