AJR 2001; 176:1589-1595
© American Roentgen Ray Society
MR Imaging Findings After Stereotactic Radiosurgery Using the Gamma Knife
David P. Friedman1,
Robert E. Morales1 and
H. Warren Goldman2,3
1
Department of Radiology, Jefferson Medical College and Thomas Jefferson
University Hospital, 132 S. 10th St., Ste. 1072, Main Bldg., Philadelphia, PA
19107.
2
Department of Neurosurgery, Neurosensory Institute—Wills Eye Hospital,
Jefferson Medical College and Thomas Jefferson University Hospital,
Philadelphia, PA 19107.
3
Present address: Department of Neurosurgery, MCP-Hahnemann University, 245 N.
15th St., Philadelphia, PA 19102.
Received September 29, 2000;
accepted after revision November 8, 2000.
Address correspondence to D. P. Friedman.
Introduction
The gamma knife is a radiosurgical tool developed to perform closed-skull
destruction of a stereotactically defined target. In modern units, 201
cobalt-60 sources are distributed over the surface of a sphere, and the
geometric focus of the emitted gamma rays is at the sphere's center. Using a
variety of helmets, collimators, and plugs, the gamma rays are further focused
to a specific target. While wearing the helmet, the patient's head is
positioned within the unit so that the center of the intracranial target and
the center of the spherical array of sources coincide. The treatment dose is
determined by the duration of exposure to the radioactive sources.
A variety of intracranial diseases are treated using the gamma knife; these
include movement disorders, trigeminal neuralgia, neoplasms, and arteriovenous
malformations. MR imaging is invaluable in studying the results of therapy,
because the pathologic data is limited in this minimally invasive technique.
Examinations can be performed to verify lesion placement, determine the
effects of the radiation dose on the targeted tissue, and detect
treatment-related complications.
Stereotactic Functional Radiosurgery
The use of the gamma knife in stereotactic functional neurosurgery differs
from the treatment of intracranial neoplasms and arteriovenous malformations,
because the radiosurgical lesion is produced in macroscopically normal tissue.
As a result, the effects of radiation may be more predictable.
Movement Disorders
For the radiosurgical treatment of tremor (e.g., benign essential tremor,
Parkinson's disease), the target is the ventralis intermedius thalamic
nucleus, which is a subnucleus of the ventral lateral nucleus. The ventral
lateral nucleus receives projections from the contralateral cerebellum via the
brachium conjunctivum. The ventralis intermedius nucleus is only 3-4 mm in
anteroposterior thickness. These thalamic nuclei cannot be definitively
identified on the basis of MR imaging characteristics. Hence, localization of
these structures with stereotactic techniques is critical for optimal
placement of the radiosurgical lesion. Stereotactic coordinates are calculated
relative to a line connecting the anterior and posterior commissures. The
target is 7-8 mm anterior to the posterior commissure along the
intercommissural line, and 11-13 mm lateral and 2 mm superior to the
intercommissural line. During a radiosurgical thalamotomy, a single dose of
120-140 Gy is administered to the target during a 45-60 min interval. In our
experience, therapeutic effect usually begins approximately 4 weeks after
treatment. At 3 months posttreatment, the radiosurgical lesion most commonly
appears as a ring-enhancing focus 5 mm or less in diameter surrounded by
vasogenic edema extending less than 7 mm in radius beyond the target (Figs.
1 and
2). Patients with
ring-enhancing lesions 7 mm or more in diameter at 3 months after treatment
may have already developed, or should be considered at risk for developing,
more extensive radiation necrosis and edema
[1] (Fig.
3A,3B).
The development of symptomatic radiation necrosis appears to be an
idiosyncratic event. In our experience, it has occurred in approximately 20%
of patients and it is usually treated successfully with corticosteroids.
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Fig. 1. —Appearance of uncomplicated radiosurgical thalamotomy in
80-year-old man with essential tremor who underwent single radiosurgical dose
of 140 Gy to left thalamus. Axial T2-weighted fast spin-echo MR image
(4000/95, TR/TE) obtained 3 months after thalamotomy shows small focus of
hyperintensity (edema, arrow) in expected location of left ventralis
intermedius thalamic nucleus.
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Fig. 2. —Appearance of uncomplicated radiosurgical thalamotomy in
71-year-old woman with Parkinson's disease who underwent single radiosurgical
dose of 120 Gy to right thalamus. Enhanced axial T1-weighted MR image (500/16)
obtained 3 months after thalamotomy shows 5-mm focus of ring enhancement
(arrow) in expected location of right ventralis intermedius thalamic
nucleus.
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Fig. 3A. —Appearance of complicated radiosurgical thalamotomy in
78-year-old man with essential tremor who underwent single radiosurgical dose
of 140 Gy to left thalamus. Enhanced axial T1-weighted MR image (500/16,
TR/TE) obtained 3 months after thalamotomy shows 10-mm focus of ring
enhancement (arrow) in expected location of left ventralis
intermedius thalamic nucleus.
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Fig. 3B. —Appearance of complicated radiosurgical thalamotomy in
78-year-old man with essential tremor who underwent single radiosurgical dose
of 140 Gy to left thalamus. Axial T2-weighted MR image (2400/80) shows
radiosurgical lesion in left thalamus (arrow) surrounded by more
extensive hyperintensity (edema, asterisks). Patient developed right
hemiparesis that partially resolved after corticosteroidal treatment.
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Trigeminal Neuralgia
For the radiosurgical treatment of trigeminal neuralgia, the target can be
either the proximal root entry zone of the trigeminal nerve (Fig.
4A,4B,4C)
or the distal (retrogasserian) portion
(Fig. 5). In theory, the
portion of the nerve closer to the brainstem is myelinated by oligodendrocytes
and therefore should be more sensitive to irradiation than the retrogasserian
portion of the nerve, which is myelinated by Schwann's cells
[2]. The proximal root entry
zone is irradiated such that no more than 20% of the 50% isodose curve
includes the brainstem. However, recent data suggest that the retrogasserian
portion of the trigeminal nerve is also an adequate target. Because the 20%
isodose curve is at the brainstem surface when this target is used, higher
doses (90 Gy, as compared with 70-80 Gy for the proximal root entry zone) can
be given to the nerve, resulting in less exposure to the brainstem
[3]. Onset of therapeutic
effect begins from 3 weeks to 3 months after treatment. In our experience, no
changes were identified in the trigeminal nerve or brainstem at 3-6 months
posttreatment except in two patients with multiple sclerosis (a known cause of
trigeminal neuralgia) (Figs.
4A,4B,4C
and 5).
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Fig. 4A. —Radiosurgical treatment of root entry zone of trigeminal
nerve in 56-year-old woman with left trigeminal neuralgia resulting from
multiple sclerosis. Axial fluid-attenuated inversion recovery MR image
(10002/142, TR/TE) shows hyperintensity (edema) in left pons (straight
arrow). Left trigeminal nerve (curved arrow) is incompletely
seen on image.
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Fig. 4B. —Radiosurgical treatment of root entry zone of trigeminal
nerve in 56-year-old woman with left trigeminal neuralgia resulting from
multiple sclerosis. Enhanced axial T1-weighted MR image (616/13) shows
abnormal enhancement in root entry zone of left trigeminal nerve (straight
arrow) and adjacent pons (curved arrow).
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Fig. 4C. —Radiosurgical treatment of root entry zone of trigeminal
nerve in 56-year-old woman with left trigeminal neuralgia resulting from
multiple sclerosis. Enhanced coronal T1-weighted MR image (550/13) shows
abnormal enhancement in left trigeminal nerve (thin straight arrow)
and adjacent pons (thick straight arrow). Treated nerve is enlarged
as compared with right trigeminal nerve (curved arrow). Asterisk
denotes hyperintense fatty marrow in skull base.
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Fig. 5. —56-year-old woman with left trigeminal neuralgia caused by
multiple sclerosis who underwent radiosurgical treatment of retrogasserian
portion of trigeminal nerve. Enhanced axial T1-weighted MR image (400/15,
TR/TE) shows abnormal enhancement in retrogasserian portion of left trigeminal
nerve (straight arrow). Enhancement indicates radiation-induced
breakdown of blood—nerve barrier. Chronic plaques are identified in
right pons (curved arrow). Asterisk denotes cerebrospinal fluid in
Meckel's cavity.
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Stereotactic Radiosurgery for Neoplasms and Arteriovenous
Malformation
Neoplasms
The effects of exposing neoplastic tissue to a high dose of focal radiation
are likely to include apoptosis, cell necrosis, and vascular obliteration.
Depending on the type of tumor and the clinical circumstances, radiation
treatment may be either fractionated or administered as a single dose. The
sequential changes identified on MR imaging appear to fall into a few major
categories: temporary exacerbation of the lesion, central loss of contrast
enhancement, growth arrest, and regression or obliteration
[4].
On initial follow-up imaging of a treated neoplasm, the lesion may show an
increase in size, volume of enhancement, and surrounding T2 hyperintense
signal (Fig.
6A,6B,6C,6D,6E).
These changes should not necessarily be interpreted as signs of tumor
progression. The increased T2 signal likely represents breakdown of the
blood—brain barrier and vasogenic edema, although experiments in
primates have shown that some of this increased signal represents reactive
astrocytosis [4]. These changes
tend to develop 3-12 months after treatment and subside within 5-7 months
after onset; they may uncommonly result in permanent deficits. Treated
neoplasms may also show loss of central enhancement
[5,
6] (Figs.
7A,7B
and
8A,8B,8C),
which likely reflects apoptosis, cell necrosis, or vascular obliteration. This
appearance occurs earlier in malignant neoplasms (8-12 weeks) than in benign
neoplasms (5-18 months) and is associated with tumor regression within the
next 12 months in more than 70% of patients
[4,
6].
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Fig. 6A. —Evolution of radiosurgical changes after treatment for benign
neoplasm in 52-year-old woman who underwent single radiosurgical dose of 15 Gy
to presumed falcine meningioma. Axial T2-weighted MR image (2400/90, TR/TE)
obtained before treatment shows hypointense, dural-based mass arising from
left posterior parietal falx (arrow); no surrounding edema is
seen.
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Fig. 6B. —Evolution of radiosurgical changes after treatment for benign
neoplasm in 52-year-old woman who underwent single radiosurgical dose of 15 Gy
to presumed falcine meningioma. Enhanced axial T1-weighted MR image (400/14)
also obtained before treatment shows uniform enhancement of mass
(arrow).
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Fig. 6C. —Evolution of radiosurgical changes after treatment for being
neoplasm in 52-year-old woman who underwent single radiosurgical dose of 15 Gy
to presumed falcine meningioma. Axial T2-weighted MR image (2400/90) obtained
9 months after treatment shows new hyperintensity in mass (arrow) and
vasogenic edema in adjacent periatrial and subcortical white matter
(asterisks).
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Fig. 6D. —Evolution of radiosurgical changes after treatment for benign
neoplasm in 52-year-old woman who underwent single radiosurgical dose of 15 Gy
to presumed falcine meningioma. Enhanced axial T1-weighted MR image (500/16)
also obtained 9 months after treatment shows increase in volume of enhancing
neoplasm (asterisk) and surrounding hypointense edema
(arrows).
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Fig. 6E. —Evolution of radiosurgical changes after treatment for benign
neoplasm in 52-year-old woman who underwent single radiosurgical dose of 15 Gy
to presumed falcine meningioma. Enhanced axial T1-weighted MR image (500/16)
obtained 13 months after treatment shows substantial reduction in volume of
neoplasm (arrow). Lesion has not yet returned to pretreatment size.
Corresponding T2-weighted MR image (not shown) revealed almost complete
resolution of surrounding edema.
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Fig. 7A. —Decreased central contrast enhancement after radiosurgical
treatment of benign neoplasm in 68-year-old man who underwent single
radiosurgical dose of 14 Gy to presumed right vestibular schwannoma. Enhanced
axial T1-weighted MR image (500/14, TR/TE) obtained before treatment shows
enhancing right cerebellopontine angle mass (arrow) extending into
right internal auditory canal (asterisk).
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Fig. 7B. —Decreased central contrast enhancement after radiosurgical
treatment of benign neoplasm in 68-year-old man who underwent single
radiosurgical dose of 14 Gy to presumed right vestibular schwannoma. Enhanced
axial T1-weighted MR image (400/14) obtained 7 months after treatment shows
markedly decreased central enhancement of mass (arrow).
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Fig. 8A. —Evolution of radiosurgical changes after treatment of primary
malignant neoplasm in 46-year-old man who underwent single radiosurgical boost
dose of 16 Gy to surgically proven glioblastoma multiforme. Enhanced axial
T1-weighted MR image (500/14, TR/TE) obtained before administration of boost
radiation shows enhancing mass in left posterior temporal lobe
(arrow).
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Fig. 8B. —Evolution of radiosurgical changes after treatment of primary
malignant neoplasm in 46-year-old man who underwent single radiosurgical boost
dose of 16 Gy to surgically proven glioblastoma multiforme. Enhanced axial
T1-weighted MR image (500/8) obtained 1 month after treatment shows increase
in volume of neoplasm and markedly decreased central enhancement
(arrow); this decrease in enhancement typically occurs much more
rapidly in malignant neoplasms than in benign neoplasms (compare with
Fig. 7B).
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Fig. 8C. —Evolution of radiosurgical changes after treatment of primary
malignant neoplasm in 46-year-old man who underwent single radiosurgical boost
dose of 16 Gy to surgically proven glioblastoma multiforme. Enhanced axial
T1-weighted MR image (400/8) obtained 4 months after treatment shows decrease
in volume of neoplasm (straight arrow). New focus of enhancing tumor
(curved arrow) is seen anteromedial to treated component.
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Successful treatment with the gamma knife may result in growth arrest,
regression, or obliteration of the neoplastic lesion. Growth arrest has been
reported in approximately 90% of benign neoplasms at the skull base and 85% of
solitary metastases [4]. Tumor
regression is uncommonly seen earlier than 3 months posttreatment and may take
years to fully evolve. For example, the median time for regression of
vestibular schwannomas is approximately 1 year, with a range of 3-33 months
[5,
7] (Fig.
9A,9B).
Tumor regression occurs more rapidly in malignant neoplasms (Fig.
10A,10B).
Fibroblasts and myofibroblasts aid in tumor retraction.
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Fig. 9A. —Late radiosurgical changes after treatment of benign neoplasm
in 75-year-old woman who underwent single radiosurgical dose of 12 Gy to
surgically proven right vestibular schwannoma. Enhanced coronal T1-weighted MR
image (400/8, TR/TE) obtained before treatment shows enhancing, partially
cystic mass in right cerebellopontine angle extending into right internal
auditory canal (large arrow). Diffuse pachymeningeal enhancement
(small arrows) is related to previous surgery.
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Fig. 9B. —Late radiosurgical changes after treatment of benign neoplasm
in 75-year-old woman who underwent single radiosurgical dose of 12 Gy to
surgically proven right vestibular schwannoma. Enhanced coronal T1-weighted MR
image (400/14) obtained 24 months after treatment shows substantial decrease
in volume of neoplasm and retraction of cystic component (arrow).
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Fig. 10A. —Early radiosurgical changes after treatment of secondary
malignant neoplasm in 72-year-old woman with lung carcinoma who underwent
single radiosurgical boost dose of 18 Gy to presumed cerebral metastasis.
Enhanced axial T1-weighted MR image (500/16, TR/TE) obtained before
administration of boost radiation shows heterogenously enhancing mass in left
posterior frontal lobe (arrow).
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Fig. 10B. —Early radiosurgical changes after treatment of secondary
malignant neoplasm in 72-year-old woman with lung carcinoma who underwent
single radiosurgical boost dose of 18 Gy to presumed cerebral metastasis.
Enhanced axial T1-weighted MR image (500/16) obtained 4 months after treatment
shows reduction in volume of mass.
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Arteriovenous Malformations
Stereotactic radiosurgery is an effective treatment strategy for small
arteriovenous malformations, and it can be the preferred treatment for
high-risk lesions located in the basal ganglia, thalamus, and brainstem. After
successful radiosurgery, injury to the endothelium and intervening brain
tissue causes the transnidal blood flow to gradually decrease until the
arteriovenous malformation is completely obliterated. MR imaging and MR
angiography are accurate methods to follow up regression of the nidus, as well
as to show the appearance of complications such as venous hypertension and
radiation-induced vasogenic edema, demyelination, or gliosis
[8] (Figs.
11A,11B,11C,11D
and
12A,12B,12C,12D).
Most patients have a transient period before the arteriovenous malformation is
obliterated, during which the transnidal blood flow is so slow that neither MR
imaging nor MR angiography can accurately reveal patency of the malformation.
Therefore, conventional cerebral angiography is usually required to exclude
the presence of a small residual nidus.
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Fig. 11A. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 40-year-old woman who underwent single
radiosurgical dose of 20 Gy to arteriovenous malformation. Axial T2-weighted
MR image (2120/80, TR/TE) obtained before radiosurgery shows nidus
(arrow) and deep draining veins (asterisks) of arteriovenous
malformation located in region of thalamus and posterior third ventricle.
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Fig. 11B. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 40-year-old woman who underwent single
radiosurgical dose of 20 Gy to arteriovenous malformation. Sagittal
T1-weighted MR image (355/12) also obtained before treatment shows nidus
(straight arrows) and deep draining veins (curved
arrow).
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Fig. 11C. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 40-year-old woman who underwent single
radiosurgical dose of 20 Gy to arteriovenous malformation. Sagittal
T1-weighted MR image (355/12) obtained 7 months after treatment shows decrease
in size of nidus (small straight arrows) and deep draining veins
(curved arrow). Nidus also shows increased intravascular signal,
indicating slow flow, thrombosis, or both. Defect in corpus callosum
(large straight arrow) is related to ventriculoperitoneal shunt.
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Fig. 11D. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 40-year-old woman who underwent single
radiosurgical dose of 20 Gy to arteriovenous malformation. Axial
fluid-attenuated inversion recovery MR image (9000/15), also obtained 7 months
after treatment, shows new hyperintensity in left thalamus and left
temporooccipital lobe (asterisks) that is probably related to
radiation-induced edema or venous hypertension. Flow voids of arteriovenous
malformations are decreased in size.
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Fig. 12A. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 53-year-old woman who underwent single
radiosurgical dose of 22 Gy to previously embolized arteriovenous
malformation. Enhanced axial T1-weighted MR image (380/12, TR/TE) obtained
before radiosurgery shows enhancing right medial frontal arteriovenous
malformation (curved arrow); punctate hypointense foci represent
flows voids (straight arrows).
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Fig. 12B. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 53-year-old woman who underwent single
radiosurgical dose of 22 Gy to previously embolized arteriovenous
malformation. Frontal subtraction right internal carotid artery angiogram
obtained before radiosurgery shows nidus of arteriovenous malformation
(curved arrow) and draining vein (straight arrow).
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Fig. 12C. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 53-year-old woman who underwent single
radiosurgical dose of 22 Gy to previously embolized arteriovenous
malformation. Enhanced axial T1-weighted MR image (400/12) obtained 12 months
after radiosurgery shows decrease in volume of enhancement (arrow);
flow voids are not seen. Enhancement is probably related to radiation-induced
breakdown of blood-brain barrier.
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Fig. 12D. —Evolution of radiosurgical changes after treatment of
arteriovenous malformation in 53-year-old woman who underwent single
radiosurgical dose of 22 Gy to previously embolized arteriovenous
malformation. Frontal subtraction magnified right internal carotid artery
angiogram obtained 13 months after radiosurgery shows no evidence of residual
arteriovenous malformation (arrows).
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References
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Regis J, Bartolomei F, Metellus P, et al. Radiosurgery for
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Introduction
Stereotactic Functional...
