Topical application of b-radiation to reduce intimal hyperplasia after
carotid artery balloon injury in rabbit
A possible application for brachytherapy in vascular surgery
David Rosenthala,*, Scott L. Stevensb, C.S. Skillernb, Eric D. Wellonsa, Keith Robinsonc,John H. Matsuuraa, Brian J. Gannond
aDepartment of Vascular Surgery, Atlanta Medical Center, 315 Boulevard NE, Suite 412, Atlanta, GA 30312, USAbSchool of Medicine, Department of Surgery, University of Tennessee, Nashville, TN, USA
cAtlanta Cardiovascular Research Institute, Norcross, GA, USAdPerkinElmer Life Sciences, Boston, MA, USA
Received 01 April 2002; received in revised form 01 June 2002; accepted 01 July 2002
Abstract Purpose: Endovascular brachytherapy for the prevention of intimal hyperplasia (IH) and restenosis
after balloon/stent angioplasty has proven effective both in animal preparations and clinical trials. A
variety of b-emitting isotopes and catheter-based devices have been developed for the delivery of
low-dose radiation in clinical coronary and peripheral trials. No platform, however, has yet been
developed for brachytherapy in concert with vascular surgical operations. The purpose of this study
was to evaluate the vascular histopathologic response following balloon injury to rabbit carotid
arteries with and without topically applied low-dose b-radiation.Methods: The b-emitting isotope strontium-90 (Sr-90) was conjugated onto the matrix of
polypropylene (PLYP) mesh. Rabbit carotid arteries were balloon-injured with a #2 embolectomy
catheter. Six carotid arteries were wrapped with nonradioactive PLYP mesh (controls) and Sr-90
(� 90 mCi) PLYP mesh in order to deliver low-dose radiation to the vessel wall from the external
(adventitial) surface. Tissue was harvested at 6 weeks and processed for histologic examination.
Results: There was consistent blockade of fibrocellular neointima formation with virtually no
neointima present in all treated segments, compared to moderate neointima formation in
controls. Medial thinning and smooth muscle cell (SMC) necrosis were also associated with
topical brachytherapy.
Conclusion: b-Radiation applied by an externally wrapped PLYP mesh labeled with Sr-90
markedly suppressed neointima formation in an animal vascular surgical injury model. Further
studies, however, are necessary to determine a suitable isotope and dosage for clinical
application. D 2002 Elsevier Science Inc. All rights reserved.
Keywords: b-radiation brachytherapy topical application in vascular surgery
1. Introduction
Human and experimental studies have demonstrated that
intimal hyperplasia (IH) and vascular remodeling are the
principal factors leading to restenosis after arterial catheter-
based and endovascular surgical intervention [1–4]. The
roles of the medial smooth muscle cells (SMCs) and
adventitial fibroblasts after vascular injury is well estab-
lished, and the proliferation and migration of the medial
SMCs to the luminal space is critical in the formation of
neointima. Radiation therapy has been shown to reduce
NIH in animal models and arterial restenosis after clinical
catheter-based intervention. The mechanism of this inhib-
itory effect is thought to be one of chromosomal damage
to the SMCs, creating mitotic cell death when these cells
1522-1865/02/$ – see front matter D 2002 Elsevier Science Inc. All rights reserved.
PII: S1522 -1865 (02 )00137 -3
* Corresponding author. Tel.: +1-404-524-0095; fax: +1-404-658-
9558.
E-mail address: [email protected] (D. Rosenthal).
Cardiovascular Radiation Medicine 3 (2002) 16–19
are stimulated to divide, as a consequence of vessel wall
medial injury.
Brachytherapy, or the placement of a radioactive source
on tissue surfaces, represents an ideal means of delivering a
low dose of radiation directly to the target vessel [5,6].
Means to deliver ionizing radiation from luminally posi-
tioned catheter based systems have been developed and
found effective in both animal preparations and human
clinical trials, and three systems are currently FDA-
approved and commercially marketed for coronary artery
endovascular brachytherapy. However, no system for the
‘‘extravascular’’ topical application of low-dose radiation
has been developed, but such a device would be a useful
adjunct at the time of cardiovascular operations.
We developed a prototype system to deliver b-radiationfrom the adventitial surface of an artery by conjugating
strontium-90 (Sr-90) onto a PLYP mesh substrate in order to
‘‘wrap’’ the vessel in a radiation blanket. The purpose of this
study was to evaluate the vascular histopathologic response
following balloon injury to the rabbit carotid artery, with
and without topical low-dose b-radiation.
2. Materials and methods
The animal work was approved by the Animal Care and
Use Committee of University of Tennessee College of
Medicine. All animal care was in accordance with the
‘‘Guide for the Care and Use of Laboratory Animals’’
(Office of Science and Health Reports CPRR/NIH 1996).
Six New Zealand white rabbits weighing between � 2.5 and
3.5 kg each were subjected to overstretch balloon injury of
one carotid artery by a #2 embolectomy catheter. The
animals were sedated with a combination of ketamine
hydrochloride (35 mg/kg) and xylazine (7 mg/kg). After
intravenous administration of � 1000 units of heparin, the
embolectomy catheter was introduced via the external
carotid artery and advanced retrograde into the common
carotid artery. The balloon was inflated and withdrawn to
the carotid bifurcation three times, after which the external
carotid was ligated.
PLYP mesh (Johnson and Johnson, Summerville, NJ)
was wrapped around a � 1-cm length of the injured artery
(control), while the low-dose Sr-90-labeled mesh (test) was
wrapped around an adjacent � 1-cm segment; mesh wraps
were secured with 5–0 PLYP suture. The proximal and
distal control versus test mesh sites was alternated with each
animal and the wounds closed in a standard fashion. All
animals were allowed to recover and returned to routine
care. Animals were maintained on a normal laboratory diet
and were euthanized 6 weeks after operation.
Postmortem, the arteries were isolated and flushed with
heparinized saline followed by 10% neutral buffered form-
alin. Segments of proximal and distal artery wrapping were
cut and dehydrated in graded ethanol series, exchanged with
xylene and embedded in paraffin. Sections � 5–6-mm-thick
were cut on a rotary microtome and collected on glass
slides. Adjacent sections were stained with hematoxylin and
eosin (H&E) and Verheoff-Masson elastin-trichrome to
evaluate general tissue and cell morphology. An experi-
enced observer unaware of treatment group assignment
performed histopathologic analysis and semiquantitative
scoring (0 = none, 1 =mild, 2 =moderate, 3 = severe) of each
segment for: extent of balloon-induced vascular injury,
neointima formation, thrombus, inflammation and media
necrosis was performed on each segment. Mann–Whitney
rank sum tests were used to compare irradiated to control
segments for these specific histopathologic criteria, and a
critical value of P < .05 was considered to indicate a
significant between-groups difference.
3. Results
Six animals underwent carotid artery injury. One animal
was found dead 1 day after surgery and another died at
2 weeks. The causes of death were not established. The
arteries from these rabbits were harvested and examined, but
were not included in the semiquantitative histopathologic
scoring. Both vessels contained both postmortem thrombus
and varying degrees of platelet-rich, antemortem thrombus;
in the animal found dead at 2 weeks, this showed partial
organization and IH was also present. In the animal found
dead at 1 day, the vessel segment that had been wrapped with
Sr-90-conjugated mesh displayed extensive media necrosis.
The appearances of vessel segments harvested 6 weeks
after injury and wrapping showed a distinctly bimodal
histomorphologic pattern. Artery segments, which had been
Fig. 1. Light microscopy of rabbit carotid arteries fixed 6 weeks after
balloon injury. (a, b) Wrapped with polypropylene mesh only (control);
(c, d) wrapped with polypropylene mesh incorporating Sr-90 � 90 mCi/cm2
(irradiated). (a, c) Verheoff-Masson stain, 40� ; (b, d) H&E stain, 200� .
Control segment shows meointima formation typical of this animal
preparation; irradiated vessel shows virtually no fibrocellular neointima,
but necrosis in deeper layers of the attenuated tunica media.
D. Rosenthal et al. / Cardiovascular Radiation Medicine 3 (2002) 16–19 17
wrapped with radioactive mesh, showed virtually no fibro-
cellular neointima formation and often displayed substantial
media necrosis (Fig. 1a and b). Control-wrapped segments
had no necrosis but had considerable neointima formation
consisting of stellate, round and spindle-shaped cells in an
abundant mixed extracellular matrix (Fig. 1c and d). Occa-
sionally, thrombus was present in the lumens of either
irradiated or control-wrapped segments; in the case of one
control segment, this showed partial organization by fibro-
cellular tissue of composition similar to proliferative neo-
intima. Diffuse granulocytic inflammatory infiltrates were
present in the adventitia and perivascular space of all vessel
segments and did not appear greater in irradiated samples
than controls; occasionally, more focal and intense infiltrates
were seen.
Semiquantitative scoring of vessel segments for histopa-
thologic criteria was performed with the observer unaware
of treatment group assignment (Table 1). The code was then
broken and the scores grouped, and between-groups com-
parisons made using Mann–Whitney rank sum tests. There
were no significant differences between sham-treated and
irradiated segments for vascular injury (P= .69), thrombus
(P= .88) or inflammation (P= .34). However, the irradiated
segments showed significantly reduced neointima formation
(P= .03) and a trend towards increased necrosis (P= .11).
4. Discussion
The problem of NIH, and hence arterial restenosis, is a
major clinical problem for cardiovascular surgeons. Informa-
tion from HCFA demonstrates that over 700,000 cardiovas-
cular operations were performed on Medicare-aged patients
in the United States in 1998 [7] and reported restenosis rates
from these operations approach, at a minimum 15% at 2 years
[8–10]. It is easy to comprehend, therefore, the enormity of
this problem in terms of patient suffering and healthcare
costs. Worldwide, these figures quadruple.
This study was performed to evaluate the use of topically
applied b-radiation to balloon injured rabbit carotid arteries
in an attempt to reduce IH. A b-emitting source was chosen
because it is less penetrating than gamma radiation to
surrounding tissues, requires less exposure to achieve the
desired affect, has an appropriate shelf-life and has minimal
handling risks for operating room personnel; indeed, the
only protection necessary for operating room personnel is
latex gloves.
The histologic analysis in this study demonstrated a
marked reduction in neointima formation in the irradiated
artery segments. There was virtual abrogation of neointima
formation in irradiated vessel segments without signific-
antly increased thrombus or inflammatory response. These
results are promising; however, the trend towards increased
necrosis in the tunica media suggests possible adverse
complications related to aneurysm or perforation with this
dose and isotope.
Despite the beneficial inhibitory effect on neointima
formation, Sr-90 with its half-life of 28 years, probable
associated need for radiation protection (despite relatively
low activity) and its nature as essentially a permanent
implant, could not be recommended as a desirable isotope
for this topical application.
This study demonstrates that topically placed b-emitting
source can decrease IH. Further studies are ongoing which
will aid in defining a more suitable isotope and the dosimetry
necessary to inhibit IH without excessive necrosis. If topical
brachytherapy proves efficacious, this simple and safe means
of preventing IH and, thereby, arterial restenosis will be a
significant adjunct for cardiovascular surgical patients.
5. Study limitations
This study was an initial pilot experiment to assess the
feasibility and histopathologic effects of topical b-radiationin an animal preparation of neointimal hyperplasia, and was
not powered to enable standard histomorphometric evalu-
ation of neointima formation and other vascular changes in
response to injury in the presence and absence of irra-
diation. Therefore, only a semiquantitative evaluation of
vascular morphology was performed, with between-groups
comparisons made using nonparametric tests appropriate for
such a study design and evaluation technique. Additionally,
in this pilot study, no dosimetry was undertaken, so at
present, we have no data to indicate the radiation dose
required to elicit an inhibitory effect on neointima formation
using this approach.
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Semiquantitative scoring of specific histopathologic criteria in mesh-
wrapped balloon-injured rabbit carotid arteries at 6 weeks
Rabbit Segment Group VI NF Thromb Inflam Necr
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10 dist rad 0 0 0 0 0
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