DOI:10.2214/AJR.05.0044
AJR 2006; 186:961-966
© American Roentgen Ray Society
MRI Findings of Subcutaneous Epidermal Cysts: Emphasis on the Presence of Rupture
Sung Hwan Hong1,
Hye Won Chung2,
Ja-Young Choi1,
Young Hwan Koh1,
Jung-Ah Choi1 and
Heung Sik Kang1
1 Department of Radiology, Seoul National University College of Medicine and
Institute of Radiation Medicine, 28 Yeongon-dong, Chongno-gu, Seoul, South
Korea.
2 Department of Radiology, Samsung Medical Center, Sungkyunkwan University
College of Medicine, 50 Ilwon-dong, Kangnam-gu, Seoul, South Korea.
Received January 9, 2005;
accepted after revision March 2, 2005.
Address correspondence to H. S. Kang
(kanghs{at}radcom.snu.ac.kr).
Abstract
OBJECTIVE. Our aim was to describe the MRI findings of subcutaneous
epidermal cysts with an emphasis on determining the presence of rupture.
CONCLUSION. Epidermal cysts show a fluidlike signal with variable
low-signal components on T2-weighted images and peripheral rim enhancement on
gadolinium-enhanced images. Most ruptured cysts have septa, show thick and
irregular rim enhancement, and are accompanied by a fuzzy enhancement in
surrounding subcutaneous tissues. These imaging features of a ruptured
epidermal cyst simulate a mass of infectious or neoplastic origin.
Keywords: extremities • MRI • subcutaneous epidermal cyst
Introduction
Epidermal cysts are cysts filled with keratin debris and bounded by a wall
of stratified squamous epithelium
[1]. Subcutaneous epidermal
cysts commonly involve the scalp, face, neck, trunk, and back; fewer than 10%
occur in the extremities [2]. A
few MRI reports have been issued on subcutaneous epidermal cysts
[3-7].
Shibata et al. [3] described
the MRI features of five cases of epidermal cysts in the extremities. They
reported that epidermal cysts have high signal intensity on T2-weighted images
and show no enhancement after IV gadolinium administration.
Lee et al. [8] showed the
sonographic findings of ruptured epidermal cysts, which can have lobulated
contours and show color Doppler signals, mimicking a solid mass. To the best
of our knowledge, this is the largest series of MRI cases of subcutaneous
epidermal cysts and the second report on the MRI findings of ruptured
subcutaneous epidermal cysts after Fisher's case report
[9]. The present study was
undertaken to determine the MRI features of subcutaneous epidermal cysts, with
an emphasis on the presence of rupture.
Materials and Methods
The records of 24 patients with pathologically confirmed epidermal cysts
were retrospectively reviewed. All patients in our study sample were selected
from the archives of our institution and were chosen from a review of
computer-generated patient diagnoses made between January 1998 and December
2004. The patients included 13 men and 11 women (average age, 42 years). For
the limited and anonymous review of patient data required for this study, the
institutional review board of our hospital did not require formal approval or
informed patient consent.
MRI planes, sequences, and field strengths varied because of the
heterogeneous nature of the study group. All patients included in the study
had at least one spin-echo T1-weighted MRI and one spin-echo or fast spin-echo
T2-weighted MRI. All patients also had IV gadolinium-enhanced MRI studies with
or without fat suppression.
MR images were evaluated by consensus between two experienced
musculoskeletal radiologists with respect to lesion location, size, shape, and
intrinsic characteristics as well as contrast enhancement pattern. Radiologic
evaluations included lesion size, shape (round, ovoid, lobulated, or
irregular), and border definition (well defined or poorly defined). MRI
features of the internal characteristics of epidermal cysts were analyzed in
terms of signal intensity, heterogeneity, and septation. Signal intensities of
masses were qualitatively compared with those of muscles and fluid. Lesions
were assessed on T1-weighted images relative to muscle as having high, low, or
mixed signal intensities. On T2-weighted images, signal intensities were
classified as high, low, or mixed where the signal intensities of normal
muscle were regarded as low, and bright signal intensities of fluid were rated
as high.
On gadolinium-enhanced images, we assessed the enhancement patterns of
lesions and surrounding soft tissue. We evaluated central and peripheral
enhancement in the cyst and focused on the cyst wall shape, that is, thickness
and smoothness. The presence or absence of fuzzy enhancement in soft tissue
surrounding cysts was also assessed.
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Fig. 1A —Unruptured epidermal cyst in subcutaneous layer of left palm in
45-year-old man. Axial T1-weighted image (TR/TE, 600/15) shows well-defined
mass (arrow) of slightly high signal intensity.
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Fig. 1C —Unruptured epidermal cyst in subcutaneous layer of left palm in
45-year-old man. Gadolinium-enhanced fat-suppressed T1-weighted image (600/12)
shows mass (arrow) with central nonenhancement and peripheral thin
rim enhancement.
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Fig. 2A —Unruptured epidermal cyst in back in 26-year-old man. Axial
T1-weighted image (TR/TE, 638/14) shows round subcutaneous mass
(arrow) of slightly high signal intensity. High-signal foci are shown
within mass.
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After the surgical excision of epidermal cysts, they were divided into two
categories by histopathologic examinations: unruptured or ruptured. We
compared the MRI findings of the two groups and then analyzed findings
statistically using Fisher's exact test. Significance was accepted at
p < 0.01.
Results
The locations of the 24 subcutaneous epidermal cysts were as follows: face
(n = 1), neck (n = 1), shoulder (n = 1), back
(n = 2), coccygeal area (n = 5), buttock (n = 3),
shoulder (n = 1), arm (n = 1), hand (n = 2), knee
(n = 1), thigh (n = 2), and foot (n = 4). Cysts
varied in size from 1.2 to 8.1 cm (mean, 3.5 cm), and their shapes also
varied, with round (n = 5), ovoid (n = 15), lobulating
(n = 2), and irregular (n = 2). Twenty-three of 24 cysts had
well-defined borders, and one had a poorly defined border.
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Fig. 4A —Ruptured epidermal cyst in right greater toe in 16-year-old girl
with recent pain. Axial T1-weighted image (TR/TE, 311/20) shows lobulating
subcutaneous mass (arrow) of low signal intensity.
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Fig. 4C —Ruptured epidermal cyst in right greater toe in 16-year-old girl
with recent pain. Gadolinium-enhanced fat-suppressed T1-weighted image
(578/20) shows mass with thick irregular peripheral rim enhancement and septal
enhancement (thick arrow). Fuzzy soft-tissue enhancement (thin
arrow) is also shown around mass.
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T2-weighted images showed seven (29%) lesions of high signal intensity,
three (13%) of low signal intensity, and 14 (58%) of mixed signal intensity.
Lesions of mixed signal intensity had a high background signal with a variable
amount of lower signal foci and serpentine structures (Figs.
1A,
1B,
1C,
2A,
2B,
3A, and
3B). On T1-weighted images,
epidermal cysts revealed low signal intensity in five (21%) of 24, high signal
intensity in seven (29%), and mixed low and high signal intensity in 12 (50%).
Four (17%) of the 24 had high signal foci on T1-weighted images (Figs.
2A and
2B). Ten (42%) of the 24
lesions had septa (Figs. 4A,
4B, and
4C).
On gadolinium-enhanced images, peripheral rim enhancement was seen in all
24 epidermal cysts. We were able to divide rim enhancement patterns into thin
and smooth wall enhancement (n = 17, 71%) (Figs.
1A,
1B, and
1C) and thick and irregular
wall enhancement (n = 7, 29%) (Figs.
4A,
4B,
4C,
5A,
5B, and
5C).
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Fig. 5A —19-year-old man with painful ruptured epidermal cyst in left
posterior shoulder. Axial T1-weighted image (TR/TE, 550/12) shows lobulating
subcutaneous mass (arrow) of low signal intensity.
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Fig. 5B —19-year-old man with painful ruptured epidermal cyst in left
posterior shoulder. Axial T2-weighted image (3,616/108) shows mass
(arrow) with central high signal and peripheral low signal
intensity.
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Fig. 5C —19-year-old man with painful ruptured epidermal cyst in left
posterior shoulder. Gadolinium-enhanced fat-suppressed T1-weighted image
(516/12) shows mass with very thick and irregular peripheral enhancement
(thick arrow). Subtle soft-tissue enhancement (thin arrow)
is present near mass.
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Histopathologically, the 24 epidermal cysts were classified into two
groups: unruptured (n = 16) and ruptured (n = 8).
Table 1 summarizes the
differences between the two groups in terms of MRI findings. A significant
difference between septation, enhanced rim shapes, and surrounding soft-tissue
enhancement was observed between the two cyst types (p < 0.01).
The majority of ruptured cysts had septa and thick and irregular rim
enhancement, which were accompanied by fuzzy enhancement in surrounding
subcutaneous tissues.
Discussion
Epidermal or epidermoid cysts result from the proliferation of epidermal
cells within a circumscribed dermal space. Epidermal cysts are probably formed
by several mechanisms—remnant ectodermal tissues misplaced during
embryogenesis [10], occlusion
of the pilosebaceous unit, or traumatic or surgical implantation of epithelial
elements [9,
11]. Human papillomavirus type
57 or 60 infections may be additional factors in the development of
palmoplantar epidermoid cysts
[12,
13].
Typically, unruptured epidermal cysts are well-defined round or ovoid
lesions of high signal intensity on T2-weighted MR images. In many cases,
T2-weighted images show variable low-signal components within epidermal cysts.
We speculate that variable signal intensity reflects heterogeneous
histopathologic features, as described previously by Brenner et al.
[14]. These authors reported
the various imaging findings regarding intratesticular epidermoid cysts, which
showed multiple layers of keratin debris, dense debris in cystic centers, and
intraluminal calcifications. Moreover, Cho et al.
[15] observed
high-signal-intensity foci within testicular epidermal cysts on T1-weighted
images; however, they did not prove them histopathologically. Fat-suppressed
gadolinium-enhanced images were available in two of four cases with these
foci; however, they were not suppressed. Gadolinium-enhanced MR images showed
central nonenhancement and peripheral thin rim enhancement.
The differential diagnosis of unruptured epidermal cyst includes other
subcutaneous cystic masses, some solid tumors, and vascular lesions. Fluid
cysts such as ganglion or bursitis also show bright signal intensity on
T2-weighted images and peripheral rim enhancement on gadolinium-enhanced
images. We believe that the meticulous observation of variable low-signal
components in epidermal cysts on T2-weighted images can be used to
differentiate epidermal cysts from other fluid cysts. In this study, all
unruptured epidermal cysts showed central nonenhancement and peripheral thin
rim enhancement without surrounding soft-tissue enhancement. However, using
this enhancement pattern alone, we were unable to distinguish them from other
cystic masses such as ganglion or bursitis.
Some solid tumors such as neurogenic tumors, nodular fasciitis, myxoid
tumors, and dermatofibrosarcoma protuberans may reveal bright signal intensity
on T2-weighted images and mimic cystic masses
[16-19].
However, contrast-enhanced MR images show variable central enhancement within
these masses, which is especially helpful for distinguishing solid tumors from
epidermal cysts. Hemangioma also presents as a bright mass on T2-weighted
images and reveals vascular space enhancement after contrast
administration.
Ruptured epidermal cysts were not different from unruptured cysts on T1-
and T2-weighted images by signal intensity. However, most ruptured epidermal
cysts contained septa and showed thick and irregular rim enhancement and fuzzy
adjacent soft-tissue enhancement on gadolinium-enhanced images. The peripheral
rim and septa were thick and irregular, and their enhancement patterns were
reminiscent of inflammatory lesions such as abscesses, complicated ganglion,
or bursitis. Clinically, it might be more important to differentiate between
ruptured epidermal cysts and other inflammatory or neoplastic lesions than to
determine whether an epidermal cyst is ruptured. Epidermal cysts that rupture
as a result of trauma can mimic pyogenic collar-button abscesses
[11]. Two of eight ruptured
epidermal cysts had very thick walls and septa and resembled some malignant
soft-tissue tumors with central necrosis (Figs.
5A,
5B, and
5C).
Malignant degeneration of epidermal cysts is uncommon, but there have been
some reports [20,
21]. Squamous cell carcinomas
did occur in 72 (2.2%) of 3,300 epidermal cysts examined
[20]. Although we did not have
a case of malignant transformation of an epidermal cyst, we believe that MRI
findings may be similar between squamous cell carcinomas arising from
epidermal cysts and ruptured epidermal cysts. We therefore emphasize that all
resected specimens should be scrutinized microscopically, especially in cases
with imaging features of ruptured epidermal cyst.
The limitations of our study include the relatively small size of the study
sample and the inherent limitations of its retrospective nature. Moreover, we
did not perform an MRI-pathology correlation study to explain variable signal
intensities within epidermal cysts. Because of the heterogeneous nature of the
study group, MRI sequences varied as did imaging planes and scan parameters.
Although gadolinium-enhanced MR images were available for all patients, some
of them were obtained with fat suppression whereas others were not.
Unruptured subcutaneous epidermal cysts were found to show characteristic
MRI findings including a well-demarcated mass, a high-signal-intensity
background with variable low-signal components on T2-weighted images, some
bright foci on T1-weighted images, and peripheral thin rim enhancement. An
epidermal cyst is more likely than another fluid cyst when the T2-weighted
image shows variable low-signal components within a cyst.
On MRI, ruptured epidermal cysts show thick and irregular peripheral rim
enhancement, surrounding soft-tissue reactions, and variable septa. Our
results indicate that a ruptured epidermal cyst may simulate infectious or
neoplastic lesions on MRI and should be included in the differential diagnosis
list.
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Abstract
Introduction
