On the Use Factor Analysis and Adequacy Evaluation of CyberKnife Shielding Design Using Clinical Data

Yu Ra Cho; Haijo Jung; Dong Han Lee

doi:10.14316/pmp.2018.29.4.115

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Cho, Jung, and Lee: On the Use Factor Analysis and Adequacy Evaluation of CyberKnife Shielding Design Using Clinical Data

Original Article

Progress in Medical Physics 2018;29(4):115-122.

Published online: 19 January 2018

DOI: https://doi.org/10.14316/pmp.2018.29.4.115

On the Use Factor Analysis and Adequacy Evaluation of CyberKnife Shielding Design Using Clinical Data

Yu Ra Cho^∗, Haijo Jung^†, Dong Han Lee^∗,

^∗Department of CyberKnife Center, Korea Institute of Radiological & Medical Sciences

^†Radiation Safety Office, Korea Institute of Radiological & Medical Sciences, Seoul, Korea

Corresponding author Dong Han Lee (hanny@kirams.re.kr) Tel: 82-2-970-1812 Fax: 82-2-970-2406

Received 26 October 2018 Revised 18 December 2018 Accepted 18 December 2018

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Although the current internationally recommended standard for the use factor (U) applied to CyberKnife is 0.05 (5%), the CyberKnife shielding standard is applied more stringently. This study, based on clinical data, was aimed at examining the appropriateness of existing shielding guidelines. Sixty patients treated with G4 CyberKnife were selected. The patients were divided into two groups, according to whether they underwent skull or spine tracking. Based on the results, the use factors for each wall ranged from 0.028 (2.8%) to 0.031 (3.1%) for the intracranial treatment and 0.020 (2.0%) to 0.022 (2.2%) for the body treatment. Excessive barrier thickness resulted in inefficient use of space and higher cost to the institutions. Furthermore, because the use factor is influenced by the position of the robot, the use factor determined based on the clinical data of this study would facilitate more reasonable treatment room design.

Keywords: Radiation protection, Radiation shielding design, G4 CyberKnife, Use factor

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Fig. 1

G4 CyberKnife treatment room coordinate system. Draw of the CyberKnife treatment room with indicated dimensions and coordinate system.

Fig. 2

Unit coordinate transformation through rotation by θ about z axis.

Fig. 3

The history of prescribed iso-dose line (a), the number of beam (b) and total MU of patients (c).

Fig. 4

The projection of radiation beams for each tracking mode in CyberKnife, (a) skull tracking-beam intensity, (b) spine tracking-beam intensity (Superior: Front wall, Inferior: Back wall, Anterior: Ceiling, Posterior: Floor).

Table 1.

Characteristics of CyberKnife clinical workload data.

Treatment site	Intracranial	Body
# of case	30	30
Tracking type	Skull tracking	Spine tracking
Avg. prescribed treatment dose (cGy)	2,301.33	3,460.00
Avg. maximum dose (cGy)	2,876.72	4,438.33
Prescribed iso-dose line (%)	80.23	77.60
Avg. # of fractions per treatment	1.90	3.63
Avg. total treatment MU	18,163.5	47,942.46
Avg. Gy per fractions	12.11	9.53
Avg. MU/fractions	9,559.74	13,207.29
Avg. MU/cGy	7.89	13.86
Avg. # of beams	147.20	166.63
Collimator size (mm)	7.5~20	7.5~35

Table 2.

The analysis of MU and use factor (U) for each projection in CyberKnife treatment room.

Direction	Intracranial (skull tracking)		Body (spine tracking)
Direction	MU of hot zone	Use factor	MU of hot zone	Use factor
Superior (Back wall)	0.5%	0.031 (3.1%)	0.3%	0.020 (2.0%)
Inferior (Front wall)	0.6%	0.030 (3.0%)	0.5%	0.020 (2.0%)
Left	3.3%	0.031 (3.1%)	0.6%	0.022 (2.2%)
Right	0.2%	0.028 (2.8%)	0.1%	-
Anterior (Ceiling)	-	-	-	-
Posterior (Floor)	75.3%	0.031 (3.1%)	89.0%	0.022 (2.2%)

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