Vascular
Proc 9th Int Meeting Leksell Gamma KnifeÒ Society,
Hong Kong, November 1998
Stereotact Funct Neurosurg 1999;72(suppl 1):168±174

A Case of Very Large Cyst Formation with
Gamma Knife Radiosurgery for an
Arteriovenous Malformation
M.S Kim, S.I. Lee, J.H. Sim
Department of Neurological Surgery, Inje University, Pusan Paik Hospital,
Pusan, Korea

Key Words

Arteriovenous malformation ´ Stereotactic radiosurgery ´ Cyst ´
Ommaya reservoir ´ Gamma Knife ´ Complications

Abstract

A 17-year-old male patient underwent Gamma Knife radiosurgery
(GKRS) for a left parietal arteriovenous malformation (AVM), which presented with hemorrhage. The 15.0 cm3 nidus was covered with the 50%
isodose. The maximum dose was 50 Gy and the margin dose was
25 Gy. Eleven months later he developed a right hemiparesis and MRI
showed a large cyst. Cerebral angiography showed partial obliteration
of the AVM nidus. Stereotactic removal of cyst fluid (about 70 cm3) was
performed, and an Ommaya reservoir was inserted. Cyst formation
after GKRS for cerebral arteriovenous malformation is a is side effect of
radiosurgery about which we need to learn more.
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Moo Seong Kim, MD
Neurosurgical Department, Inje University, Pusan Paik
Hospital, 633-165 KaeKeum Dong Jin-Ku, Pusan (Korea)
Tel. +82 51 890 6085, 6874; Fax +82 51 893 9368, 898 4244
E-Mail nsmsk@ijnc.inje.ac.kr; KIMMS@mediKorea.net

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Stereotactic Bragg peak proton beam irradiation of arteriovenous malformations (AVMs) was begun in the 1960s by Kjellberg and associates [1].
Over 5,000 cases of AVM have been treated with stereotactic radiosurgery
and a favorable outcome has been obtained in most cases. It takes 2±3 years
for complete angiographic obliteration to occur. The treatment is uncom-

monly associated with complications, including brain swelling, cerebral
ischemia, radionecrosis and demyelination. Delayed cyst formation after
Gamma Knife radiosurgery (GKRS) for cerebral arteriovenous malformation is very rare, with about 10 cases reported in the literature [2±4]. The
mechanism underlying delayed cyst formation after GKRS for cerebral arteriovenous malformation is obscure.
From October 1994 to October 1998, 82 patients with AVM were treated by the authors with GKRS. In one of these cases, a 17-year-old male patient, the treatment was associated with an intracerebral hematoma. Eleven
months later, a huge cyst developed. Stereotactic cyst aspiration was performed and an Ommaya reservoir was inserted.

Case Report
A 16-year-old male was admitted in September 1996, with an acute onset right hemiparesis and dysphasia. A computerized tomographic scan revealed a large hematoma in
the left parietal region. A cerebral angiogram showed a left-sided AVM fed by the left
middle cerebral artery. The diameter of the nidus was about 5 cm (fig. 1).
The patient (now ages 17) underwent GKRS in February 1997. The maximum dose
was 50 Gy, and the margin dose was 25 Gy. The AVM volume was 15.01 cm3. At follow-up
in January 1998, he had developed a progressive right hemiparesis. MRI showed a huge
cyst associated with the left parietal AVM. Angiography showed approximately 80% reduction in the size of the AVM nidus (fig. 2).
In January, 1998, stereotactic cyst aspiration was performed and 60 cm3 of xanthochromic fluid was aspirated. An Ommaya reservoir was inserted. The cytology of the cystic
fluid was negative for tumor cells. After surgery the symptoms improved and CT showed
that the cyst had disappeared.

Discussion

Very Large Cyst Formation with GKRS for an
Arteriovenous Malformation

Stereotact Funct Neurosurg
1999;72(suppl 1):168±174

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The progressive occlusion of an AVM after GKRS usually takes from 1
to 3 years. The mechanism of the occlusion of AVM is thought to be radiation induced endothelial proliferation, collagen deposition, progressive endothelial hypertrophy and finally luminal closure [5, 6]. Pollock et al. [7] reported 64% complete angiographic obliteration, 28% decreased nidus, and
seven no change in cases. They observed a 4.8% annual risk of hemorrhage
during the first 2 years after GKRS. Steiner et al. [8] reported 35 cases
(3.5%) where delayed complications, such as headache and radiation necrosis, occurred during a 4±56 months follow-up (mean 9.9 months) in 1,000
cases of AVMs. Lunsford et al. [9] reported a complete obliteration rate at

a

b

c

d
Fig. 1. a, b CT scan and MRI revealed a large hematoma with on AVM in the left frontotemporal. c, d Four-vessel angiography disclosed an AVM at the location of the hematoma.

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2 years, which varied according to lesion volume; for 1 cm3 100%, 1±4 cm3
85%, and 4±10 cm3 58%. Moreover, he reported a 4.4% complication rate.
Lindquist [10] observed three AVM patients with delayed cyst formation
who remained asymptomatic. Forster [3] also observed three AVM patients
who developed asymptomatic cysts. One of these underwent surgery to rule

a

b

c

d
Fig. 2. a, b Sequential T1-weighted MR images obtained 11 months after radiosurgery
showing an enlarging hypointense cyst. c, d Angiogram obtained 11 months after radiosurgery showing partial obliteration of the AVM nidus.

Very Large Cyst Formation with GKRS for an
Arteriovenous Malformation

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out the possibility of neoplastic changes. However, the pathological study
disclosed nothing of note other than gliosis and a spongiform changes.
Tanaka et al. [2] reported two cases of cyst formation after GKRS for
AVM. A 26-year-old male had microsurgery for an AVM with a left frontal

a

b
Fig. 3. a MRI showed an enlarged huge cyst. b Two Ommaya reservoirs inserted in the
cyst.

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intracerebral hematoma. GKRS was performed for a residual AVM, the diameter of which was 20 mm. The maximum dose was 34 Gy and the margin
dose was 16 Gy. At 15 months after the treatment, the AVM had completely
disappeared. However, about 3 years later, the patient developed a headache and MRI showed a large cyst in the irradiated region. A biopsy was taken and a cystoperitoneal shunt was inserted. Histological examination
showed gliosis. The other case reported by this group concerned an 11-yearold boy. Ventricular drainage had been performed following a left thalamic
and intraventricular hemorrhage. The diameter of the nidus was 23 mm. At
GKRS, the maximum dose was 44.5 Gy and the margin dose was 20 Gy.
After 24 months the AVM was obliterated. About 1 year later, the patient
developed a headache and a tremor in his right hand. The MRI showed a
large cyst. A biopsy was taken and an Ommaya reservoir was inserted. Histological examination showed gliosis.
Hara et al. [11] reported two symptomatic patients. The location of each
of the cysts corresponded to the location of an initial hematoma. Surgical intervention was undertaken and histological examination of the nidus showed
the majority of vessels were obliterated and hyalinized. Some thick-walled

recanalizing vessels and/or patent blood vessels were observed within these
obstructed vessels.
Yamamoto et al. [12, 13] described AVMs where late cysts developed.
One was a 25-year-old person. The region which had received a high radiation dose became surrounded by edema at the 79th month after radiosurgery. The other cyst in this material was observed at autopsy. Follow-up angiogram had shown complete nidus obliteration. Histological examination of
the resected nidus showed that many of the AVM vessels were occluded
due to intimal hypertrophy. Nonetheless, there were still patent vascular
channels in the nidus, which were not observed angiographically.
The mechanism of delayed cyst formation remains obscure. Four different mechanisms are cited for the formation of cysts associated with brain tumors [14]:
(1) An intratumoral cyst may be due to degenerative changes of the
neoplastic tissue; as seen in astrocytomas and hemangioblastomas.
(2) A peritumoral cyst may arise because of a block in CSF circulation,
as occurs with meningiomas.
(3) A cyst with a wall composed of glial or connective tissue may be
caused by a glial reaction.
(4) A cyst may be formed by intratumoral or intracerebral hemorrhage.

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Arteriovenous Malformation

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These same mechanisms may occur with an AVM. It is thought that
there may have been three different mechanisms leading to cyst formation
in presence of an AVM. These are hemorrhage, exudation or a breakdown
of the blood-brain barrier. Once hemorrhage occurs, however, it may produce a pseudo-aneurysm with an extremely thin wall. Then serum may be
exuded from its wall. Furthermore, relatively high blood flow values and increased permeability through incompletely occluded blood vessel walls within the treated nidus, whether angiographically visible or not, may enhance
cyst formation within the region. Radiosurgically-induced degeneration, coagulation necrosis to liquefaction necrosis continues for several years after
radiosurgery [14]. Furthermore, the location of the initial hematoma may facilitate the accumulation of cystic fluid due to increased permeability
through the residual nidus following treatment. An autopsy report from an
AVM patient who died from an unrelated cause 27 months after treatment
was reported by Fujino et al. [15]. This report disclosed exudation of albuminous fluid within the treated nidus.
Cyst formation after GKRS for cerebral AVM can occur and long-term
MRI follow-up studies should be performed.

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