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Focal myositis: a clinicopathologic study of 115 cases of an intramuscular mass-like reactive process.
[focal myositis]
Focal
myositis
is
an
uncommon
inflammatory
pseudotumor
of
skeletal
muscle
that
can
be
confused
with
a
variety
of
neoplastic
and
inflammatory
diseases
.
It
is
often
misunderstood
because
it
presents
as
a
tumor
-like
mass
,
but
histologically
resembles
a
skeletal
muscle
myopathy
or
dystrophy
.
We
wanted
to
discuss
the
detailed
morphologic
and
immunophenotypic
features
of
the
largest
reported
group
of
focal
myositis
patients
.
Two
hundred
and
six
cases
coded
as
"
focal
myositis
"
were
culled
from
our
files
.
Only
115
cases
with
adequate
material
,
a
solitary
lesion
,
and
correct
diagnosis
were
included
.
A
variety
of
immunohistochemical
studies
were
performed
,
as
were
polymerase
chain
reaction
for
T
cell
receptor
gene
rearrangement
and
immunoglobulin
heavy
chain
rearrangement
.
Age
ranged
from
7
to
94
years
(
mean
41
,
median
36
y
)
.
Most
patients
were
otherwise
healthy
,
and
with
the
exception
of
10
cases
,
lacked
antecedent
trauma
.
Masses
that
ranged
in
size
from
1
.
0
to
20
.
0
cm
(
median
3
.
0
cm
,
mean
3
.
9
cm
)
were
reported
in
specific
muscles
of
the
lower
extremities
(
including
vastus
lateralis
,
adductor
muscle
,
and
groin
muscles
,
n
=
39
;
gastrocnemius
,
n
=
22
)
,
followed
by
the
trunk
,
neck
(
mentalis
,
n
=
8
;
sternocleidomastoid
muscle
,
n
=
8
)
,
and
upper
extremity
.
Histologically
,
these
were
solitary
intramuscular
processes
composed
of
variable
myopathic
(
93
%
)
and
focal
neurogenic
(
89
%
)
changes
,
fibrosis
,
and
inflammation
(
97
%
)
,
occasionally
accompanied
by
prominent
eosinophils
(
n
=
20
)
.
By
immunohistochemistry
,
most
cases
had
CD
163
-
positive
macrophages
that
were
negative
for
S
100
protein
and
CD
1
a
.
Lymphocytes
were
mostly
CD
3
,
CD
4
-
positive
lymphocytes
that
were
negative
for
cytotoxic
markers
,
TIA
-
1
and
granzyme-
B
.
Polymerase
chain
reaction
did
not
show
B
cell
or
T
cell
rearrangement
.
In
situ
studies
for
Epstein-
Barr
-encoded
receptor
were
negative
,
as
was
ALK
-
1
immunohistochemistry
.
Major
histocompatibility
complex
-
1
and
weak
IgG
4
were
focally
positive
in
skeletal
muscle
.
Cases
with
severe
inflammation
had
increased
numbers
of
CD
2
0
-
positive
B
cells
and
CD
123
-
positive
plasmacytic
dendritic
cells
.
S
100
was
strongest
in
skeletal
muscle
fibers
with
vacuolar
change
.
Clinical
diagnostic
considerations
ranged
from
benign
entities
such
as
rhabdomyoma
,
intramuscular
lipoma
,
fibromatosis
,
myositis
ossificans
,
proliferative
myositis
,
inflammatory
myofibroblastic
tumor
,
and
inflammatory
myopathy
to
malignant
entities
such
as
rhabdomyosarcoma
,
leiomyosarcoma
,
liposarcoma
,
and
lymphoma
.
Available
follow-up
revealed
spontaneous
regression
.
Focal
myositis
occurs
in
specific
muscle
groups
of
young
adults
of
both
sexes
without
significant
trauma
.
It
is
a
largely
unrecognized
entity
with
specific
histology
including
myopathic
,
focal
neurogenic
,
fibrosis
,
and
inflammatory
features
.
It
can
be
easily
mistaken
for
an
inflammatory
myopathy
,
dystrophy
,
alternate
reactive
,
or
even
neoplastic
process
.
Focal
myositis
seems
to
be
a
macrophage
and
T
-
cell-rich
lesion
that
changes
to
B
cell
and
dendritic
plasmacytoid
cells
when
markedly
inflamed
,
but
does
not
seem
to
have
a
known
viral
or
molecular
etiology
.
IgG
4
presence
may
be
linked
to
the
fibrosis
in
these
lesions
;
a
possible
transient
autoimmune
etiology
can
not
be
excluded
.
Careful
attention
to
reproducible
clinicopathologic
features
can
aid
diagnosis
and
spare
patients
from
excessive
surgery
or
adverse
therapy
.
Diseases
Validation
Diseases presenting
"but histologically resembles a skeletal muscle myopathy or dystrophy"
symptom
focal myositis
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