Full text of DOI:10.1007/PL00011324

Lasers Med Sci 2000, 15:252–256
2000 Springer-Verlag London Limited
©
Preserving the Integrity of Oesophageal Stents
with Laser Therapy and Argon Plasma Coagulation:
An In Vitro Study
N. Rajendran and N. Krasner
Gastrointestinal Unit, University Hospital Aintree, Liverpool
Abstract. Thermal lasers and argon plasma coagulation are widely used in the treatment of stent
overgrowth in patients with advanced oesophageal malignancy. The aim of treatment is to achieve patency
while avoiding damage to the prosthesis. This experimental study was designed to determine the power and
duration of application that can be safely tolerated by four di#erent types of oesophageal prostheses. Five
stents were studied: wall stent; open metal mesh stent (uncovered Ultraflex); covered metal mesh stent
(
covered Ultraflex); Gianturco (Z-stent); Esophagocoil. Nd-YAG Laser, GaAlAs diode laser and argon plasma
coagulation were applied in non-contact mode at gradually increasing power levels and duration and the
e#ects were observed. The use of argon plasma coagulation on Esophagocoil stent seems safe in power settings
of 100 W up to 10 s. The diode laser is intermediate in that Gianturco and Esophagocoil stents can withstand
pulses of up to 50 W for about 2 s. The Nd-YAG laser is detrimental to most stents at power levels of 20 W. Only
the Esophagocoil withstands Nd-YAG pulses of 60 W but only up to 1 s. Wallstent, open and membrane-
covered mesh stents perform poorly in that they can only tolerate up to 1.5 s of power at 25 W with the Diode
and 1.0 s of power at 20 W with Nd-YAG laser. The use of di#erent thermal modalities on the five stents has
indicated safe power limits and duration. Membrane-covered stents are always damaged by thermal laser
application unless the membrane is truly transparent.
Keywords: Argon plasma coagulation; Expandable metal stents; GaAlAs diode laser; Nd-YAG laser; Stent
overgrowth
BACKGROUND AND AIM
terminal uncovered portions through which
ingrowth can occur.
Expandable metal stents (EMS) are rapidly
replacing semirigid stents as the prostheses of
choice for the palliation of inoperable oeso-
phageal cancer [1–3]. They can be placed with
relative ease and safety and give a good func-
tional result in terms of relief from dysphagia
Whatever type of tumour ingress occurs, the
result is dysphagia and further therapy is
required. One option is coaxial placement
of another stent [8–10]. Alternatively the
obstructing tumour can be destroyed by
thermal ablation using laser or argon plasma
coagulation (APC) [8,11–14]. Given the high
cost of EMS, this is likely to be a more cost-
e#ective solution and is the preferred
approach in many centres. Despite the wide-
spread use of thermal destructive methods
for clearing stents, little is known about the
ability of the prostheses themselves to with-
stand damage when exposed to laser or APC
energy.
[
4–6]. Nonetheless, as experience with these
stents grows, it has become clear they are not
without problems. One frequent reason for loss
of stent patency is tumour overgrowth [7–9]. In
those stents designed with an open mesh or
coil, tumour may grow in through the sides of
the stent. Others incorporate a plastic sheath
or membrane covering to prevent ingrowth but
are still susceptible to overgrowth of tumour
at the proximal or distal ends of the stent.
Furthermore, many covered stents have
Stents in common use are constructed from
surgical grade stainless-steel alloy filaments
woven in a tubular mesh. The membranes used
in covered stents are made from silicone. Both
the metal strands and the membranes can be
damaged by thermal laser [7,11,13]. Very little
Correspondence to: Professor N. Krasner, Consultant
Gastroenterologist, University Hospital Aintree, Lower
Lane, Liverpool L9 7AL, UK.

Preserving the Integrity of Oesophageal Stents with Laser Therapy
253
information is available on the e#ects of APC
on stent components.
The longer the EMS is in situ, the greater
the likelihood of ingrowth/overgrowth and
hence the need for laser ablation or APC
therapy [11,13,14]. Thus the e#ect of the
thermal ablation process on the metal stents
needs to be assessed. We studied the e#ects of
laser and APC on five types of stents to deter-
mine the power and duration that can be safely
applied without damaging the stents.
MATERIALS AND METHODS
The following instruments were used.
1
.
.
High power neodymium–yttrium–aluminium–
garnet (Nd-YAG laser), 50–70 W operating
at 1064 nm. Laser pulses were usually
applied for 1–3 s.
Gallium–aluminium–arsenide (GaAlAs) semi-
conductor diode laser operating at 810 nm
Fig. 1. Expanding metal stents (top to bottom): open-
mesh metal stent; covered-mesh metal stent; Esophagocoil;
Gianturco (Z stent).
2
(
25–45 W). Although short pulses of 1–2 s
can be applied, continuous application for
up to 10 s is more usual. Both forms
of laser have been shown to be e#ective,
relatively safe, and easy to apply with the
patient under intravenous sedation.
3
. Argon plasma coagulation (APC) is a means
of applying a monopolar electrosurgical
current to tissue via an ionised argon gas
stream (argon plasma). The argon gas
remains ionised for 2–10 nm distal to the tip
of the applicator where the energy delivery
to the tissue is uniform, contact-free and
penetrates to a maximum coagulation depth
of about 3 mm. The usual power level is
Fig. 2. Laser beam damage on the membrane of the
covered stent.
65 W and continuous application is used.
The following stents were tested (Fig. 1):
Fig. 3. ‘Pitting’ on metal coil of Esophagocoil.
1
2
. Wall stent;
. Open mesh metal stent (uncovered
Ultraflex, Boston Scientific);
. Covered mesh metal stent (covered
Ultraflex, Boston Scientific);
The lasers were applied to the stents at
gradually increasing power settings, starting
at 20 W and increasing to 100 W. The duration
was also increased in increments of 0.5 s up to
10 s. The e#ects on the stents were then
observed. The earliest sign of damage was
noted. The membrane covered stents developed
tiny holes in the membrane (Fig. 2) whereas
the Esophagocoil stents developed pitting on
the metal surface (Fig. 3). The final endpoint
was taken when the stents visibly melted.
Open mesh stents did not show any early signs,
and the first sign of damage was melting of the
strands of the stent.
3
4
5
. Esophagocoil (Medtronic);
. Gianturco (or ‘Z-stent’) (Wilson Cook).
Nd-YAG laser (Flexilase, Living Tech-
nology, Pilkington) and GaAlAs diode laser
Diomed, Cambridge) were applied in a non-
(
contact mode at about 10 mm from the stent
surface. The argon plasma coagulator (ERBE,
Tubingen) was used in a non-contact mode, as
close as possible to the surface of the stent.

254
N. Rajendran and N. Krasner
Table 1. Thermal application of APC, Nd-Yag and GaAIAs diode lasers to five stents, showing time up to
which the stents are not significantly damaged
Stent type
Power setting
APC
Wallstent
Open
Cover
Giant.
Esoph.
Diode laser
Wall stent
Open
Cover
Giant.
50 W
1 s
1 s
1 s
5 s
10 s
30 W
*
65 W
1 s
1 s
1 s
5 s
10 s
40 W
*
75 W
*
*
*
3 s
10 s
50 W
*
*
*
100 W
*
*
*
3 s
10 s
60 W
*
20 W
1.5 s
1.5 s
1.5 s
5 s
10 s
20 W
1 s
25 W
35 W
*
*
*
0.5 s
3 s
35 W
*
45 W
*
*
*
55 W
*
*
*
1 s
1 s
1 s
3 s
10 s
25 W
*
*
*
*
*
*
*
*
1 s
5 s
30 W
*
0.5 s
3 s
40 W
*
*
*
*
Esoph.
Nd-Yag laser
Wall stent
Open
3 s
45 W
*
*
3 s
50 W
*
3 s
55 W
*
1 s
60 W
*
*
1 s
*
*
*
*
*
*
Cover
Giant.
Esoph.
1.5 s
1.5 s
5 s
*
*
*
*
*
*
2 s
*
*
2 s
*
*
1.5 s
*
*
1 s
*
*
1 s
*
*
5 s
2 s
1 s
Open, open-mesh metal stent; cover, covered-mesh metal stent; Giant, Gianturco or Z stent; Esoph, Esophagocoil.
*Stent destruction.
A glowing e#ect on the metal surface of the
Esophagocoil and up to 2.5 s with the Z-stent
but the ‘safe’ limits of power setting and
duration may not be su$ciently clinically
stent was sometimes noted, but as this was
seen only during the laser application and
appeared to result in no permanent damage it
was taken as safe from the point of view of
integrity of the stent although the patient may
experience discomfort from heat transference
during the clinical procedure.
e#ective
to
treat
tumour
ingrowth/
overgrowth. Wall stents, open and covered
mesh stent could be safely treated to an upper
limit of 1.5 s exposure at a power level of 25 W.
Nd-YAG laser damages the stents at much
lower settings (Table 1). At a power setting of
2
0 W, the Wall stent and open mesh stent
lasted for only 1 s whereas the covered mesh
and Gianturco were maintained intact up to
RESULTS
1
.5 s. The Esophagocoil was undamaged up to
The e#ects of laser and APC on the stents
are shown in Table 1. For the purpose of
brevity, only those power settings causing
stent damage are recorded. According to this
in vitro study, argon plasma coagulation
appears to have less potential for damaging
stents than the other two laser modalities
a maximum of 60 W but for only 1 s. The
Nd-YAG laser must be used with great care in
patients with oesophageal stents irrespective
of the stent type.
(
Table 1c). APC can be applied for 10 s at a
DISCUSSION
power setting of 100 W in the presence of the
Esophagocoil and 6 s at a power setting of
Patients with overgrowth of oesophageal
stents usually present with recurrence of dys-
phagia. Once metal stents are in place for
longer than 2–3 weeks they cannot be readily
removed because they are incorporated in to
the wall of the oesophagus due to epithelialis-
ation and granulation formation. A method of
tumour clearance must be found. Treatment
75 W in the presence of the Z-stent. Wallstent,
open, and covered mesh stents could only
withstand APC at a setting of 65 W for up to
1
s.
Diode laser causes less damage than
Nd-YAG laser (Table 1b). At a power setting of
0 W this can be applied for up to 5 s with the
3

Preserving the Integrity of Oesophageal Stents with Laser Therapy
255
o#ered needs to be e#ective, safe and prefer-
ably done on an outpatient basis and capable
of being repeated. These objectives can cer-
tainly be achieved by laser therapy to the
overgrowth. However, during the process
there is a possibility that the metallic stent
may be damaged [11]. We have investigated the
safe level of power and duration with three
thermal modalities on five commonly used
stents.
Semirigid tubes were initially used for pal-
liation of oesophageal malignancies [15,16].
Nd-YAG laser and diode lasers have been a
mainstay of oesophageal tumour ablation
therapy since the early 1980s, and before the
development of EMS, were the primary means
of palliation [17–20]. APC is a more recent
development but is rapidly gaining in popu-
larity because of its ease of use and safety
stent). For each laser modality, it is possible to
define a ‘safety limit’ of power and duration
below which damage does not occur (Figs 2
and 3). When lasers were applied direct to the
transparent membrane of the Z stent, no
damage occurred to the membrane, but the
heat energy was still transmitted through
the membrane, as evidenced by the burning of
the cloth beneath the stent observed in our
studies. At clinically useful powers and dur-
ation, limited stent damage was always seen
with all the stents that we used, although the
integrity of the stents was not necessarily
compromised. With APC, however, which is
usually applied at 65 W, the metallic com-
ponent of all stents survived brief exposure but
only the Z stent and Esophagocoil can survive
more than 5 s or so. The membrane is consist-
ently damaged by any direct thermal contact
except for the briefest exposures.
[21–23]. Photodynamic therapy is another safe
modality in tumour ingrowth of coated and
non-coated stents but the process of treatment
is inconvenient for the patient and also expen-
sive when compared with thermal ablation
Other considerations that may determine
the choice of a particular modality in this
situation include extent of overgrowth,
presence of bleeding (APC preferred choice),
type of stent in place and local availability of
the type of laser/APC.
[24,25].
Nd-YAG lasers are bulky and require special
electrical and plumbing connections if water is
used as a coolant, but they have a higher tissue
penetration and are useful for ablation of large
exophytic tumours.
Semiconductor diode lasers are small and
portable and do not require special electrical
or plumbing facilities. In non-contact mode
there is good absorption of diode laser produc-
ing more localised vaporisation than with
Nd-YAG although penetration is less because
of its lower wavelength. It can be used in
continuous mode or pulsed application.
APC results in better visualisation because
of low volume of smoke, but depth of pene-
tration is limited. It can be used in pulses or
continuously for coagulating large bleeding
surfaces but is less e#ective in debulking large
tumours [21]. This is less likely to be a problem
since the volume of tumour blocking a stent is
typically quite small.
Our studies confirmed that for a given
power setting the diode laser was tolerated
for longer periods than Nd-YAG laser before
stent damage occurred. When di#erent
stents are compared, Esophagocoil is more
resilient and the open mesh stent the most
easily damaged. The relative vulnerability of
the five varieties of stent was the same for both
laser modalities. (Esophagocoil>Gianturco>
covered mesh stent>open mesh stent/wall-
It is apparent that individual stents have
di#erent abilities to withstand radial forces
[26,27]. It follows, therefore, that weakening
of the stent caused by heat damage from the
laser while clearing obstructing tumour may
be an additional fact diminishing the capacity
of the stent to resist compression and ingress
by encasing tumour. There is, to date, little
information to indicate which stent is most
suitable for a particular patient or type of
tumour.
Availability of di#erent types of lasers
depends largely on their cost. Lasers are
expensive (Nd-YAG laser about £45 000–
60 000; GaAlAs diode laser about £45 000) and
may be available only in special centres. APC
is relatively cheap (£15 000) and hence more
readily available. Even though its use in pri-
mary ablation therapy may be limited, it
undoubtedly has a place in the treatment of
stent overgrowth [28].
Our preliminary studies have shown APC to
be safe when used with stents. This in vitro
study has shown that Nd-YAG laser is likely
(in higher power settings) to destroy the metal
stent and this is true with diode laser but to a
lesser extent. However, perhaps it is worth
pointing out that in clinical practice, the diode
laser is unlikely to cause any significant
damage [11].

256
N. Rajendran and N. Krasner
ACKNOWLEDGEMENTS
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4. Hochberger J, May A, Bauer R et al. Laser, monopolar,
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tumour ingrowth or overgrowth of coated and non-
coated metal endoprosthesis. Gastrointest Endosc
The authors wish to thank ERBE, Tubingen for the use of
the APC and Lasers for Life for its support of the study.
1
995;41:304.
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