Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

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Reports of Practical Oncology and Radiotherapy
journal homepage: http://www.elsevier.com/locate/rpor

Original research article

Comparison of CT artifacts and image recognition of various ﬁducial
markers including two types of thinner ﬁducial markers for
CyberKnife treatment
Toshihiro Suzuki a,∗ , Masahide Saito b , Hiroshi Onishi b , Koji Mochizuki a ,Kenichiro Satani a , Akihiro Yamazaki c , Kenichi Miura c , Shinji Taka d , Naoki Sano b ,Takafumi Komiyama b , Hiroshi Takahashi aaKasugai CyberKnife Rehabilitation Hospital, Yamanashi, Japan
Department of Radiology, University of Yamanashi, Yamanashi, Japanc
Shizuoka City Shimizu Hospital, Shizuoka, Japand
Shizuoka General Hospital, Shizuoka, Japanba r t i c l e

i n f oArticle history:
Received 7 May 2019
Received in revised form 4 September 2019
Accepted 2 December 2019
Available online 9 December 2019
Keywords:
Fiducial marker
CyberKnife
VISICOIL
Gold Anchor
SBRT
CT artifacta b s t r a c t
Aim: The aim of the study was to evaluate computed tomography (CT) artifacts and image recognition of
the CyberKnife system. Regarding ﬁducial markers, VISICOIL of 0.5 mm × 5.0 mm and 0.75 mm × 5.0 mm,
ball-shaped Gold Anchor (GA) of 0.28 mm × 10 mm and 0.28 mm × 20 mm, were compared with thestandard cylinder marker of 0.9 mm × 3.0 mm (ACCULOC).
Background: Recently, various kinds of commercial ﬁducial markers have been available in CyberKnife
treatment.
Materials and methods: The CT images of a water equivalent gel with each ﬁducial marker were acquired
for the evaluation of CT artifacts. The evaluation was performed using the standard deviation of Hounsﬁeld
Unit (HU) value for a rectangle region near the ﬁducial marker. Then, to evaluate the image recognition,
each ﬁducial marker was located to overlap in the target locating system (TLS) for the two sites; thevertebral bone and the pubic bone.
Results: For CT artifacts, the standard deviations of the VISICOIL of 0.5 mm × 5.0 mm was the smallest.
The image recognition of four ﬁducial markers had a value close to the standard cylinder marker and was
feasible for common use, but was slightly poorer when using GA of 0.28 mm × 10 mm in the dynamic
conditions.
Conclusion: Our results indicated that VISICOIL 0.5 × 5.0 mm and the GAs can be used nearly always for
CyberKnife treatment in spite of their much thinner needles than those of cylinder types.
© 2019 Greater Poland Cancer Centre. Published by Elsevier B.V. All rights reserved.

1. Introduction
Stereotactic body radiotherapy (SBRT) using CyberKnife (Accuray, Sunnyvale, California, USA) has been recently introduced asa treatment modality for tumors outside the central nervous system and is already performed in many institutions. Speciﬁcally,
CyberKnife involves using a 6 MV linear accelerator mounted ona robotic arm and allows for irradiation from a speciﬁed location
with six degrees of freedom.1–3 To deliver treatment, a 2D image∗ Corresponding author at: Kasugai CyberKnife Rehabilitation Hospital, 436 Kou,
Kasugai-cho, Fuefuki-city, Yamanashi, 406-0014, Japan.
E-mail address: suzuki@kasugai-reha.com (T. Suzuki).

can also be acquired at 5–150 seconds inter- and intra-fraction byusing two kV X-ray devices (target locating system, TLS), resulting in a high-precision image-guided radiotherapy.4 In addition,a dynamic tracking method using ﬁducial markers placed in or
around the target is available. SBRT using CyberKnife is particularly effective for the treatment of tumors located in organs withrespiratory movement such as the lung or liver. It is also effective
for the treatment of the prostate, which moves depending on urine
volume in the bladder or gas and feces in the rectum. Various kindsof commercial ﬁducial markers for radiation treatment are available, namely, VISICOIL (RadioMed Corporation, Bartlett, TN, USA),
GA (Naslund Medical AB, Huddinge, Sweden), ACCULOC, Fusioncoil, PolyMark (CIVCO Medical Solutions, Kalona, IA, USA) and Goldhttps://doi.org/10.1016/j.rpor.2019.12.005
1507-1367/© 2019 Greater Poland Cancer Centre. Published by Elsevier B.V. All rights reserved.

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T. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

Table 1
Characteristics of various kinds of commercial ﬁducial markers.
Fiducial marker

Coil diameter, mm (needle size (G)or introducer diameter (mm))

VISICOIL (RadioMed Corporation,
Bartlett, TN, USA)Gold Anchor (Naslund Medical AB,
Huddinge, Sweden)
ACCULOC (CIVCO Medical Solutions,
Kalona, IA, USA)
FusionCoil (CIVCO Medical Solutions,
Kalona, IA, USA)
PolyMark (CIVCO Medical Solutions,
Kalona, IA, USA)Gold Marker (Olympus Co. Ltd., Tokyo,
Japan)

0.35 (19 G, 22 G), 0.5 (18 G,19 G,
21 G),a 0.75 (18 G),a 1.1 (17 G)
0.28 (22 G, 25 G)aa

0.9 (18 G),a 1.2 (17 G), 1.6 (14 G)
1.0 (18 G)
1.0 (18 G)
1.5 (1.94 mm)

Our study evaluated the markers.

Marker (Olympus Co. Ltd., Tokyo, Japan). The characteristics of eachmarker are summarized in Table 1.
However, the clinical use of these ﬁducial markers for
CyberKnife treatment is limited by the following factors (1) poor
image recognition in TLS, (2) high degree of invasion such as pneumothorax, bleeding and infection for the marker placement, and(3) artifacts of treatment-planning computed tomography (CT).5–8
Therefore, each ﬁducial marker must be sufﬁciently validated
before it can be used in the clinical setting. In general, (1) might
be improved with the use of large markers, while (2) and (3) might
be improved by using thinner needles and ﬁducial markers.
Recently, two types of thinner needles have been introduced for
CyberKnife treatment: VISICOIL of 0.5 mm × 5.0 mm with 21 G needle and Gold Anchor with 25 G needle. In this study, we evaluated
the CT artifacts and image recognition for CyberKnife treatment offour kinds of markers including these two types of ﬁducial markers compared with those of a standard cylinder marker ACCULOC(0.9 mm × 3.0 mm).
2. Materials and methods
2.1. Characteristics of ﬁducial markers
Four ﬁducial markers, the straight-shaped VISICOIL(0.5 mm × 5.0 mm and 0.75 mm × 5.0 mm) and the ball-shaped GA(0.28 mm × 10 mm and 0.28 mm × 20 mm), were evaluated in thisstudy. However, only the ball-shaped GA was evaluated because
the straight-shaped type cannot be recognized by the CyberKnifesystem. The features of these markers were compared with astandard cylinder marker ACCULOC (0.9 mm × 3.0 mm), which has
obtained a regulatory approval in Japan, and it is conventionallyused in CyberKnife treatment.9 Fig. 1 shows the appearance ofeach ﬁducial marker. The VISICOIL is made of gold (above 99.95%), 0.35–1.1 mm wide, and 5–30 mm long. Meanwhile, the GA is
made of alloy comprising 0.5 % iron and has two types according to
the insertion method, namely, ball-shaped and straight-shaped.10
The ball-shaped is formed by inserting the coil while holding theneedle, and the straight-shaped is formed by inserting the coilwhile pulling the needle. The ACCULOC is made of gold (above
99.99 %). In the TLS system, template matching is performed
between a template image reconstructed from the DRR image andthe live image on the system. The template image is deﬁned as
the expansion of 10 pixels in all directions from the selected partof the ﬁducial marker. The pixel size of both images is approximately 0.4 mm, indicating that the maximum detection range isapproximately 8 mm. Therefore, the ﬁducial marker of more than
8.0 mm in diameter might be misrecognized on the CyberKnifesystem. The straight-shaped GA is over 8.0 mm on the TLS system;
therefore, only the ball-shaped GA was evaluated in this study.

2.2. Evaluation of CT artifacts
CT artifacts were evaluated for each ﬁducial marker. First, theCT images of a water equivalent gel (40 mm × 40 mm × 25 mm
cylinder-shaped) with each marker were acquired. A water equivalent gel was used to make the same conditions as the body.
All CT images were acquired using Optima CT660 (GE Medical
Systems, Milwaukee, WI, USA) in the following settings: 120 kV,
400 mA, 1.25-mm slice thickness, 100-mm ﬁeld of view (FOV), and
512 × 512 pixels. These were the same settings used clinically andin our study, the CT values of the markers were approximately 10
pixels. This feature was the same for all markers. Therefore, a CT
slice, which had a maximum standard deviation of the HU value fora region of 30 pixels from the center of the marker (namely, 61 × 61
pixels) as an area where artifacts can be evaluated sufﬁciently, wasselected to evaluate the CT artifacts. For all markers, Wilcoxon
signed-rank test was performed. Moreover, the Hounsﬁeld Unit
(HU) value of the region of ﬁducial marker was converted from
more than 250 HU to 0 HU to evaluate only the outside region by
excluding the high CT value of the ﬁducial marker. These analyses were performed via MATLAB 2016a (Mathworks, Natick, MA,USA).
2.3. Evaluation of the image recognition of CyberKnife system
To evaluate the accuracy of ﬁducial tracking of the CyberKnifesystem, the image recognition of each ﬁducial marker on TLS wasinvestigated. We used an uncertainty value, which was used in
TLS for the evaluation of ﬁducial tracking. The uncertainty value
between the DRR image and the live image was calculated using the
Zero-means Normalized Cross-Correlation (ZNCC) which is a similarity index. ZNCC is a general index of image similarity, and theimage intensity and X–Y coordinates of DRR image and live image
are used for calculation, respectively. ZNCC can range from −1 to
1, and the value is 1 if the two images are matched completely.
The analysis was performed in a region of 21 × 21 pixels, which
included a ﬁducial marker. The uncertainty value improved to as
close to 0 % as possible, and the threshold of the value is generally
set as 40 % in the clinical use of the CyberKnife system.
The image recognition of each ﬁducial marker in TLS was evaluated under two regions in which the ﬁducial markers often overlap
with the bone in TLS (1) pelvic region and (2) abdominal region.
2.3.1. Pelvis
The ﬁducial marker is often overlapped in TLS with the pubic
bone during CyberKnife treatment to the pelvic region, such as inprostate cancer. Therefore, the evaluation for image recognition
under the condition that the ﬁducial marker is overlapped withthe pubic bone in TLS was performed using an in-house phantom(Fig. 2(a) and (b)). Each marker was inserted into the Rand phantom (arrow) on the condition that the marker is overlapped withthe pubic bone in TLS (a). Rand phantom was ﬁxed on a vacuum
immobilization system for planning CT and veriﬁcation of overlapping with the pubic bone in TLS (b). Set-up of in-house phantom;
the gel with the marker was lifted on the acrylic board, which was
attached to a driven stick (c). The Rand phantom was placed under
the condition that the ﬁducial marker was overlapped with thevertebral bone in TLS (d). First, the CT images of the Rand phantom (The Phantom Laboratory, Salem, NY, USA) with each ﬁducial
marker (straight-shaped VISICOIL, ball-shaped GA and ACCULOC)
were acquired. A water equivalent gel (5 mm × 5 mm × 12 mm),involving each ﬁducial marker, was inserted in the Rand phantom near the bone. A water equivalent gel was used to eliminate
the cavity in the insert section. The CT scans were performed
with Optima CT660, with 120 kV, 400 mA, 1.25-mm slice thickness, 500-mm FOV, and 512 × 512 pixels. The DRR images wereT. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

119

Fig. 1. Fiducial markers used in this study.

Fig. 2. Experimental set-up of each marker, evaluation of tracking accuracy for the pelvis (a,b), abdomen (c,d), respectively.

generated via the obtained CT images, and treatment beams werecreated to validate the image recognition for each ﬁducial marker
in TLS.
To acquire live images, X-ray irradiation was performed using
the same clinical parameters (120, 130, and 140 kV and 100 and
125 mA with 100 ms exposure time). The uncertainty value wasmeasured ten times.
2.3.2. Abdomen
The ﬁducial marker is often overlapped in TLS with a vertebral
bone during CyberKnife treatment to the abdominal region, suchas for liver cancer. For CyberKnife treatment of lung cancer, theXsight Lung Tracking System is usually used in our institute, whichis performed without any other ﬁducial markers.11,12 Therefore,
in this study, only the abdominal region was evaluated. Moreover, the ﬁducial marker is applicable regardless of the presenceor absence of respiratory movement. Therefore, the evaluation
was performed for two kinds of conditions, namely, static anddynamic.

Fig. 2(c) and (d) show the phantom for the evaluation. First, thegel with the marker was lifted on the acrylic board, which was
attached to a driven stick. Then, the Rand phantom was placed insuch a way that the ﬁducial marker was overlapped with the vertebral bone in TLS. Irradiation conditions and procedure were thesame as in C.1.
For evaluation of the dynamic pattern, a simulate respiration
waveform (SI: ± 10 mm; respiration width: AP ± 10 mm; cycle
time: 4 s of sine wave) was used to move the ﬁducial marker fromthe center of the vertebral bone. The waveform was recognizedby the CyberKnife system using the LED markers attached in thephantom. The uncertainty value was measured once at eight points(Fig. 3), which were obtained from the respiration waveform via the
CyberKnife system under each of the above irradiation conditions.
In addition, a Wilcoxon signed-rank test was performed using
the mean of the uncertainty values obtained in C.1 and C.2 (staticand dynamic) to evaluate the image recognition for each marker.
All data analyses were performed using the IBM SPSS statistics ver.
24 software (IBM, Armonk, NY, USA).

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T. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124
Table 4
The rate and number of cases where ﬁducial markers overlapped with the bone in
TLS in our institution. 159 cases of all 229 cases (69 %) were eligible.
Treatment site

Patient num.

Overlap num.

Rate (%)

Liver
Prostate
Lymph nodes
Pancreas
KidneyAdrenal gland
Bile duct
All

145
34
20
16
8
4
2
229

101
32
6
6
8
4
2
159

70
94
30
38
100
100
100
69

Patient num.: The number of patient received with body radiation therapy in our
institution; Overlap num.: The number of the markers overlapping with bone in TLS.

Fig. 4 shows the 2D proﬁle curves of the selected CT slice
and the standard deviations of the CT value for each marker.
The CT slice including the maximum standard deviation was
selected as the CT artifact was the most pronounced. The standard deviation was calculated for 61 × 61 pixels near the ﬁducial
marker with converting the region of more than 250 to 0 HU.
The standard deviations of the two VISICOILs and two GAs weresigniﬁcantly smaller than that of the ACCULOC (p < 0.05). The
standard deviations were 120.6, 203.6, 181.1, 205.5, and 441.9
HU for VISICOIL (0.5 mm × 5.0 mm), VISICOIL (0.75 mm × 5.0 mm),
GA (0.28 mm × 10 mm), GA (0.28 mm × 20 mm), and ACCULOC(0.9 mm × 3.0 mm), respectively. The standard deviations of thetwo VISICOILs and two GAs were signiﬁcantly smaller than thatof the ACCULOC (p < 0.05).

in TLS, and all markers (arrows) can also be visually conﬁrmed
similarly as in 3.2.1.
The mean values of the uncertainty and each standard
deviation for all irradiation conditions to the static phantom
in the vertebral bone are also shown in Table 2. There wasa signiﬁcant difference between VISICOIL (0.5 mm × 5.0 mm)
and ACCULOC (0.9 mm × 3.0 mm) (p < 0.05), and VISICOIL(0.75 mm × 5.0 mm) and ACCULOC (0.9 mm × 3.0 mm) (p < 0.05),
and GA (0.28 mm × 10 mm) and ACCULOC (0.9 mm × 3.0 mm)(p < 0.05) respectively. By contrast, no signiﬁcant differences
between GA (0.28 mm × 20 mm) and ACCULOC (0.9 mm × 3.0 mm)
were observed under the following condition: 120 kV/125 mA(p > 0.05). For GA (0.28 mm × 20 mm), the deviation of the average
VISICOIL (0.75 mm × 5.0 mm) and GA (0.28 mm × 20 mm) were
smaller than that of the other markers.
Fig. 6 shows the results of the dynamic conditions. Regarding
GA (0.28 mm × 10 mm), the uncertainty values of two points (iiiand vii) increased compared with other points. Table 3 shows thedetail of the mean uncertainty values for all irradiation conditionsof each marker for each respiratory point. The uncertainty valuesof GA (0.28 mm × 10 mm) at two points (iii and vii) were especiallyhigher than those of other markers. At other points, changes of theuncertainty value of all markers were only minimal, while somepoints had signiﬁcant differences.

3.2. Evaluation of the image recognition of the CyberKnife system

4. Discussion

3.2.1. Pelvis
For the pelvis phantom with the overlapped region of eachmarker and the pubic bone, the 2D image acquisitions were performed using TLS under some irradiation conditions. Fig. 5 shows
the live images obtained in TLS. All markers (arrows) can be visuallyconﬁrmed in the images.
The mean values of the uncertainty and each standard
deviation for all irradiation conditions in the pubic bone areshown in Table 2. Signiﬁcant differences were observed between
VISICOIL (0.75 mm × 5.0 mm) and ACCULOC (0.9 mm × 3.0 mm)(p < 0.05). Signiﬁcant differences were observed between VISICOIL (0.5 mm × 5.0 mm) and ACCULOC (0.9 mm × 3.0 mm) withthe exception of under the following condition: 140 kV/100 mA(p > 0.05), while the deviation of the average value was similar in
all markers. By contrast, no signiﬁcant differences were observed
between GA (0.28 mm × 10 mm and 0.28 mm × 20 mm) and ACCULOC (0.9 mm × 3.0 mm) (p > 0.05).

Commercial ﬁducial markers of various diameters are available
not only for CyberKnife, but also for linear accelerators, which is themost commonly used device for external beam radiation. In general, using large markers can improve the image recognition andvisibility, but CT artifacts might also increase with the use of lager
markers. The GA can be indwelled via a minimally invasive process because it has the thinnest diameter among all commerciallyavailable markers, and a 25 G needle can be applied to it. However, the straight-shaped GA is a 10 or 20-mm long marker; thus,
straight indwelling cannot be recognized in the CyberKnife systembecause it tends to misrecognize coils longer than 8.0 mm. Moreover, the position of ﬁducial markers varies for each patient andtreatment site; thus, they do not always provide a high accuracy ofimage recognition in TLS. In particular, in the event that the thinmarker overlaps with the bone in TLS, the image recognition may
be signiﬁcantly decreased. Table 4 shows the rate and number ofcases in which the ﬁducial markers overlapped with the bone in TLS
in our institution, which generally occurs in 69 % of overall cases.
The ACCULOC with a larger marker diameter (0.9 mm × 3.0 mm)
than VISICOIL (0.5 × 5.0 mm, 0.75 mm × 5.0 mm) and GA is sometimes used for CyberKnife treatment. In this study, we investigatedimage recognition and CT artifacts of two VISICOILs and two GAs
compared with the ACCULOC.

Fig. 3. A respiratory sine wave used in this study. Eight points on the wave aredeﬁned.

3. Results
3.1. CT artifacts

3.2.2. AbdomenTo evaluate the image recognition for the abdominal region
using the phantom with the overlapped region of a marker and avertebral bone, 2D images were also obtained using TLS under certain irradiation conditions. Fig. 5 shows the live images obtained

T. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

121

Fig. 4. Comparison of standard deviations of the CT images for each ﬁducial marker, (a) thorough (e): straight-shaped VISICOIL (0.5 mm × 5.0 mm), straight-shaped VISICOIL(0.75 mm × 5.0 mm), ball-shaped Gold Anchor (0.28 mm × 10 mm), ball-shaped Gold Anchor (0.28 mm × 20 mm) and ACCULOC (0.9 mm × 3.0 mm), respectively.
Table 2The mean values of uncertainty and each standard deviation for all irradiation conditions to the static phantom (pubic bone and vertebral bone). The position of the marker
was overlapped with the pubic bone and center of the vertebral bone in TLS, respectively.
Uncertainty (%, mean ± S.D.)
VISICOIL
Site

kV/ms
120/100

Pubic bone

130/100
140/100
120/100

Vertebralbone

130/100
140/100

Gold Anchor

ACCULOCmA

0.5 × 5.0 mm

p-value

0.75 × 5.0 mm

p-value

0.28 × 10 mm

p-value

0.28 × 20 mm

p-value

0.9 × 3.0 mm

100
125
100
125
100
125
100
125
100
125
100
125

15.0 ± 0.4
15.0 ± 0.3
15.0 ± 0.3
15.0 ± 0.4
14.8 ± 0.4
14.7 ± 0.4
21.2 ± 1.0
21.1 ± 0.8
20.6 ± 1.1
20.6 ± 1.0
20.2 ± 0.6
20.0 ± 0.7

<0.05
<0.05
<0.05
<0.05
0.12
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05

14.8 ± 0.3
14.7 ± 0.2
14.9 ± 0.4
14.6 ± 0.3
14.7 ± 0.2
14.6 ± 0.2
18.0 ± 0.2
18.1 ± 0.3
18.0 ± 0.2
18.2 ± 0.2
18.0 ± 0.3
18.1 ± 0.3

<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05

14.8 ± 0.3
14.9 ± 0.3
14.4 ± 0.5
14.3 ± 0.3
14.3 ± 0.4
14.3 ± 0.5
17.5 ± 0.6
17.3 ± 0.6
17.4 ± 0.7
17.3 ± 0.6
17.7 ± 0.5
17.3 ± 0.8

0.11
0.11
0.11
0.34
1.00
0.51
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05

14.6 ± 0.3
14.5 ± 0.3
14.2 ± 0.3
14.1 ± 0.2
14.1 ± 0.2
14.0 ± 0.2
17.0 ± 0.2
17.2 ± 0.2
17.3 ± 0.2
17.3 ± 0.2
17.2 ± 0.2
17.0 ± 0.2

0.11
0.07
0.34
0.73
1.00
0.18
<0.05
0.75
<0.05
<0.05
<0.05
<0.05

14.3 ± 0.2
14.1 ± 0.2
14.1 ± 0.2
14.0 ± 0.2
14.0 ± 0.3
13.9 ± 0.1
16.5 ± 0.6
16.7 ± 0.9
16.2 ± 0.5
16.2 ± 0.4
16.0 ± 0.5
16.0 ± 0.6

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Fig. 5. Fluoroscopic images with each ﬁducial marker overlapped with the vertebral bone (left) and the pubic bone (right) in TLS. The straight-shaped VISICOIL with
0.5 mm × 5.0 mm, straight-shaped VISICOIL with 0.75 mm × 5.0 mm, ball-shaped Gold Anchor with 0.28 mm × 10 mm, ball-shaped Gold Anchor with 0.28 mm × 20 mm, and
ACCULOC (0.9 mm × 3.0 mm) are showed from (a) to (e), respectively (arrow).
Table 3The mean values of uncertainty and each standard deviation of all irradiation conditions of each marker for eight respiratory points. For the evaluation of the dynamic pattern,simulate respiration waveform (SI: ± 10 mm; respiration width: AP ± 10 mm; cycle time: 4 s of sine wave) was used to move the ﬁducial marker from the position of centerof vertebral bone.
Uncertainty (%, mean ± S.D.)
VISICOIL

Gold Anchor

ACCULOC

Respiratory point

0.5 × 5.0 mm

p-value

0.75 × 5.0 mm

p-value

0.28 × 10 mm

p-value

0.28 × 20 mm

p-value

0.9 × 3.0 mm

i
ii
iii
iv
v
vi
vii
viii

20.3 ± 1.2
15.4 ± 2.4
18.0 ± 1.6
14.2 ± 0.8
20.1 ± 1.4
26.9 ± 0.9
17.3 ± 1.1
26.0 ± 1.3

0.22
0.22
<0.05
<0.05
0.22
<0.05
1.00
<0.05

18.7 ± 2.2
21.0 ± 1.3
20.5 ± 1.2
20.6 ± 1.8
21.0 ± 2.0
20.1 ± 1.7
21.3 ± 1.7
21.4 ± 2.5

0.69
0.69
<0.05
1.0
0.22
0.22
<0.05
<0.05

19.7 ± 2.3
21.7 ± 1.8
36.6 ± 1.2
20.9 ± 3.4
20.7 ± 5.0
27.1 ± 3.4
37.9 ± 2.0
32.1 ± 3.0

0.69
0.69
<0.05
0.22
<0.05
<0.05
<0.05
<0.05

19.2 ± 0.5
18.7 ± 1.0
20.9 ± 1.2
18.3 ± 0.8
19.2 ± 0.7
22.0 ± 1.4
16.9 ± 0.7
23.1 ± 1.2

0.69
0.69
<0.05
1.00
0.22
<0.05
0.69
0.22

17.1 ± 3.8
20.1 ± 3.8
15.7 ± 1.2
18.3 ± 2.9
14.6 ± 3.8
17.1 ± 1.2
17.2 ± 1.5
19.3 ± 1.9

Metal artifacts inhibit accurate image reconstruction, and are
the result of a signiﬁcantly reduced amount of X-ray incident on thedetector due to metal having a high X-ray absorption rate. Therefore, large markers absorb more X-rays, which increase artifacts.13
Due to CT artifacts, inaccuracies in the HU values result in error inelectron density or stopping power estimate, which may signiﬁcantly affect the dose calculation and optimization processes.14–16
Therefore, it is important to reduce the artifacts. Tanaka et al. evaluated the visibility of CT artifacts relative to the presence of eachﬁducial marker.17 For a more detailed investigation, the CT arti-

T. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

123

Fig. 6. Values of uncertainty for X-ray irradiation under each condition of each marker at the point of i to viii on respiratory waveform, 120 kV (a), 130 kV (b), 140 kV (c).

facts were quantitatively evaluated in this study. The standard
deviations of the two VISICOILs and two GAs were signiﬁcantly
smaller than that of the ACCULOC (p < 0.05), indicating that thesemarkers could reduce the metal artifacts compared to the standard cylinder marker. However, the ball-shaped GA also had higher
density per pixels of CT images than the straight-shaped VISICOIL(0.5 mm × 5.0 mm); as such, the standard deviations of ball-shaped
GA were larger than those of VISICOIL (0.5 mm × 5.0 mm). Furthermore, VISICOIL (0.75 mm × 5.0 mm) had a value close to GA(0.28 mm × 20 mm). Future studies should investigate the ﬁducial
markers under other conditions and investigate their effects ondose distribution.
Regarding image recognition, there have been no studies to evaluate the image recognition of ﬁducial marker under the difﬁcult
tracking conditions in CyberKnife system, while Marsico et al. haveevaluated the visibility of markers with the subjective visual evaluation on the CT images.18 Therefore, we focused on the image
recognition of each ﬁducial marker in CyberKnife treatment.
For the image recognition in the pelvic bone with the overlapped
region of each marker in TLS, our results demonstrated that both GA(0.28 mm × 10 mm and 0.28 mm × 20 mm) and ACCULOC can provide better image recognition than VISICOIL (0.5 mm × 5.0 mm and0.75 mm × 5.0 mm). However, although two VISICOILs had a relatively high uncertainty value, this was not clinically problematic.
Therefore, we recommend using VISICOIL (0.5 mm × 5.0 mm) andGA (0.28 mm × 10 mm) among four ﬁducial markers for treatmentsin the pelvic region. However, these results were obtained in only
one insertion place. In actual treatment, the insertion site, size ofthe prostate, and position of the prostate vary. As such, the ﬁducial
marker in TLS can be overlapped with not only the pubic bone, butalso the ischium and bone cortex. The ischial bone and bone cortex tend to have a higher contrast than the pubic bone used in thisstudy. In addition, the value of uncertainty may change depending
on rectal movement and stomach gas. Further studies are needed
to evaluate the performance of ﬁducial markers in these various
conditions.
On the other hand, for evaluation at the abdominal region forthe static phantom, our results demonstrated that both VISICOIL
and GA had high uncertainty values that were not clinically problematic, which was similar to that in the pelvic region. Meanwhile,124

T. Suzuki et al. / Reports of Practical Oncology and Radiotherapy 25 (2020) 117–124

for evaluation at the abdominal region for the dynamic phantom,
the uncertainty value of GA (0.28 mm × 10 mm) reached up to 40% near the cortex of the vertebral bone, and this might be because
the vertebral bone was thicker than the pubic bone. Another possible reason is that although the GA density (0.28 mm × 10 mm) was
increased by rolling, the size of the marker itself became small.
Therefore, we recommend using VISICOIL (0.5 mm × 5.0 mm) andGA (0.28 mm × 20 mm) among the four ﬁducial markers for static
and/or dynamic treatments in the abdominal region. However,
because a phantom was used in this study, the image recognitionof the system and the visibility might change, depending on the
body thickness, bone shape, and respiratory wave of the patient.
Moreover, the shape of the ﬁducial marker can vary, depending on
the insertion method. Future studies should investigate the ﬁducial
markers under other conditions.
5. Conclusion
In this study, we evaluated the CT artifacts and image recognition on TLS for four ﬁducial markers; straight-shaped VISICOIL of
0.5 mm × 5.0 mm, straight-shaped VISICOIL of 0.75 mm × 5.0 mm,
ball-shaped Gold Anchor (GA) of 0.28 mm × 10 mm and0.28 mm × 20 mm, in comparison with the standard cylinder
marker of 0.9 mm × 3.0 mm (ACCULOC). These four ﬁducial markers can be used for CyberKnife treatment. In particular, VISICOIL(0.5 mm × 5.0 mm) and GA (0.28 mm × 10 mm, 0.28 × 20 mm) are
quite useful in terms of less invasiveness according of their muchthinner needles than those of cylinder types. The results of ourstudy can be useful in understanding and selecting the appropriate
ﬁducial marker for clinical CyberKnife treatment.
Financial disclosure
None declared.
Conﬂict of interest
There are no conﬂicts of interest to declare.
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