|The dot-in-circle sign of mycetoma on MRI
|Jyoti Kumar1, Atin Kumar1, Pradeep Sethy2, Somesh Gupta2
|1From the Departments of Radiology, All India Institute of Medical Sciences, New Delhi, India
2From the Departments of Dermatology, All India Institute of Medical Sciences, New Delhi, India
|Keywords: • maduromycosis • magnetic resonance imaging
Mycetoma is a chronic granulomatous disease prevalent
in tropical countries, but it also occurs in Europe
and the United States. Early diagnosis is important as
it has therapeutic implications. Although biopsy and
microbiological culture provide the definitive diagnosis,
these are difficult to achieve in many instances.
The dot-in-circle sign is a recently proposed magnetic
resonance imaging (MRI) sign of mycetoma, which
is likely to be highly specific. We present a case of
mycetoma of the thigh with characteristic MRI features.
To the best of our knowledge, only 2 cases of
mycetoma of the foot demonstrating this sign have
been previously published.
M ycetoma is a debilitating chronic granulomatous disease prevalent
in tropical and subtropical regions, but it also occurs in
Europe and the United States. It was first described in Madura,
India, in 1846; hence, the eponym Madura foot [1
]. The disease can be
caused by 2 groups of organisms, the Eumyces, or true fungi (eumycetoma),
and the Actinomyces, which are filamentous bacteria of the order
Actinomycetales (actinomycetoma) [1
]. The evolution of the disease is
slow and mostly painless. Patients present many years after the onset
of infection, often with extensive soft tissue and bone involvement [2
The organism first lodges in the soft tissues. Bones are almost always attacked
from outside, in contrast to bacterial osteomyelitis, and periosteal
reaction and cortical erosion may then be seen. Early diagnosis, before
the appearance of sinuses and grains, is difficult. If left untreated, it may
result in severe disability, often necessitating amputation. Although biopsy
or microbiological culture of the discharge will yield the definitive
diagnosis, both may be difficult to achieve with fastidious organisms.
Imaging can aid in the early diagnosis of the disease. We present the
magnetic resonance imaging (MRI) characteristics of a patient with mycetoma
that demonstrated the recently described dot-in-circle sign, suggesting
the possible diagnosis before the histological diagnosis.
A 28-year-old man complained of a mass in the right thigh for 4
years. He previously underwent a wide excision biopsy with skin grafting
in another institution, based on the clinical suspicion of a soft tissue
tumor. Histopathological evaluation revealed it to be an inflammatory
process with chronic abscess formation. The patient was treated with
multiple courses of oral and intravenous antibiotics over the following
6–8 months, to which he did not respond. During this period the patient
developed multiple discharging sinuses with which he presented to our
hospital. On physical examination (Fig. 1
), non-tender swelling involving
mainly the posteromedial aspect of the right thigh was noted. It
was associated with numerous chronic sinuses. No grains, however, were
seen from any of the discharging sinuses. General examination was unremarkable,
with no other soft tissue masses. Blood and serum chemistry
were also unremarkable.
Click to Enlarge
|Figure 1: Photograph of the posterior right thigh shows soft tissue thickening with multiple
Plain radiograph of the right thigh showed soft tissue swelling with superficial
skin ulceration. No calcification or bone destruction was seen.
MRI was performed to characterize and evaluate the extent of the disease.
T2-weighted and T2-weighted fat-saturated MRI revealed diffuse
hyperintensity involving subcutaneous tissue, muscles, and intermuscular
fascial planes, with multiple focal fluid collections and overlying skin
ulceration. In addition, multiple small discrete spherical hyperintense lesions
were noted, which were divided by a network of low-intensity soft tissue. In the center of some of these
lesions, there was a tiny hypointense
focus, resulting in the dot-in-circle sign
(Figs. 2, 3). Small conglomerated lowintensity
foci and microabscesses were
also seen at other sites (Figs. 4, 5). The
underlying bones were normal. The diagnosis
of mycetoma was made based
on these findings.
Click to Enlarge
|Figure 2: T2-weighted transverse MR image of the right
thigh reveals extensive inflammatory changes. Multiple small
spherical hyperintense lesions separated by tissue of low signal
intensity are noted. Some of these lesions (arrows) show a
central tiny hypointense focus, resulting in the dot-in-circle sign.
Click to Enlarge
|Figure 3: T2-weighted fat-saturated transverse MR image
of the right thigh shows inflammatory soft tissue changes
with fluid collections. At least 3 spherical hyperintense
lesions, each with a tiny central hypointense focus, can be
Click to Enlarge
|Figure 4: T2-weighted fat-saturated transverse MR image of
the right thigh shows multiple microabscesses separated by
a low-intensity matrix seen posteriorly (black arrows), distinct
from the adjacent musculature. Note the presence of marked
inflammatory changes with multiple fluid collections (white
Click to Enlarge
|Figure 5: T2-weighted transverse MR image of the right
thigh shows multiple small adjoining low-intensity lesions
(arrows) that may represent a conglomerate of grains in the
background of diffuse hyperintense inflammatory changes.
Subsequently, a biopsy was performed,
which revealed granulomatous
inflammation. There were multiple
neutrophilic microabscesses surrounded
by granulation tissue composed of
acute and chronic inflammatory cells.
There was dense connective tissue,
containing plasma cells, macrophages,
lymphocytes, eosinophils, and occasional
giant cells, separating the foci
of inflammation. Although fungal organisms
could not be isolated, the histopathological
features were consistent
The patient was put on itraconazole
therapy and significant clinical improvement
was observed at the 11th
The term mycetoma is a clinical
entity, which applies to a chronic inflammatory
process of soft tissue, usually
of the foot, resulting from the implantation
of one or various fungi or
actinomycetes. Initially there is the formation of soft tissue swelling with
induration due to the underlying granulation
tissue. It usually progresses to
the formation of sinuses and extrusion
of grains. The lesion may be confined
to the soft tissue for years before bone
involvement occurs. The diagnosis of
mycetoma should not be considered
by physicians when presentation is
limited to soft tissues, without sinus or
bone involvement. Although mainly a
disease of the tropics, patients living in
temperate regions may also be affected
by this entity [3
], though they are
often misdiagnosed as soft tissue tumors
in the early stage.
Histopathologically, the inflammatory
reaction of mycetoma is non-specific
and, in the absence of isolation of
fungal grains, it is difficult to differentiate
from other inflammatory soft tissue
processes and cold abscesses, which is
not an uncommon occurrence.
Although various radiographic bone
changes have been described in cases
of mycetoma, bone involvement occurs
late in the course of the disease,
when non-surgical cure is unlikely.
Non-invasive imaging with MRI can
characterize the soft tissue masses of
mycetoma and aid in early diagnosis.
Czechowski et al.  described the MR
appearance of mycetomas and found
small low-signal intensity lesions on
T1-weighted and T2-weighted MR images in 16 of 20 patients. They suggested
that these appearances were due to susceptibility
from the metabolic products
within the grains. They observed lesions
showing a conglomerate of low-intensity
foci, as was seen in the presented case.
The dot-in-circle sign is a recently
described sign reflecting the unique
pathological feature of mycetoma. It is
seen as a tiny hypointense focus within
high-intensity spherical lesions.
This sign was proposed by Sarris et al.
 in 2003 on T2-weighted, STIR, and
T1-weighted fat-saturated gadoliniumenhanced
images. They correlated the
MRI and histological findings in 2 cases
of mycetoma and concluded that
the small central hypointense foci
represented the fungal balls or grains,
while the surrounding high-signal
intensity foci represented the inflammatory
granulomata . The low-intensity
tissue seen surrounding these
lesions represented the fibrous matrix.
They proposed that it is likely to be a
highly specific sign for mycetoma . We were able to demonstrate similar
MRI findings in the presented case.
Although the histopathological specimen
in our case showed features of
chronic granulomatous inflammatory
response consistent with the clinical
and imaging diagnosis of mycetoma,
fungal grains were not visualized. We
postulate that the grains may not have
been present in the biopsy specimen,
which represents only a small portion
of the extensively involved area. MRI
clearly depicted the conglomerated
hypointense foci scattered in a few
regions, which likely represent the
susceptibility effect due to the grains.
The marked clinical response to itraconazole
therapy also substantiated
the diagnosis in our case.
Few radiographic bone changes have
been described to distinguish between
actinomycetoma and eumycetoma
. Eumycotic lesions tend to form a
few cavities in bone ≥1 cm in diameter,
while actinomycetes often form
smaller, but more numerous cavities.
In a study by Lewall et al. , a motheaten
appearance caused by a combination
of irregular periosteal reaction,
periosteal erosion, and small cavities
within bone were seen in 25% of cases
of actinomycetoma, but in none of
the patients with eumycetoma. The
distinction between the 2 forms of
soft tissue mycetoma was not possible
To conclude, we stress the importance
of MRI in the early diagnosis
of mycetoma, even before the development
of sinuses and/or extrusion
of grains. Furthermore, as these
fastidious organisms may be difficult
to demonstrate either on biopsy or
microbiological culture, the clinical
picture often necessitates multiple
surgical biopsies, thus exacerbating
morbidity due to delays in diagnosis
and therapeutic intervention. MRI
can be useful in such situations. It
can strongly suggest the diagnosis of
mycetoma when it demonstrates the dot-in-circle sign, conglomerated lowsignal
intensity foci, or microabscesses
in the background of a hypointense
matrix as described above.
Lewall DB, Ofole S, Bendl B. Mycetoma.
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