We will be discussing the varied ultrasonographic appearances of subcutaneous epidermal inclusion cyst and its complications and highlighting the imaging features of other common subcutaneous lesions which will help us differentiate them from epidermal inclusion cyst.
Ultrasonographic characteristics to be evaluated in cases of epidermal inclusion cyst are size,
posterior acoustic enhancement,
internal echoes and vascularity.
- Size: It can vary from few millimeters to centimeters in size (the longest diameter),
however they have a mean size of 2-3cms.
 Fig. 1
Fig. 1: Size of epidermal inclusion cyst: (A) Longest dimensions measuring 2.5cm. (B) Large cyst with longest dimension measuring 7.5cm.
- Shape: Three types of shapes have been described ovoid,
lobulated and tubular,
ovoid being the commonest seen in approximately 70% of the cases. Fig. 2
Fig. 2: Shape of epidermal inclusion cyst: (A) Oval shaped lesion (B) Lobulated shaped lesion (C) Tubular shaped lesion
- Margin: Margins can be either well circumscribed lesion or not circumscribed,
with >90% of the lesions being well circumscribed. Fig. 3
Fig. 3: Margins of epidermal inclusion cyst: (A) Well circumscribed (B) Not circumscribed
- Posterior acoustic enhancement: About 90-95% of the cases of epidermal inclusion cysts demonstrate posterior acoustic enhancement.
 (Fig. 4) This feature of posterior acoustic enhancement or increased through transmission is an important characteristic which enables us to differentiate them with other subcutaneous lesions.
Fig. 4: Posterior acoustic enhancement seen in epidermal inclusion cyst (A) and (B)
- Echogenicity: Lesions can be either echogenic or hypoechoic to the surrounding tissue.
Fig. 5: Echogenicty of epidermal inclusion cyst: (A) Hypoechoic (B) Hyperechoic
- Internal echoes: Presence of varying amount of keratin within the cyst alters the appearance of internal echoes which have been classified by Lee et al.
as follows: Fig. 6
(I) Alternating hypoechoic and hyperechoic eccentric rings
(II) Target sign (hyoechoic lesion with hyperechoic center)
(III) Hypoechoic lesion with scattered echogenic reflectors
(IV) Inhomogeneous lesion,
(V) Areas of varying echogenicity.
Type III is the commonest type which gives it the typical appearance.
Fig. 6: Types of internal echoes in epidermal inclusion cyst:
(A) Type 1- Alternating hypoechoic and hyperechoic eccentric rings.
(B) Type 2- Target sign (hyoechoic lesion with hyperechoic center).
(C,D) Type 3- Hypoechoic lesion with scattered echogenic reflectors.
(E) Type 4- Inhomogeneous lesion.
(F) Type 5- Areas of varying echogenicity.
- Vascularity: On colour doppler,
about 80% cases show no evidence of internal vascularity[2,3] .
Vascularity may be seen in cases of rupture or if the cyst gets infected.[3,4] Fig. 7
Fig. 7: Vascularity of epidermal inclusion cyst: (A,B) No internal vascularity (C) Mild vascularity is noted.
Based on the above ultrasound characteristics the typical appearance of an unruptured epidermal inclusion cyst is:
predominantly hypoechoic lesion with posterior acoustic enhancement,
varying internal echo and without internal vascularity”.
After evaluating the lesion,
a focused scan for the surrounding soft tissue should be done to see for any changes of inflammation or signs of rupture.
Various complications such as infection,
rupture or malignant transformation can occur which further alters the appearance of the lesion making the diagnosis difficult.
Repeated trauma or enlargement often precipitates rupture leading to the formation of foreign body granuloma type of inflammatory reaction in the surrounding soft tissue.
 These pericystic changes such as hypoechogenicity of the surrounding soft tissue and increased peripheral vascularity are seen near the protruding portions of the cyst. Fig. 8
Fig. 8: Ruptured epidermal inclusion cyst:
(A) Well defined hypoechoic lesion with moderate perilesional hypoechogenicty implying rupture of the cyst.
(B) On colour doppler interrogation, mild perilesional vascularity is noted representing inflammatory response.
Cysts are usually lobulated in shape in case of rupture with uncircumscribed margins (Fig. 9) and may demonstrate some internal vascularity.[2,4,5]
Fig. 9: Ruptured epidermal inclusion cyst – The cyst is lobulated with uncircumcribed margins and a rent in the inferior wall with surrounding hypoechogenicity suggesting rupture.
Patients having rupture present with severe tenderness of the soft tissue swelling and a local rise of temperature.
These symptoms are probably due to secondary infections and surrounding reactive changes after rupture of the epidermal inclusion cysts.
Rate of malignant transformtion into squamous-cell carcinoma is about 2.2% according to the study by Bauer with the majority of the carcinomas being well differentiated.
Complete excision of the cyst with its wall must be done to reduce the risk of recurrence which is about 3%. Also proper care must be taken avoid spillage of the contents into the soft tissue,
as an inflammatory reaction is then likely to occur.
Various subcutaneous lesions such as lipoma,
lymph node and peripheral nerve sheath tumours are the common differentials which may mimic the appearance of epidermal inclusion cysts  and cause confusion in its diagnosis.
- Lipoma: Lipomas are by far the most frequently encountered subcutaneous masses. They can be located in the subcutaneous,
intramuscular or intermuscular plane.
Typically they are well encapsulated,
homogeneous,predominantly isoechoic to hyperechoic lesions without posterior acoustic enhancement.[7,8] (Fig. 10) Majority of the lesions show no colour flow on doppler study.[7,8] Hypoechoic lesions with ill defined margins are also found in some cases.
Fig. 10: Lipoma : An oval, well encapsulated, homogeneous, isoechoic to hyperechoic lesion showing no posterior acoustic enhancement.
- Fibrolipoma: Fibrolipoma is one of the infrequently reported subtypes of lipoma characterized by the presence of prominent bundles of mature fibrous tissue traversing the fatty lobules. Ultrasound shows a globular,
hyperechoic mass with heterogeneous internal echoes due to presence of fibrous tissue within and absent vascularity. Fig. 11
Fig. 11: Fibrolipoma : A well defined hypoechoic lesion with heterogeneous internal echoes and absent vascularity.
- Hemangioma: Hemangiomas are frequently encountered vascular malformations which constitute 7% of all soft-tissue masses.
Frequently seen in children and women.
Hemangiomas can be of two types,
capillary and cavernous.
they appear hypoechoic with prominent vascular channels,
phleboliths can be seen. (Fig. 12) Doppler evaluation may show low-resistance arterial flow with forward flow during both systole and diastole.
Arteriovenous malformations (AVMs) shows high vascularization with high peak velocity.
Fig. 12: Hemangioma : (A) An ill defined heterogeneous hypoechoic soft tissue lesion with multiple cystic areas. (B) Doppler evaluation shows multiple venous channels.
- Abscess: It is a manifestation of a spectrum of soft tissue skin infection which includes cellulitis and necrotizing fascitis. It appears as irregular-walled,
complex cystic lesions containing pus with debris or internal echoes inside.
(Fig. 13) Prominent flow around the abscess on colour doppler confirms the diagnosis.
Fig. 13: Abscess: An irregular-walled cystic lesions with dense internal echoes within.
- Ganglion cyst: Ganglion cysts commonly occur in the hand,
Patients have a history of trauma.
Ganglion cysts are frequently anechoic on ultrasound and demonstrate no vascularization on color doppler study.
It may extend into the joint space with a thin neck. Fig. 14
Fig. 14: Ganglion cyst: (A) An ill defined anechoic cystic structure overlying the left wrist joint. (B) Another cystic structure on the dorsal part of the right wrist with echoes within, extending to the joint space with a thin neck.
- Nerve sheath tumours: Schwannomas and neurofibromas are common nerve sheath tumours.
Schwannomas tend to be hypoechoic,
and well-defined masses on US that taper distally and have an eccentric cystic component. (Fig. 15) Neurofibromas often appear elongated and parallel to the long axis of the nerve.
Both tumors show vascularization on doppler ultrasound.
Fig. 15: Schwannoma: (A) A well-defined hypoechoic, round with heterogeneous internal echoes which is seen to be continuous with the nerve distally. (B) Colour doppler shows presence of internal vascularity.
- Hematoma: Is usually seen following a history of trauma.
Their appearance depends on the age of the hematoma.
They are spherical,
ovoid or lentiform,
hypo-echoic with irregular margins and posterior acoustic enhancement or even anechoic in the first 48 hours.
After 48 hours they are heterogeneous,
with moving echoes within.
Hematoma becomes anechoic,
similar to a cyst after approximately one month.
 Fig. 16
Fig. 16: Hematoma : (A) A small well defined anechoic lesion in a patient with history of trauma 2 months back. (B) A fairly well defined heterogeneous lesion with dense internal echoes in a patient presenting with history of trauma 1 week back.
- Lymph node: They have variable appearances depending upon the etiology of the lymph node being reactive,
malignant or infective.
- Reactive nodes tend to be hypoechoic compared with adjacent muscles and oval (short axis–to–long axis ratio [S/L] < 0.5) and have an echogenic hilum.
On color doppler,
reactive nodes predominantly show hilar vascularity. Fig. 17
Fig. 17: Reactive lymphadenopathy: (A) Oval hypoechoic lesion with echogenic hilum in the left cervical region (B) On color doppler interrogation, hilar vascularity is noted in another reactive lymph node.
- Malignant lymph nodes are usually hypoechoic,
and without echogenic hilum.
Intranodal necrosis is common in metastatic nodes.
Metastatic nodes with necrosis may show irregular margins.
Non necrotic nodes usually show peripheral vascularity. Fig. 18
Fig. 18: Malignant lymphadenopathy: (A) Round hypoechoic lesion with intranodal necrosis in the left submandibular region, in a known case of buccal mucosa carcinoma. (B) On color doppler interrogation, minimal peripheral vascularity is noted.
- Tuberculous nodes tend to be hypoechoic,
and without echogenic hilum and tend to show intranodal necrosis,
and adjacent soft-tissue edema.
On color Doppler it shows varied distribution with displacement of vascular hilum due to the high incidence of intranodal necrosis.
The characteristic imaging appearances of these lesions are summarized (Fig. 19 ) for a quick review.
Fig. 19: Table highlighting the ultrasound characteristics of various subcutaneous lesions.