Remote radioactive seed-loading device for permanent brachytherapy of oral cancer with Au-198 grains
Sato et al. Robomech J
Remote radioactive seed-loading device for permanent brachytherapy of oral cancer with Au-198 grains
Mukau Sato 2
Yukari Saito 2
Toshio Takayama 0
Toru Omata 0
Hiroshi Watanabe 1
Ryoichi Yoshimura 4
Masahiko Miura 3
0 School of Engineering, Tokyo Institute of Technology , 4259-G5-27 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502 , Japan
1 Department of Oral and Maxillofacial Radiology, Tokyo Medical and Dental University , 1-5-45 Yushima, Bunkyo-ku, Tokyo , Japan
2 Department of Mechano-Micro Engineering, Tokyo Institute of Technology , 4259-G5-27 Nagatsuta-cho, Midoriku, Yokohama, Kanagawa 226-8502 , Japan
3 Department of Oral Radiation Oncology, Tokyo Medical and Dental University , 1-5-45 Yushima, Bunkyo-ku, Tokyo 113-8510 , Japan
4 Department of Radiation Therapeutics and Oncology, Tokyo Medical and Dental University , 1-5-45 Yushima, Bunkyo-ku, Tokyo , Japan
This paper presents a remote radioactive seed-loading device for permanent brachytherapy of oral cancer with Au-198 grains to reduce the exposure dose to the doctors. Permanent brachytherapy is a treatment which involve implanting of radioactive Au-198 grains, and requires neither long-term needle insertion nor diet-restriction. This is its advantage over temporary brachytherapy. However, the doctors that perform this therapy are exposed to radiation during the treatment procedures. This hinders permanent brachytherapy from being commonly used. The developed device shoots an Au grain from a distant position using air pressure and loads it onto the applicator. This can reduce the exposure dose of the doctors compared with that of the traditional procedure because the doctors are able to stay as far away from the radioactive seeds as possible. We conducted experiments of motion simulation of the proposed and traditional treatment procedures, and compared the exposed doses. The device can reduce the exposed dose by 62% on average.
Brachytherapy; Oral cancer; Loading device
In recent years, radiotherapy has attracted attention as a
treatment for oral cancer because it can maintain a
better quality of life (QOL) of the patient in comparison
with surgery. Among the radiotherapies, brachytherapy
has a number of advantages; it can intensively
irradiate cancer tissue and reduce radiation exposure to
normal tissue around the cancer tissue. With regard to oral
cancer, two brachytherapy methods are performed in
Japan. One is a temporary method using Ir-192 or Cs-137
needles, and the other is a permanent method using
Au-198 grains [
]. The latter requires implanting small
radioactive Au grains (2.5 mm in length and 0.8 mm in
diameter), and involves no diet-restriction, whereas the
former requires a 3–5 day diet-restriction period during
which needles are inserted intra-orally to the cancerous
tissue. Therefore, the permanent method can be applied
to aged or handicapped patients for whom the
temporary method is not suitable. Moreover, it can be applied
to oral cancers other than carcinoma of the tongue, such
as carcinomas of the floor of the mouth and the buccal
mucosa, and oropharyngeal cancer .
The doctors are exposed to radiation during the
treatment procedures. Consequently, there are few hospitals
conducting permanent brachytherapy, and the number
of treatments is limited at these hospitals. Brachytherapy
devices for prostate and lung cancers that reduce the
radiation dose to the doctors have been developed [
]. However, devices that can be applied to permanent
brachytherapy for oral cancer have not been developed.
As mentioned next, oral cancer is quite different from
prostate and lung cancers including their surrounding
environments. Therefore, brachytherapy methods for
them are also different and devices developed for
prostate and lung cancers are not applicable for oral cancers.
Therefore, the purpose of this paper is to develop such
a device to make permanent brachytherapy standard in
Japan and worldwide. In south and south-east Asian
countries, such as India where chewing-tobacco is
popular, it is said that 30% of cancers are oral cancers.
Therefore, there is a potential demand for the device developed
in this paper.
In the treatment, the doctor implants 8–15 Au grains in
and around the cancer tissue with the applicator shown
in Fig. 1. The doctor is exposed to radiation during the
preparation process (Fig. 2a) in which the Au grains are
loaded into the applicator, and during the operating
process (Fig. 2b) in which the Au grains are implanted in the
patient. In the preparation process, the doctor loads Au
grains one by one with a tweezer into applicators arranged
on a tray. In this process, the doctor needs to stand close
to the radioactive seeds for a long time. In the operating
process, the doctor is exposed to radiation from
applicators arranged on a tray, holding the applicator to implant
an Au grain, and grains already implanted in the patient.
The differences between oral cancer and prostate or
lung cancer lead to differences in the design of devices,
which can reduce exposure dosage. The permanent
brachytherapy treatment for prostate cancer uses
approximately 15–20 applicators, wherein each applicator is
loaded linearly with radioactive seeds and spacers. Each
applicator implants them linearly at a time to surround
the cancer tissue. To reduce the exposure dose of the
doctors, implantation robots were developed [
use an ultrasonic probe, CT, or MRI to implant the grains
accurately, as prostate cancer cannot be observed directly
via visual inspection. A device that automates the
loading process of the seeds and spacers into the applicators
was also developed. The device developed by Green 
arranges the seeds and spaces on the ditches of cartridges
and slides them into a needle using stylets. In the
brachytherapy treatment for lung cancer, linearly arranged
seeds and spacers are implanted in a manner similar to
the method observed in brachytherapy for prostate
Unlike prostate cancers, oral cancers occur on the
surface or close to the surface of the oral organs. Au grains
need to be implanted one by one. Therefore, the devices
developed for prostate or lung cancer cannot be used for
the brachytherapy of oral cancer. For temporary
brachytherapy, Remote After Loading System (RALS) is
commercially available [
]. However, it cannot be used for
the permanent brachytherapy of oral cancer.
In our previous study, we developed a remote needle
insertion manipulator for the permanent brachytherapy
of oral cancer [
]. The manipulator can generate pivot
motion by a Remote Center-of-Motion Mechanism
(RCM) so that it can insert a needle into the narrow oral
space. The whole manipulator system becomes large
and complicated resulting in a high-class medical device
according to the medical law in Japan. This makes its
early introduction to clinical practice difficult.
In this study, we develop a remote radioactive
seed-loading device. It shoots an Au grain using air pressure from a
large distance and loads it into a traditional applicator, as
shown in Fig. 3. It can reduce exposure dose by
minimizing the time for which the doctor is close to the
radioactive seeds. The device itself does not make contact with the
patient, which is advantageous for early introduction to
clinical practice. It can also be used to load seeds into the
remote needle insertion manipulator, as mentioned.
This is because the manipulator system uses the same
applicator. The developed seed loading device can also
automate the seed loading process for the manipulator
system. Otherwise, manual loading is necessary, which
would increase the exposure dose of the doctor.
Therefore, to develop the remote loading device prior
to the remote needle insertion manipulator is a
“Remote loading device” section discusses the design of
the remote radioactive seed-loading device and describes
its components. “Experiment” section shows the
experiment of simulation motions by subjects to evaluate the
exposure dose in the traditional and proposed
brachytherapy procedures. The effectiveness of the exposure
dose reduction is confirmed. “Discussion” section
discusses the effectiveness of the proposed device and why
commercially available devices, such as parts feeder
systems and pneumatic tube conveyors cannot be employed
for this remote seed-loading device.
Remote loading device
The remote loading device is composed of a
delivering unit, an applicator holder, and a pipeline connecting
them, as shown in Fig. 4. The delivering unit shoots the
arranged seeds one by one. The applicator holder catches
the shot seed to load it into the applicator. The pipeline
transmits the seed, and is typically 10 m long.
The proposed procedure using the device is as follows:
1. First, the doctor puts seeds into the aligning
subdevice, as shown in Fig. 5, and inserts it into a
vibrator that aligns the seeds by vibration. During the seed
alignment, the doctor can stay away from the seeds
and exposure dose can be reduced.
2. The doctor sets the aligning sub-device to the
delivering unit. The delivering unit is set far from the
dispensary; for example, it can be set in the next room.
Consequently, the exposure dose from the
applicators placed on the tray in the traditional method can
3. In the treatment to the patient, the doctor sets an
applicator into the holder and switches to trigger
seed sending. Then, a seed is shot and loaded into the
Outlet side ildhSSDeenlivdeirningg usniidte
Aphpollidcaetror Switch Seed Cartridge
Fig. 4 Concept of the remote radioactive seed-loading device
applicator passing through the pipeline. The doctor
implants the seed and repeats this process until all
seeds are implanted.
Figure 6 shows the developed remote seed-loading
device, which satisfies the following design
requirements. When a novel device is introduced to the clinical
practice, the change in clinical environment or manner
increases the fatigue of the doctor and, therefore, this
should be avoided. A novel device should also be easy to
operate for the doctor. Therefore, it should not require
expertise and complex techniques to use. Moreover, to
send a seed from far away, it needs to be shot by air
pressure. To prevent back-flow, sealing is necessary. If sealing
materials make sliding motions, there is possibility that
small powder dusts will adhere to the seed. Therefore,
dust generation must be avoided. The next subsections
describe details of the developed device.
Components of the device
Figure 7 shows the V-shaped aligning sub-device. We
conducted experiments to verify that seeds can be
aligned in the bottom groove of the V-shape with a
vibrator and measure the time needed to align them.
Throughout this paper, we use mock seeds made of tungsten with
the same dimensions as the Au grain. The density of
tungsten is close to that of gold. The measurement was
repeated 5 times. Figure 8 shows the result. The
horizontal axis shows the number of seeds to align and the
vertical axis the time needed to align them. All seeds can be
aligned, although the time needed increased as the
number of seeds increased.
Figure 9 shows the overview of the delivering unit and
Fig. 10 its cross sections: the front view (left) and side
Number of seeds
view (right). From the front of the unit, four sub-devices
are arranged. Those are the seed pushing, aligning,
setting, and shooting sub-devices. The seed pushing
subdevice has a pushing rod pushed by a spring, and it is
connected to the seed-aligning sub-device with two
pins. The pushing rod pushes the aligned seeds into the
seed-setting sub-device with the spring. The head seed
is entered into the hole of the lifter in the seed-setting
Figure 11 shows the mechanism of the seed-setting
sub-device. As shown in Fig. 11b, air pressure is supplied
to its cylinder and a cam mechanism of the piston head
lifts up the lifter. The lifter carries the head seed to the
route of the pipeline one by one. The seed-shooting
subdevice driven by air pressure carries the seed into the
pipeline and shoots it, as shown in Fig. 11c. Finally, the
seed-shooting sub-device moves backward; the
seed-setting device moves down its lifter, and the next seed is set
in the lifter, as shown in Fig. 11d.
The linearly arranged seeds may cause a jam when
a seed is lifted up to the route of the pipeline. To avoid
such a jam, a taper is cut in the lifter, as shown in Fig. 12.
The length of the Au grain is 2.50 ± 0.01 mm, and the
width of the lifter is set to be 2.6 mm. Thus, in addition
to the head seed, the front of the next seed is entered
into the hole of the lifter. When the lifter moves upward,
the surface of the taper collides with the front end of the
next seed, which generates a friction force at the tapered
surface. If the angle of the taper is sufficiently large (for
example, 70°), the lifter can overcome the friction and
push the next seed back by the taper and therefore the
jam can be avoided (Fig. 12b).
Figure 13 shows the motion of the seed-shooting
subdevice. When solenoid valve 1 is activated (Fig. 13b), the
pressured metal bellows push forward the piston. The
piston has a thin pipe that pushes the seed to a prescribed
position. At the same time, a small projection from the
piston pushes a switch attached to the seed-setting
subdevice, which activates solenoid valve 2 and supplies air
to shoot the seed (Fig. 13c).
Figure 14a shows the metal bellows used to generate
the reciprocation motion of the pipe. It is structurally
simple and has some advantages, such as its high
airtightness, its ability to extend by air pressure without
friction, and its ability to spring back by its resilience
(Fig. 14b). Therefore, an additional spring to return it to
the initial position is not required.
In the motion procedure in Fig. 13, a tube tightening
mechanism is also activated. Its conceptual diagram is
shown in Fig. 14. It tightens the front tip of the pipe to
prevent backward air leak when the seed is shot. Usually,
O-rings are used to prevent air leak from sliding portions.
However, O-rings generate abrasion and some dusts
could attach to the seed. Therefore, to avoid the
generation of dusts, we propose the tightening tube mechanism.
It is composed of a silicone rubber tube with
cylindrical spacers in its inside, and a fixture with an air
channel that provides air pressure to the silicone rubber tube
from the outside. First, the pipe pushes the seed into the
pipeline at a prescribed position (Fig. 15a-1). Next, the
air chamber is pressurized and the silicone rubber tube
is deformed from its outside (Fig. 15b-2), which prevents
backward air leak when the nozzle of the pipe blows air
to shoot the seed (Fig. 15b-3). It does not rub against the
nozzle while the nozzle pushes the seed into the pipeline.
Only when it is activated does it tightens up the pipe and
prevents the backward air leak.
The air that shoots seeds also requires the factor of
cleanliness. To ensure clean air, we use air filters used for
medical and food automation devices. Moreover, we use
solenoid valves for a clean room.
The applicator holder that loads a seed into an
applicator has a box-like shape and the pipeline is connected to
its lid (Fig. 16). The shot seed falls into the loading hole
of the applicator (Fig. 17a), and by pushing the piston by
half of its length, the seed is loaded into the needle of the
applicator (Fig. 17b). When the needle is inserted into
the skin and the piston is pushed completely, the seed is
implanted under the skin (Fig. 17c).
We developed the remote loading device out of stainless
steel and verified its operation as follows:
1. The piston of the seed-setting sub-device can
generate reciprocation motion so that its cam mechanism
works with an air pressure of 0.25 MPa, as shown in
2. The hole of the lifter of the seed-setting sub-device
is positioned accurately with respect to the pipe of
the seed-shooting device by penetrating them with a
3. The seed-shooting sub-device can push a seed into
the pipeline and can shoot it. It passes through a
pipeline of 10 m in less than 2 s.
Next, we connected the seed aligning, setting, and
shooting sub-devices, to confirm that the sub-devices can work
together continuously. The lifter of the seed-setting
subdevice lifted up a seed to the route of the pipeline, and
the nozzle of the shooting sub-device pushed the seed
into the pipeline, and the seed was shot. This test
succeeded continuously more than 100 times.
Evaluation experiment of the device
To estimate the exposure dose, using real radioactive
Au grains should be avoided. Instead, we estimate it by
measuring the time and distance from the mock seeds
made of tungsten in motion simulations performed by
participants. It can be estimated by
E = f D2 dt (1)
where E [μSv] is the exposure dose, f [μSv m2/s]is the
radioactive dose coefficient, and D(t) [m] is the distance
between a radioactive seed and a reference point on the
participant. It is on the left chest where the doctor wears
a dosimeter in the traditional brachytherapy treatment.
Fig. 16 Applicator holder
The radioactive dose coefficient f is defined as a
coefficient for dose evaluation from external exposure, and
it can be calculated by the product of the effective dose
ratio constant and nominal radioactivity value. From the
physical characteristics of the gold grain, they are 0.0576
μSv m2/(MBq h) and 185 MBq, respectively. Thus,
f = 0.0576 × 185/3600 = 2.96 × 10−3μSv m2/s (2)
The participants separately performed motion
simulations for the three processes: (1) the seed-aligning
process, (2) the seed-loading process, and (3) the traditional
seed-loading process. These are described next. We
recorded with a stereo camera to measure the time and
distance. The attached software can calculate the
distance between two arbitrary points on the basis of
triangulation. We obtained the distance at every second from
the pictures sampled from the movie.
This experiment was approved by the Ethical
Committee of Tokyo Institute of Technology, and advertised for
the participants of the experiment. Written informed
consent was obtained from each participant. The
number of the participants is five. They are men and women
in their 20s. The participants practice two to five times
to become skillful before recording the experiment. The
number of seeds is 8. The participants wore a fatigue
uniform (Fig. 19) that has a mark at the left chest, which
helps recognize the position of the left chest in the
Motion simulation of the seed-aligning process
Carry a case containing seeds from a
pre-positioned location (about 3 s).
Throw the seeds into the aligning sub-device, set
it in a vibrator, and turn it on (Fig. 20a).
Keep at least 1 m away from the vibrator, and
wait until the seeds are aligned automatically
(about 40 s).
Turn off the vibrator and put the top cover on
the seed-aligning sub-device, and set it to the
seed-setting sub-device (Fig. 20b).
The obtained exposure dose is denoted as PA in Table 2.
Motion simulation of the seed-loading process
Set an applicator in the applicator holder and
switch it on to trigger the seed shooting.
Open the lid of the applicator holder and
confirm that the seed falls in the loading hole of the
applicator. Then push the piston of the
applicator to load the seed into its needle.
In actual treatment, seed implantation to the patient
would follow B. The participant repeats processes A and
B n times, where n is the number of seeds.
Meanwhile, the doctor is exposed to radiation from
the seeds implanted in the patient. We estimate the
exposure dose of implanted seeds during the
seedloading process by using the distance and time
measured from a video on the traditional actual treatment.
Figure 21 shows the outline figures of the applicator
exchange and the needle insertion in the traditional
implanting process, while Table 1 shows the data of the
distance and time. Based on the distance between the
doctor and the target tumor, we estimate the exposure
dose radiated from the tumor. The obtained exposure
dose is denoted as PL in Table 2.
We estimate the exposure dose during the seed
implantation to the patient from Table 1. The exposure dose in
the proposed method is only from the holding applicator
and the already implanted seeds around the target tumor
because no applicators are put on a tray.
We applied these data to eq. (1) and obtained the
exposure dose, which is denoted as PH in Table 2. Therefore,
the total exposure during process PI is calculated by
PI = PL + PH.
The total exposure dose of the proposed method is
the sum of those of the seed-aligning process and the
Motion simulation of the traditional seed-loading
Carry a case containing seeds from a
pre-positioned location (about 3 s).
First, push the piston into the applicator
completely. Using tweezers, drop the first seed into
the loading hole of the applicator, as shown in
Fig. 22a. For the rest of the seeds, put them at
the indicated places on the paper, as shown in
Fig. 22b (this is because we have only one
Pick up the applicator and pull its piston to drop
the seed into the bottom of the loading hole.
Then, push the piston again. This motion is
repeated n times.
Carry the applicator to a hanger (about 10 s).
The exposure dose dureing traditional seed-loading
process is denoted as TL in Table 2.
Moreover, we calculated the exposure dose during the
traditional implantation process from the data of Table 1.
Applicator exchange process 900 450
In this calculation, the radiation from the applicators on the
tray is added. The obtained data is denoted as TI in Table 2.
The total exposure dose of the traditional method
is the sum of those of the traditional seed-loading
process and the implantation process. The proposed remote
loading device can reduce the exposure dose by 44–68%
and by 62% on average. The heights, postures, and
progress degrees of the participants are the factors of this
The acceptable radiation dose per year is recommended
by the International Commission on Radiological
Protection (ICRP). ICRP recommends 100 mSv/5 year (with
a maximum of 50 mSv/year). This restriction limits the
number of treatments. If the 62 % of exposure dose can
be reduced, doctors engaging in oral brachytherapy can
increase the number of treatment by 2.6(= 1/(1 − 0.62)).
This is the contribution of the proposed device.
Possibility to employ commercially available devices:
Parts feeder systems are commercially available [
They can feed parts when a large number of similar parts
are supplied in its resource area. They direct parts
initially at random orientations to a feed track and sends
out the aligned parts one by one. In this process, parts
Implanting process 300 1000
PA exposure dose during the seed-aligning process
PL exposure dose from the applicator in which a seed is loaded and already
implanted seeds in the patient during the implantation process
PH exposure dose during the seed implantation to the patient which is
estimated from the traditional process
PI exposure dose during the implantation process (PL+PH)
TL exposure dose during the traditional seed-loading process
TI exposure dose from the handling applicator, implanted seeds and prepared
applicators beside the doctor during the traditional implantation process
that are at undesirable orientations are dropped into the
resource area and sent back to the track.
Therefore, conventional parts feeders have to prepare
more parts than the ones required.
Au grains that are used for the brachytherapy
treatment of oral cancer are radioactive materials, and a large
number of grains cannot be provided because the
radiation dose and cost increase.
A few special parts feeder systems can send out each
and every part. Such parts feeder systems may be used
instead of the vibrator used in this study. However, they
align the parts by repeating the trial process
aforementioned, and hence, the time required to send out all the
Au grains is not guaranteed. In addition, the number
of radioactive Au grains must be controlled. Before the
treatment, the number of aligned Au grains should be
confirmed via visual observation.
Therefore, Au grains need to be aligned and saved in
a buffer prior to the treatment even if such commercial
parts feeder systems are used. A supplying device that
sends the aligned Au grains in the buffer into a pneumatic
tube conveyor one by one is necessary during the
treatment. The pushing subdevice and the setting subdevice
that we developed serve as a supplying device, and
therefore, they are essential.
Pneumatic tube conveyors that shoot parts supplied by
parts feeder systems are slso commercially available [
However, they are designed to shoot specific parts, e.g.,
bolts or nuts. Air leak in a pneumatic tube conveyor may
be negligible for sufficiently heavy parts such as bolts and
nuts. Au grains are small and lightweight, and an air leak
can blow away subsequent grains in the buffer.
Moreover, in the commercial pneumatic tube conveyor
combined with a parts feedersystem, parts are supplied
via the pushing force of a parts feeder system. However,
as mentioned, Au grains must be aligned before the
treatment, and cannot be supplied by the pushing force of a
part feeder system during the treatment. Therefore, a
special pneumatic tube conveyor needs to be designed to
shoot Au grains. The shooting subdevice that we
developed satisfies the requirement for shooting Au grains.
Conclusion and future work
To reduce the exposure dose to the doctor in the
permanent brachytherapy of oral cancer, we developed a
remote radioactive seed-loading device that delivers
radioactive seeds from a distance to the doctor using air
pressure to load the seed into the applicator. To confirm the
effectiveness of the device, we conducted motion
simulations of the proposed and traditional brachytherapy
procedures and compared their exposure doses. The results
show that the proposed procedure using the developed
device can reduce the exposure dose by 62% on average.
In future, we will apply the proposed procedure for
MS, YS, TT and TO equally contribute to the development of the device and
conducting the experiments. HW, RY and MM equally contribute to the overall
system design from the medical point of veiw. All authors read and approved
the final manuscript.
Mukau Sato, Yukari Saito, Toshio Takayama and Toru Omata: Tokyo Tech
contributes to the development of the device and conducting the experiments.
Hiroshi Watanabe, Ryoichi Yoshimura and Masahiko Miura: Tokyo Medical and
dental university contributes to the overall system design from the medical
point of view.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
We have been approved by ethics committee of Tokyo Institute of Technology
to conduct experiment in this paper.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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