One of the possibilities in our differential diagnosis for soft tissue masses is a lipoma.
Lipomas are the most common soft tissue tumors, often found in the subcutaneous tissue. A great article by Wagner, et al.1 discusses the results of a sonographic study of 72 lesions in 62 patients. Although only one lesion was located in the lower extremity, the study pointed out salient sonographic features of all lipomas that can aid in lower extremity situations. The standard of reference was the surgical pathology report. The ultrasound used was an Acuson Sequoia 512 system with an 8-15 MHz capability probe. The lesions imaged on ultrasound were assessed prospectively (prior to surgery) by a radiologist. Three radiologists provided a blinded, independent, retrospective review of the images as well. The study was highly accurate for lipomasInflatable Planes Combo 7 commercial, having a high sensitivity and specificity. These results were higher than studies done years ago. The authors felt that this had to do with the better resolution available on the newer ultrasounds and the availability of power and color Doppler.
Lipomas typically will be hyperechoic or isoechoic. They tend to be elliptical masses parallel to the skin surface with internal echogenic lines perpendicular to the sound beam. These lines tend to be gently curved. Although possible, hypoechogenicity is not common. Unless ruptured, they will have a well delineated border. Lipomas usually have no acoustic shadowing, which is a hypoechoic signal deep to the lesion. They will have minimal to no color or power Doppler.
Abscess is another possibility. Abscesses usually are hypoechoic and have an irregular border. They will have internal hyperechoic debris with no pattern. When you compress them, the debris can swirl around. An abscess will have a clinical history of infection and typically be in an environment of cellulitis, which has a “cobblestone” appearance on the surrounding tissue. Cellulitis will show on ultrasound as fat lobules floating in hypechoic fluid. (This is why ultrasound is helpful in determining if it is just cellulitis or if there is also an abscess involved that needs to be drained.) There may be a slight increase in vascularity, as seen on color Doppler.
Benign peripheral nerve tumors are another possibility. They can be classified into 2 types:
1) schwannomas (neurolemma)
Schwannomas are eccentrically located on the nerve, while neurofibromas are within the nerve. Surgically, it matters what you are dealing with. Unfortunately, you cannot tell with any significant accuracy between the two on ultrasound. Thus, if surgical intervention is needed, an MRI is warranted. One may ask, “why even do an ultrasound if an MRI is more specific?” Well, if you remember, clinically this mass would not have been diagnosed without the ultrasound. Also, we get a great deal of information just from the ultrasound alone. Schwannomas and neurofibromas will be elliptical hypoechoic masses with some mild internal echotexture. Usually you will be unable to compress the mass completely. The mass will abut the nerve and approximately 50% of time you won’t see the mass if it is a part of the nerve. There will be acoustic enhancement, although it may be hard to see if it the mass is sitting on the nerve. There will be vascularity on power Doppler and color Doppler.
As you can see, a benign peripheral tumor is definitely in our differential diagnosis and, I think, at the top of the list. Tasi, et al. wrote a great article on the topic.2
Ganglions are another group of soft tissue masses that certainly are in our differential diagnoses. Commonly seen coming off tendon sheaths or joints, they can also be associated with muscle and nerve, and thus cannot be excluded in our case. They tend to be anechoic or hypoechoic masses with a clear margin. They are not solid lesions, but have a more fluid internal environment. Ganglions usually have no vascularity, as assessed on color Doppler.
A mass of fibrous tissue origin, such as a fibroma, has a solid content. Although masses can be hypoechoic in regard to the surrounding tissue, they are not fluid filled. They appear as a solid lesion that is ovoid to lobulated. Fibromas are well defined and have moderate vascularity on Doppler.
Hematomas will have a clinical history of trauma and will have signs of inflammation upon clinical exam. The mass will by hypoechoic with borders that can be ill defined or, in some cases, defined. They will be avascular on Doppler.
So we see a working diagnosis:
- Benign peripheral nerve tumors – I put this above ganglion because of some vascularity seen and its association with the nerve
- Ganglion – would be listed first, but ganglia tend to be associated with tendon and have no vascularity on Doppler
- Fibroma – unlikely because fibromas tend to be solid and this is not
- Lipoma – unlikely because of the lack of internal echogenic lines
- Abscess – unlikely because of lack of clinical and sonographic signs of infection
- Hematoma – no clinical signs of injury or history of injury
This is a partial list. I recommend the following site, which shows nice examples of sonographic images of soft tissue masses. Look for the site’s search tool on the left side of screen.
Another article that addresses sonographic appearances of soft tissue masses is:
2. Tsai WC, Chiou HJ, Chou YH, et al. Differentiation between schwannomas and neurofibromas in the extremities and superficial body: the role of high-resolution and color Doppler ultrasonography. J Ultrasound Med. 2008 Feb;27(2):161-166; quiz 168-169.