Ultrasound of soft tissue pseudo-tumors and “whoops lesions”
Kil-Ho Cho, M.D., Department of Diagnostic Radiology
Yeungnam University School of Medicine, Daegu, Korea
The ratio of benign vs. malignant soft tissue masses is about 100:1. Fortunately, most of
them are non-neoplastic pseudotumors which are small and located superficially. Thus
ultrasound (US) is useful to evaluate various tumor-like soft tissue lesions.
Non-neoplastic pseudotumors we may encounter in everyday practice are: (1) peri-articular
cystic or solid masses, (2) inflammation/infection-related, (3) trauma-related, (4) foreign
bodies, (5) vascular/lymphatic lesions, (6) bone lesions, and (7) normal variants such as
accessory muscles or sesamoids. Recently, implanted materials for cosmetic purpose,
peri-prosthetic soft tissue changes and injection site granuloma may also cause confusion in
diagnosis.
US can be the first-line exam in conjunction with radiography in evaluation of soft-tissue
masses. US can be used to define the relationship of the mass to adjacent neurovascular
bundles, even though MR was already performed.
Knowledge of clinical data combined with ultrasonic physical exam is essential to interpret
the US findings for accurate diagnosis and for narrowing differential diagnosis. In
diagnosis of trauma-related pseudotumors, clinical history is very useful. A mass with
typical US findings in common predilection sites (for an example, bursitis) is mostly easy
to diagnose. However, a mass at unusual location with atypical findings even though it is
pathologically benign is quite difficult or impossible in making a correct diagnosis. For
examples, heterotopic ossification (=myositis ossificans), traumatic soft tissues, infection,
hypervascular lesion, and a mass with dirty calcification are very complex in echogenicity
according to the clinical stage, commonly resulting in wrong impression of malignant
masses.
US parameters for predicting malignancy are the echotexture, size, shape, number,
characteristics of the margin, peri-tumoral edema, and involvement of neurovascular
bundles &/or bone. The most reliable criterion among them in differentiation of a benign
from a malignant lesion is the lesion size. When a mass is larger than 5 cm, the possibility
of malignancy is relatively high (about 60%). When a mass is smaller than 3 cm, it may be
benign in 88% of specificity.
The better defined margin of a mass, the more likely benign. The more heterogeneous
echotexture of a mass, the more likely malignant. The findings, however, are commonly
overlapped and non-specific. In the situation, percutaneous needle biopsy under
US-guidance is tremendously useful. It is essential to evaluate the soft tissue tumors
radiologically before any invasive procedures such as biopsy. Because interpretation of the
imaging findings following biopsy is much less reliable owing to local soft tissue
alterations such as local inflammation, traumatic changes, and possible complications such
as hematoma.
With US, we can obtain unique useful information which is vague or missed on MR/CT or
other imaging modalities. The objectives of US for soft tissue tumors are (1) detection of
lesions, (2) diagnosis and differential diagnoses, if possible, (3) evaluation of relationship
between a mass and adjacent neurovascular channels, and (4) guidance for percutaneous
needle biopsy &/or aspiration for final histopathologic diagnosis.