A. TAKEMURA ET AL. 75

The points in the graphs in Figure 4 represent errors of

the movements. For instance, Figure 4(a) shows five

points for each of non-weighted and weighted and these

points represents the error for −20 mm, −10 mm, 10 mm

20 mm and 29 mm translations. The 3D errors were rep-

resented in two 2D graphs. The Y coordinate errors in the

TX and TY for the weighted couch were more widely

spread than for the non-weighted couch, although there

was no significant difference in the mean error of the

translation motion, as shown in Table 1. The error dis-

tributions for the rotation motion, RX, RY and RZ, for

both the non-weighted and weighted couch were un-

evenly spread. Thus, a significant difference between the

non-weighted and weighted couch in terms of the X and

Y coordinate errors for the rotation motion would occur.

4. Discussion

The mean X coordinate error in the rotation and the mean

Y coordinate error in the rotation and the overall error for

the weighted couch significantly differed from those for

the non-weighted couch. However, all differences did not

exceeded 0.1 mm which is the correctable minimum dis-

tance of the 6D couch system.

With regard to the overall results, the mean distance of

the 3D error vectors for the weighted couch was 0.21 ±

0.11 mm, which was larger than the mean distance of the

3D error vectors of 0.16 ± 0.09 mm for the non-weighted

couch. There was a significant difference between these

values. However, the difference was only 0.05 mm. The

difference in the mean 3D distance for the rotation mo-

tion between the non-weighted and the weighted couch

was also low at only 0.03 mm.

Significant differences were observed in the results of

the mean distance of the 3D error vectors with regard to

the rotation and overall and the mean coordinate error

along X and Y axes. All of these differences between the

non-weighted and weighted couch were smaller than the

minimum correctable distance of the 6D couch system.

Thus, a 60 kg weight does not actually affect accuracy of

the 6D couch motion in the clinical situation.

Although the mean error of the Y coordinate in the

translation did not differ significantly, the distribution of

the Y coordinate errors with regard to the weighted

couch seemed to spread more widely than was the case

for the non-weighted couch. The standard deviations of

the Y coordinate errors for the non-weighted and wei-

ghted couches were 0.06 mm and 0.16 mm, respectively.

A significant difference was found between the variances

of these distributions using the F-test (p < 0.01). This

suggested that the uncertainty of motion, especially con-

cerning the Y coordinate, could be increased by the at-

tachment of the 60 kg weight, and that this uncertainty

might possibly be further increased by the use of a heav-

ier weight.

The variance of a coordinate of the needle tip in

three time manual measurements from an image was up

to one pixel (approximately 0.015 mm). It was enough

smaller than the motion errors of the 6D couch.

5. Conclusion

We found that attaching a weight of 60 kg to the 6D

couch only slightly increased the error regarding the ac-

curacy of motion. The amount of increase of the error is

smaller than the minimum correctable distance of the 6D

couch system. Thus, the 6D couch can correct setup error

for a patient with body weight of about 60 kg or lighter.

However, we also found that the distribution of the Y

coordinate errors for the weighted couch was greater than

those for the non-weighted couch. This might suggest

that a weight of more over 60 kg would increase the un-

certainty of motion.

REFERENCES

[1] J. Rahimian, J. C. Chen, A. A. Rao, M. R. Girvigian, M. J.

Miller and H. E. Greathouse, “Geometrical Accuracy of

the Novalis Stereotactic Radiosurgery System for Tri-

geminal Neuralgia,” Journal of Neurosurgery, Vol. 101,

Supplement 3, 2004, pp. 351-355.

[2] J. Y. Jin, F. F. Yin, S. E. Tenn, P. M. Medin and T. D.

Solberg, “Use of the BrainLAB ExacTrac X-Ray 6D Sys-

tem in Image-Guided Radiotherapy,” Medical Dosimetry,

Vol. 33, No. 2, 2008, pp. 124-134.

doi:10.1016/j.meddos.2008.02.005

[3] J. Ma, Z. Chang, Z. Wang, W. Q. Jackie, J. P. Kirkpatrick

and F. F. Yin, “ExacTrac X-Ray 6 Degree-of-Freedom

Image-Guidance for Intracranial Non-Invasive Stereotac-

tic Radiotherapy: Comparison with Kilo-Voltage Cone-

Beam CT,” Radiotherapy and Oncology, Vol. 93, No. 3,

2009, pp. 602-608. doi:10.1016/j.radonc.2009.09.009

[4] T. Takakura, T. Mizowaki, M. Nakata, S. Yano, T. Fuji-

moto, Y. Miyabe, M. Nakamura and M. Hiraoka, “The

Geometric Accuracy of Frameless Stereotactic Radiosur-

gery Using a 6D Robotic Couch System,” Physics in

Medicine and Biology, Vol. 55, No. 1, 2010, pp. 1-10.

doi:10.1088/0031-9155/55/1/001

[5] J. Meyer, J. Wilbert, K. Baier, M. Guckenberger, A. Rich-

ter, O. Sauer and M. Flentje, “Positioning Accuracy of

Cone-Beam Computed Tomography in Combination with

a Hexapod Robot Treatment Table,” International Jour-

nal of Radiation Oncology, Biology, Physics, Vol. 67, No.

4, 2007, pp. 1220-1228. doi:10.1016/j.ijrobp.2006.11.010

[6] N. Hayashi, Y. Obata, Y. Uchiyama, Y. Mori, C. Hashi-

zume and T. Kobayashi, “Assessment of Spatial Uncer-

tainties in the Radiotherapy Process with the Novalis Sys-

tem,” International Journal of Radiation Oncology, Bi-

ology, Physics, Vol. 75, No. 2, 2009, pp. 549-557.

doi:10.1016/j.ijrobp.2009.02.080

Copyright © 2013 SciRes. IJMPCERO