Soft tissue lumps and bumps

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Introduction

Palpable lumps and bumps are frequently encountered. Although most of lumps are benign, the appearance of a new lump can cause worry in a patient due to the uncertainty of its nature. While many lumps can be assessed clinically by an experienced physician, there is a growing trend to increase the utilisation of imaging to evaluate soft tissue lumps. 

Ultrasound is a fast and effective tool to evaluate superficial soft tissue lumps. Soft tissue lumps are very common, but overall, the incidence of malignant/cancerous lumps is very low. Nevertheless, they frequently cause anxiety, especially when newly discovered, requiring an imaging diagnosis. In most cases, ultrasound can distinguish a superficial soft tissue lump as benign, non-concerning or potentially concerning lump requiring further imaging or biopsy.

In a UK-based study from 2018, out of 2399 lumps assessed on ultrasound, only 0.7% proved to be malignant[1]. In 95% of all lumps, ultrasound confirmed a benign diagnosis and reassured the patient. The remainder of the lumps required further investigation or biopsy but consequently had a benign diagnosis. This review demonstrated that most superficial palpable lumps are ultimately benign.   

In the same study, most soft tissue lumps are diagnosed as lipomas and ganglion. Other, less frequent benign lumps include epidermal cysts, nerve sheath tumours, and haemangiomas. The study also demonstrated that even those lumps referred to biopsy were mostly diagnosed as benign (epidermal cysts or lipomas).

During ultrasound evaluation, the experienced radiologist or sonographer will look for several features that might help to categorise a lump as benign, malignant, or sometimes indeterminate (meaning not definitely benign and not definitely malignant).

Important features on ultrasound imaging are[2]:

  1. The location of the lesion. Generally, a lesion in the superficial subcutaneous tissues is more likely benign compared to a lesion located deep to the deep fascia.
  2. The margin of the lesion
  3. The echogenicity, whether it appears ‘bright’ or ‘dark’ compared to other surrounding structures.
  4. The vascularity of the lesion. With a few exceptions, benign lesions often show only minimal or no vascularity.

Lipoma

A lipoma is a soft tissue lump composed of fatty tissue, often encapsulated by a thin fibrous membrane. On Ultrasound, they have a varied appearance depending on the degree of fibrous septa, from hypoechoic to hyperechoic. The fibrous septa are often aligned parallel to the skin. Most lipomas will show no internal blood flow. They are often compressible, giving them a soft feeling and touch. If a presumed lipoma is painful, increases in size, or is large and located very deep, it may warrant further evaluation.

Angiolipomas are related lesion to lipomas[3]. In addition to fatty tissue, they contain higher vascular components. Angiolipomas are more often multiple than lipomas.  Although they are benign, they can present with pain or increase in size. On ultrasound, they appear more echogenic and heterogeneous than lipomas and may have subtle internal blood flow.

Epidermal cysts

Epidermal cysts are benign cysts filled with keratin[4]. Keratin is a product of the skin and has a white-yellow ‘cheesy’ appearance. These cysts go under different names, including epidermal inclusion cyst, infundibular cyst, keratin cyst and epidermoid cyst. Often, they come from a hair track. They often present as a hard nodule under the skin and may have a punctum. On ultrasound, these cysts are hypoechoic and have well-defined borders. They are located along the deep surface of the dermis. The punctum can often be seen on ultrasound as a thin extension of the cyst to the skin surface, an appearance referred to as the ‘submarine sign.’ When they get infected or inflamed, they can become more ill-defined with increased blood flow, potentially mimicking a concerning lesion.

Ganglion cysts

Ganglions are Cystic structures containing thick viscous mucoid material. The often present has hard lumps around typical locations including the wrist and the foot. Diagnosis on ultrasound is usually straightforward. Ganglions demonstrate well-defined margins, are relatively anechoic and have no internal vascularity. Often, a tail can be demonstrated on ultrasound, indicating the site of origin of the cyst, usually a joint, or a tendon sheath. Ultrasound be useful in demonstrating the spatial relation of the cyst to surrounding nerves and blood vessels. Ultrasound guidance is often used to aspirate the cyst. Especially if the cyst is deeper, or near neurovascular structures, ultrasound guidance can protect from damage to those structures.

Potentially similar in appearance, but different in nature are other fluid-containing structures, for example, fluid in bursae or synovial recesses around joints. A good example is the commonly seen Baker’s cyst at the back of the knee.

Fat Necrosis

Fat necrosis is a self-limited condition characterized by tissue damage and inflammation in the subcutaneous fatty tissue due to vascular impairment or trauma. Other causes include surgery, injections, sickle cell disease, autoimmune conditions, and vasculitis.

On ultrasound, fat necrosis has a non-specific appearance. It ranges from areas of hyperechoic and poorly marginated fatty tissue to ill-defined hypoechoic lesions and does not demonstrate any vascularity. It can undergo cystic degeneration or calcification.

Because the appearance is non-specific, a preceding history of injury or injection to the area can help in making the diagnosis.

Peripheral Nerve Sheath Tumour

Peripheral nerve sheath tumours (PNSTs) are tumours of protective layer around the nerves.  The two common types of benign tumours are schwannomas and neurofibromas. Malignant peripheral nerve sheath tumours are very rare.

On ultrasound, PNSTs appear as solid uniform, hypoechoic masses. An important diagnostic feature is their close connection to peripheral nerves. While it can be challenging to differentiate between neurofibromas and schwannomas, there are some distinguishing features. Neurofibromas often have a more irregular shape and less blood flow, while schwannomas are rounder and smoother with more vascularity. Additionally, the position of the nerve in relation to the tumour is different between the two types: Schwannomas are more eccentric to the nerve, while neurofibromas often involve the nerve more centrally.

Applying slight pressure to the nodule can sometimes cause pain or other nerve-related symptoms, further confirming the diagnosis.

Lymph nodes

Lymph nodes are specialised tissues that play a role as part of our normal immune function. We all have lymph nodes at the common lymph node station, for example along the sides of the neck, the groin, and in the axilla. We are unaware of these nodes and only tend to feel them if they enlarge or become painful. Because they are part of the immune system, lymph nodes react whenever the immune system is activated, for example during a mild illness, or when bacteria enter through a skin wound. Rarely, lymph nodes can enlarge because of a more concerning illness.

An ultrasound scan can show the normal morphology of a lymph node, consisting of a hyperechoic fatty hilum and a hypoechoic rim. A healthy lymph node only demonstrates vascularity around the hilum at Doppler imaging. A reactive lymph node can become swollen but maintains its architecture. Ultrasound can look for any more concerning features, such as marked size increase, increased vascularity, and loss of the normal architecture. As the appearance of a normal lymph node, a reactive node and a diseased node are not clear-cut categories, sometimes blood tests, a follow-up scan after a time interval or a biopsy becomes necessary.

Conclusion

Ultrasound is an excellent first-line test to evaluate superficial soft tissue lumps and bumps. Most of the time, it can characterise a lump as benign or potentially malignant. Most of the lumps referred for ultrasound by the primary care turn out to be benign.

References

[1]        M. Charnock, N. Kotnis, M. Fernando, and V. Wilkinson, “An assessment of Ultrasound screening for soft tissue lumps referred from primary care,” Clin Radiol, vol. 73, no. 12, pp. 1025–1032, Dec. 2018, doi: 10.1016/J.CRAD.2018.07.102.

[2]        O. Catalano et al., “A bump: what to do next? Ultrasound imaging of superficial soft-tissue palpable lesions,” J Ultrasound, vol. 23, no. 3, p. 287, Sep. 2020, doi: 10.1007/S40477-019-00415-Z.

[3]        T. Omori and S. Nakamura, “Angiolipoma of the chest wall: a case report,” Surg Case Rep, vol. 8, no. 1, p. 32, Dec. 2022, doi: 10.1186/S40792-022-01384-Y.

[4]        C. B. Weir and N. J. St.Hilaire, “Epidermal Inclusion Cyst,” StatPearls, Aug. 2023, Accessed: Oct. 07, 2023. [Online]. Available: https://www.ncbi.nlm.nih.gov/books/NBK532310/