Benign lesions:
The most common myxoid soft-tissue benign lesions are the intramuscular myxoma,
with no malignant potential,
and the aggressive angiomyxoma,
a locally aggressive entity.
1. Intramuscular myxoma:
It is a rare benign mesenchymal type of soft-tissue tumour which consists of an abundant myxoid stroma with bland undifferentiated stellate cells in it.
(Fig.
2) (Fig.
3) (Fig.
4) (Fig.
5)
It is said to have an approximate incidence of one case per million people,
affecting middle-aged women around 40-70 years.
Most of them are intramuscular in location,
appearing more frequently in the large muscles of the thigh (51%),
followed by the upper arm (9%),
the calf (7%) and the buttocks (7%).
Patients will refer a slow growing enlarging,
either painless or painful mass.
It has been described its association with the Mazabraud syndrome,
characterized by the combination of multiple intramuscular myxomas,
affecting predominantly lower limbs,
and a monostotic or polyostotic skeletal fibrous dyplasia.
These lesions are usually well circumscribed and can come to measure up to 17-20 cm in size.
Although well-marginated,
they may reveal mycroscopic infiltration at pathologic analysis,
irregular or ill-defined borders,
through where part of myxoid material may scape and extend to adjacent muscles leading to muscle atrophy,
oedema and fat deposition around the lesion.
Radiographs will show no pathologic findings in up to 55% of the cases,
being the remaining 45% of the findings nonspecific.
On US they will appear as well-defined hypoechoic masses with some internal heterogeneous echoes.
Anechoic cystic areas are present in 85% of the cases,
and in an 83% of the occasions we will see the “bright rim sign”; a rim with increased echogenicity around the lesion,
which is comparable to the fatty tissue around the myxoma detected on the MR.
Similar to the bright rim sign is the “bright cap sign”,
which represents a prominent adipose tissue deposit at either one or both poles of the lesion.
On Colour Doppler US lesions will be avascular or hypo-vascular.
On CT we will identify these lesions as homogeneous and well-described soft-tissue masses,
with low attenuation in relation to the surrounding muscle,
and mild diffuse,
peripheral or septal enhancement after contrast administration.
On MR,
just like on other imaging techniques,
lesions will be well-circumscribed,
homogeneous or mildly heterogeneous.
T1 weighed images will show homogeneous low to intermediate signal intensity.
This sequence will be useful to demonstrate the “rim of fat”,
present in 65-89% of the cases,
and that represents muscle atrophy and fatty infiltration owing to slow tumour growth.
T2 weighed images and fluid sensitive sequences will appear hyper-intense.
Using these sequences we may notice perilesional high intensity secondary to myxomatous stroma leakage.
These findings,
the rim of fat and the perilesional high signal intensity,
are unique MR features and highly suggestive of intramuscular myxoma.
On contrast enhanced images,
myxomas will show mild-moderate,
diffuse or thick peripheral and septal enhancement.
The differential diagnosis includes benign lesions such as ganglion,
synovial cyst or nerve sheath tumour,
and other myxoid neoplasms including liposarcoma and myxofibrosarcoma.
There are two main treatment options; firstly observation and surveillance,
because of the benign behaviour; and secondly surgical excision with wide margins,
with recurrence being rare.
If possible,
the latter one is preferred.
2. Aggressive angiomyxoma:
Uncommon mesenchymal locally aggressive tumour,
with abundant myxo-oedematous matrix and scattered spindle cells and medium-sized vessels.
(Fig.
6) (Fig.
7)
It affects predominantly women (90%),
with a wide range age of appearance (16-70 years),
and a peak incidence in forth to fifth decades of life.
It usually arises from deep soft tissues of the perineum and lower pelvis,
involving soft tissues of pelvis,
perineum,
vulva,
buttock,
retroperitoneum and inguinal regions.
Patients usually present with a slow-growing pelvic-perineal lump that can be asymptomatic or associate vague discomfort,
pressure-like sensation,
dull pain and/or dyspareunia.
Due to the ambiguity of the symptoms differential diagnosis includes entities that affect the same anatomic area,
such as Bartholin or Gartner cysts,
vulvar abscess,
vaginal prolapse,
lipomas,
hernias,
neurofibromas,
hydrocele or spermatocele.
It is often a well-defined,
lobular,
gelatinous mass that adhereres to the surrounding fat,
muscle and other surfaces.
At diagnosis most of these tumours are larger than 10 cm in size,
although they can grow larger.
It is important to notice that it tends to grow around the structures rather than penetrate them.
On histopathology,
they are generally positive for oestrogen and progesterone receptors,
suggesting a hormonal influence.
Radiographs may show mass-effect with displacement of pelvic structures as a nonspecific sign.
It will appear markedly hypoechoic heterogeneous with cystc and solid components on US.
On CT we will appreciate a well-defined hypodense mass with fat planes preservation and no invasion of muscular layers of vagina and rectum.
Similarly on T1-weighed MR images it has usually a low to intermediate signal intensity.
And on T2-weighed images it typically appears hyper-intense,
and rarely isointense.
Aggressive angiomyxomas have a characteristic finding,
a “swirling” or “layered pattern”,
which consists of hypo-intense strands with transelevetor spread,
caused by the fibro-vascular stroma as it grows and protrudes through pelvic diaphragm.
This pattern can also be noted at CT and T1-weighed MR images after the contrast administration.
Another characteristic feature is diffusion sequences is to show no restriction on ADC maps.
Because of its high recurrence rate after surgery,
it is considered a locally aggressive lesion.
This high recurrence rate may be attributed to initial incomplete resection.
Hence,
accurate description of lesions and their extension is necessary to allow for a complete resection,
since surgery is the mainstay of treatment.
As above mentioned,
they generally have positive oestrogen and progesterone receptors.
Therefore,
gonadotropin-releasing hormone receptor agonist may be of use to try to reduce tumour size.
Malignant lesions:
1. Myxoid liposarcoma:
Myxoid liposarcoma is an intermediate-high-grade malignant tumour composed of oval-shaped,
non-lipogenic mesenchymal cells,
a variable number of signet-ring lipoblasts,
surrounded by prominent myxoid stroma with arborized vascular network.
The percentage of round-cell high-grade component is a very important prognostic factor; >5% of round cells indicating a higher risk for recurrence and metastases.
(Fig.
8) (Fig.
9)
It is the second most common and prevalent subtype of liposarcoma,
representing 30-40% of all liposarcomas in the extremities.
It shows no gender predilection,
and usually affects younger patients than other types of liposarcomas,
with peak prevalence around 30-50 years.
Lesions most often occur in the lower extremities; medial thigh and popliteal location,
followed by the torso.
They are located in the intermuscular space,
with intramuscular and subcutaneous location being less frequent; which helps differentiate it from intramuscular myxoma.
It develops as a large,
larger than 15 cm at presentation,
painless,
slow growing,
and relatively soft mass.
These tumours exhibit predilection for extra-pulmonary metastases and progression with synchronous or metachronous multifocal disease.
Radiographs will show nonspecific soft-tissue mass,
with calcifications in well-differentiated liposarcomas.
US will demonstrate a complex,
well-defined,
hypoechoic but solid mass with posterior acoustic enhancement.
CT and MR studies will show a large,
multi-lobulated,
intermuscular lesion,
with high water content,
demonstrated as a low attenuation area on CT,
low signal intensity on T1-weighted images,
and markedly high signal intensity on T2-weighted images.
Myxoid liposarcoma has a pathognomonic feature on MR images; the identification of "fatty component or adipose tissue" within the mass.
The detection of this fatty component is difficult,
thus careful comparison of images,
as well as fat-suppression or inversion-recovery sequences may be useful.
The enhancement pattern when using contrast is variable and can be divided into three groups: homogeneous,
heterogeneous enhancement and no-enhancement.
If liposarcomas exhibit no-enhancement,
its differentiation from a cyst-like lesion may not be possible.
In these cases,
US will help us distinguish between a solid and a cyst lesion,
and its use is mandatory.
The differential diagnosis of myxoid liposarcomas include benign lesions,
such as cyst-like lesions,
ganglions or intramuscular myxoma,
and other malignant myxoid soft-tissue lesions like extra-skeletal myxoid chondrosarcoma.
Surgical resection is the treatment of choice,
with neoadjuvant or adjuvant chemotherapy or chemo-radiation being sometime necessary,
especially in high-risk or metastatic tumours.
Other neoplasms that may contain myxoid tissue:
Many other neoplasms may contain myxoid tissue,
but it will be neither their only nor their main component.
Herein we present a intraneural neurofibroma, whcih belongs to peripheral nerve sheath tumour group.
1. Intraneural neurofibroma:
It is a benign tumour identified in young adults,
20-30 years of age,
with no gender predilection.
(Fig.
10) (Fig.
11)
There are three types: localised,
diffuse and plexiform.
Localised account for 90% of all neurofibromas and are the type more easily confusable with myxoid lesions.
They usually present as slow-growing,
painless,
large masses,
involving cutaneous nerves.
Similar to lesions above described,
radiographic signs will be nonspecific,
showing,
in some cases soft-tissue mass-effect.
A hypoechoic,
well-defined,
fusiform-shaped mass will be seen through US,
with intrinsic blood flow on Colour Doppler.
Complete or incomplete echogenic rim surrounding the mass,
representing the entering of the nerves,
is rare,
but highly suggestive of neurofibroma.
At unenhanced CT neurofibromas have homogeneous low attenuation appearance,
probably due to; myelin,
fat,
cystic areas and myxoid tissue with high water content.
At MR a "target-like pattern" may be identified,
which refers to different signal intensities at the peripheral and central areas due to the distribution of components.
Thus,
on T1-weighted images central myxoid-tissue part will appear hypo-intense with no or mild contrast enhancement after Gadolinium,
whereas on T2-weighted images high intensity central signal will be seen,
where the myxoid degeneration tissue is located.
On CT-PET the myxoid part of the tumour will not show high densities,
whilst the cellular peripheral part will,
indicating hyper-metabolism.
If myxoid tissue is the major component,
it may be confused with other myxoid soft-tissue lesions.